WHAT IS .NET ??

C# 

1.    What is .NET?

·         It is a platform neutral framework.

·         It is a layer between the operating system and the programming language.

·         It supports many programming languages, including VB.NET, C# etc.

·         .NET provides a common set of class libraries, which can be accessed from any .NET based programming language. There will not be separate set of classes and libraries for each language. If you know any one .NET language, you can write code in any .NET language!!

·         In future versions of Windows, .NET will be freely distributed as part of operating system and users will never have to install .NET separately.

2.    What is Not?

·         .NET is not an operating system.

·         .NET is not a programming language.

         3. “.NET is a framework”

         Confused with this definition?

·         We cannot define .NET as a ‘single thing’.

·         It is a new, easy, and extensive programming platform.

·         It is not a programming language, but it supports several programming languages.

·         By default .NET comes with few programming languages including C# (C Sharp), VB.NET, J# and managed C++.

·         .NET is a common platform for all the supported languages. It gives a common class library, which can be called from any of the supported languages.

·         So, developers need not learn many libraries when they switch to a different language. Only the syntax is different for each language.

·         When you write code in any language and compile, it will be converted to an ‘Intermediate Language’ (Microsoft Intermediate Language – MSIL).

·         So, your compiled executable contains the IL and not really executable machine language.

·         When the .NET application runs, the .NET framework in the target computer take care of the execution. (To run a .NET application, the target computer should have .NET framework installed.)

·         The .NET framework converts the calls to .NET class libraries to the corresponding APIs of the Operating system.

·         Whether you write code in C# or VB.NET, you are calling methods in the same .NET class libraries.

·         The same .NET framework executes the C# and VB.NET applications.

·         So, there won’t be any performance difference based on the language you write code.

                 3.1 Is it platform independent?

Many people ask this question “Java is platform independent, what about .NET?”.

The answer is “Yes” and “No”!

·         The code you write is platform independent, because whatever you write is getting compiled into MSIL.

·         There is no native code, which depends on your operating system or CPU. But when you execute the MSIL, the .NET framework in the target system will convert the MSIL into native platform code.

·         So, if you run your .NET exe in a Windows machine, the .NET framework for Windows will convert it into Windows native code and execute.

·         If you run your .NET application in Unix or Linux, the .NET framework for Unix/Linux will convert your code into Unix/Linux native code and execute.

·         So, your code is purely platform independent and runs anywhere!

·         But wait, we said it wrong… there is no .NET framework for UNIX or Linux available now.

·         Microsoft has written the .NET framework only for Windows.

·         If you or some one else write a .NET framework for other platforms in future, your code will run there too. So, let us wait until someone write .NET framework for Linux before you run your .NET code in Linux.

                  3.2 Major Issues before .NET

·         Registration of COM components.

·         Unloading COM components

·         Versioning Problem (DLL Hell)

The .NET Platform

The .Net platform is a set of technologies. Microsoft .NET platform simplify software development (Windows or WEB) by building applications of XML Web services.

The .NET platform consists of the following core technologies which are refer as components of .NET: -

·         The .NET Framework

·         The .NET Enterprise Servers

·         Building block services

·         Visual Studio .NET

A programming model (.NET framework) enables developers to build Extensible Markup Language (XML) Web Services and applications.

Developer Tools

.NET Framework

Application

Libraries

CLR

.NET Enterprise Servers

.NET Building Block Services

C#, Visual Basic .NET

Visual Studio .NET

Windows OS (Win 32)

The .NET Platform Architecture

3.3 Components of .NET Framework

The .Net Framework consists of:

·         Common Language Runtime

·         Class Libraries

·         Support for Multiple Programming Language

.NET Compliant Languages (VC++, VB.NET, ASP.NET, C# and other third party languages)

Common Language Runtime (Memory Management, Common Type System, Garbage Collector)

Windows Forms Web Forms Web Services

.NET Framework Base Class Library (ADO.NET, XML, Threading, Diagnostics, IO, Security, etc.)

Components of .NET Framework

4. Application Development and Execution

.NET is a multilingual platform then any .NET based language can be chosen to develop applications.

4.1 Choosing a Compiler

According to the language we can choose its run time aware compiler for .NET platform. Because it is a multilingual execution environment, the runtime supports a wide variety of data types and language features.

4.2 Compiling to MSIL

Source Code

Compiler

Class Libraries (IL & Metadata)

EXE/DLL (IL & Metadata)

Class loader

Security checks

Managed Native Code

Execution

JIT Compiler

Runtime Engine

Call to an un- compiled method

Trusted pre-JIT code only

Source code to native code and code execution

When compiling source code, the compiler translates it into an intermediate code represented in MSIL. Before code can be run, MSIL code must be converted to CPU-specific code, usually by a just-in-time (JIT) compiler. When a compiler produces MSIL, it also produces metadata. Metadata includes following information: -

·         Description of the types in your code, including the definition of each type.

·         The signatures of each type’s members,

·         The members that your code references.

·         Other data that the runtime uses at execution time.

The MSIL and metadata are contained in a portable executable (PE) file that is based on and extends the published Microsoft PE and Common object file format (COFF) used historically for executable content. The file format, which accommodates MSIL or native code as well as metadata, enables the operating system to recognize common language runtime images.

4.3 Compiling MSIL to Native Code

Before you can run Microsoft intermediate language (MSIL), it must be compiled against the common language runtime to native code for the target machine architecture. The .NET Framework provides two ways to perform this conversion:

•A .NET Framework just-in-time (JIT) compiler.

•The .NET Framework Ngen.exe (Native Image Generator).

4.4 Compilation by the JIT Compiler

JIT compilation converts MSIL to native code on demand at application run time, when the contents of an assembly are loaded and executed. Because the common language runtime supplies a JIT compiler for each supported CPU architecture, developers can build a set of MSIL assemblies that can be JIT-compiled and run on different computers with different machine architectures. However, if your managed code calls platform-specific native APIs or a platform-specific class library, it will run only on that operating system.

JIT compilation takes into account the possibility that some code might never be called during execution. Instead of using time and memory to convert all the MSIL in a PE file to native code, it converts the MSIL as needed during execution and stores the resulting native code in memory so that it is accessible for subsequent calls in the context of that process. The loader creates and attaches a stub to each method in a type when the type is loaded and initialized. When a method is called for the first time, the stub passes control to the JIT compiler, which converts the MSIL for that method into native code and modifies the stub to point directly to the generated native code. Therefore, subsequent calls to the JIT-compiled method go directly to the native code.

Install-Time Code Generation Using NGen.exe

Because the JIT compiler converts an assembly’s MSIL to native code when individual methods defined in that assembly are called, it affects performance adversely at run time. In most cases, that diminished performance is acceptable. More importantly, the code generated by the JIT compiler is bound to the process that triggered the compilation. It cannot be shared across multiple processes. To allow the generated code to be shared across multiple invocations of an application or across multiple processes that share a set of assemblies, the common language runtime supports an ahead-of-time compilation mode. This ahead-of-time compilation mode uses the Ngen.exe (Native Image Generator) to convert MSIL assemblies to native code much like the JIT compiler does. However, the operation of Ngen.exe differs from that of the JIT compiler in three ways:

•It performs the conversion from MSIL to native code before running the application instead of while the application is running.

•It compiles an entire assembly at a time, instead of one method at a time.

•It persists the generated code in the Native Image Cache as a file on disk.

4.5 Summary of Managed Code Execution Process

The process of compiling and executing managed code is given below: -

1.    When you compile a program developed in a language that targets the CLR, instead of compiling the source code into machine-level code, the compiler translates it into Microsoft Intermediate Language (MSIL) or Intermediate language (IL). This ensures language interoperability.

2.    In addition to translating the code into IL, the compiler also produces metadata about the program during the process of compilation. Metadata contains the description of the program, such as classes and interfaces, the dependencies and the versions of the components used in the program.

3.    The IL and the metadata are linked in assembly.

4.    The compiler creates the .EXE or .DLL file.

5.    When you execute the .EXE or .DLL file, the code (converted to IL) and all the other relevant information from the base class library is sent to the class loader. The class loader loads the code in the memory.

6.    Before the code can be executed, the .NET framework needs to convert the IL into native or CPU-specific code. The Just-in-time (JIT) compiler translates the code from IL to managed native code. The CLR supplies a JIT compiler for each supported CPU architecture. During the process of compilation, the JIT compiler compiles only the code that is required during execution instead of compiling the complete IL code. When an uncompiled method is invoked during execution, the JIT compiler converts the IL for that method into native code. This process saves the time and memory required to convert the complete IL into native code.

7.    During JIT compilation, the code is also checked for type safety. Type safety ensures that objects are always accessed in a compatible way. Therefore, if you try to pass an 8-byte value to a method that accepts a 4-byte value as a parameter, the CLR will detect and trap such an attempt. Type safety also ensures that objects are safely isolated from each other and are therefore safe from any malicious corruption.

8.    After translating the IL into native code, the converted code is sent to the .NET runtime manager.

9.    The .NET runtime manager executes the code. While executing the code, a security check is performed to ensure that the code has the appropriate permissions for accessing the available resources.

Common Language Infrastructure (CLI)

The Common Language Infrastructure (CLI) is an open specification developed by Microsoft that describes the executable code and runtime environment that allows multiple high-level languages to be used on different computer platforms without being rewritten for specific architectures.

The common language Infrastructure (CLI) is a theoretical model of a development platform that provides a device and language independent way to express data and behavior of applications.

The CLI specification describes the following four aspects: -

·         The Common Type System (CTS)

The language interoperability and .NET Class Framework are not possible without all the language sharing the same data type. CTS is an important part of the runtime support for cross-language integration. The CTS performs the following functions:

·         Establishes a framework that enables cross-language integration, type safety and high performance code execution.

·         Provides an object-oriented model that supports the complete implementation of many programming languages.

The CTS supports two general categories of types: -

1. Value Types

Value types directly contain their data, and instances of value types are either allocated on the stack or allocated inline in a structure. Value types can be built-in, user-defined or enumerations types.

2. Reference Types

Reference types store a reference to the value’s memory address, and are allocated on the heap. Reference types can be self-describing types, pointers type or interface types. The type of a reference type can be determined from values of self-describing types. Self-describing types are further split into arrays and class types are user-defined classes, boxed value types, and delegates.

 A set of data types and operations that are shared by all CTS-compliant programming languages.

·         Metadata

Information about program structure is language-agnostic, so that it can be referenced between languages and tools, making it easy to work with code written in a language you are not using.

·         Common Language Specification (CLS)

A set of base rules to which any language targeting the CLI should conform in order to interoperate with other CLS-compliant languages. The CLS rules define a subset of the Common Type System.

·         Virtual Execution System (VES)

The VES loads and executes CLI-compatible programs, using the metadata to combine separately generated pieces of code at runtime.

The Common Language Runtime

The CLR is one of the most essential components of the .NET framework. The CLR or the runtime provides functionality such as exception handling, security, debugging, and versioning support to any language that targets it. The CLR can execute programs written any language. You can use the compilers to write the code that runs in the managed execution environment provided by the CLR. The code that is developed with a language compiler that targets the CLR is managed code. On the other hand, the code that is developed without considering the conventions and requirements of the common language run time is called unmanaged code.

CLR activates objects, performs security checks, lays them out in memory, executes them and garbage collects these objects as well.

The CLR is a runtime engine that loads required classes, performs just in time compilations, and enforces security checks and a bunch of other runtime functions.

The CLR executables are either exe or DLL files that consist mostly of metadata and code. These executables must adhere to a file format called the Portable Executable (PE) file format.

Features Provided by CLR

Some of the features provided by the CLR are as follows: -

·         Automatic Memory Management: The CLR provides the garbage collection feature for managing the lifetime of an object. This process relieves a programmer of the task of manual memory management by deallocating the blocks of memory associated with objects that are no longer being used. The objects whose lifetime is managed by the garbage collection process are called managed data.

·         Standard Type System: The CLR implements a formal specification calledCommon Type System (CTS). The CTS is an important part of the support provided by the CLR for cross-language integration because it provides a type system that is common across all programming languages. It also defines the rules that ensure that objects written in different languages can interact with each other.

·         Language Interoperability: Language interoperability is the ability of an application to interact with another application written in a different programming language. Language interoperability helps maximize code reuse. For example, you can write a class in Visual Basic and inherit it in a code written in Visual C++  or c#.

·         Platform Independence: When you compile a program developed in language that targets the CLR, the compiler translates the code into an intermediate language. This language is CPU-independent. This means that the code can be executed from any platform that supports the .NET CLR.

·         Security Management The traditional operating system security model provides permissions to access resources, such as memory and data, based on user accounts. In .NET platform security is achieved through the Code Access Security (CAS) model. The CAS model specifies what the code can access instead of specifying who can access resources.

·         Type Safety: This feature ensures that objects are always accessed in compatible ways. Therefore the CLR will prohibit a code from assigning a 10-byte value to an object that occupies 8 bytes.

Advantages of the .NET Framework

·         Consistent programming model

·         Multi-platform applications

·         Multi-Language integration

·         Automatic Resource Management

·         Ease of deployment

OOPS & C#    

The skeleton of object – oriented programming is of course the concepts of class. The C# on OOPS explains classes and their importance in implementation of object oriented principles.

   Any language can be called object oriented if it has data and method that use data encapsulated in items named objects. An object oriented programming method has many advantages; some of them are flexibility and code reusability.

Key Concepts of Object Orientation

·         Abstraction

·         Encapsulation

·         Inheritance

·         Polymorphism

Abstraction is the ability to generalize an object as a data type that has a specific set of characteristics and is able to perform a set of actions.

Object-oriented languages provide abstraction via classes. Classes define the properties and methods of an object type.

Examples:

 You can create an abstraction of a dog with characteristics, such as color, height, and weight, and actions such as run and bite. The characteristics are called properties, and the actions are called methods.

 A Recordset object is an abstract representation of a set of data.

Classes are blueprints for Object.

Objects are instance of classes.

Object References

When we work with an object we are using a reference to that object. On the other hand, when we are working with simple data types such as Integer, we are working with the actual value rather than a reference.

When we create a new object using the New keyword, we store a reference to that object in a variable. For instance:

Draw MyDraw = new Draw;

This code creates a new instance of Draw. We gain access to this new object via the MyDraw variable. This variable holds a reference to the object.

Now we have a second variable, which also has a reference to that same object. We can use either variable interchangeably, since they both reference the exact same object. The thing we need to remember is that the variable we have is not the object itself but, rather, is just a reference or pointer to the object itself.

Early binding means that our code directly interacts with the object, by directly calling its methods. Since the compiler knows the object’s data type ahead of time, it can directly compile code to invoke the methods on the object. Early binding also allows the IDE to use IntelliSense to aid our development efforts; it allows the compiler to ensure that we are referencing methods that do exist and that we are providing the proper parameter values.

Late binding means that our code interacts with an object dynamically at run-time. This provides a great deal of flexibility since our code literally doesn’t care what type of object it is interacting with as long as the object supports the methods we want to call. Because the type of the object isn’t known by the IDE or compiler, neither IntelliSense nor compile-time syntax checking is possible but we get unprecedented flexibility in exchange.

If we enable strict type checking by using Option Strict On at the top of our code modules, then the IDE and compiler will enforce early binding behavior. By default, Option Strict is turned off and so we have easy access to the use of late binding within our code.

Access Modifiers Access Modifiers are keywords used to specify the declared accessibility of a member of a type.

Public is visible to everyone. A public member can be accessed using an instance of a class, by a class’s internal code, and by any descendants of a class.

Private is hidden and usable only by the class itself. No code using a class instance can access a private member directly and neither can a descendant class.

Protected members are similar to private ones in that they are accessible only by the containing class. However, protected members also may be used by a descendant class. So members that are likely to be needed by a descendant class should be marked protected.

Internal/Friend is public to the entire application but private to any outside applications. Internal is useful when you want to allow a class to be used by other applications but reserve special functionality for the application that contains the class. Internal is used by C# and Friend by VB .NET.

Protected Internal may be accessed only by a descendant class that’s contained in the same application as its base class. You use protected internal in situations where you want to deny access to parts of a class functionality to any descendant classes found in other applications.

Composition of an OBJECT

We use an interface to get access to an object’s data and behavior. The object’s data and behaviors are contained within the object, so a client application can treat the object like a black box accessible only through its interface. This is a key object-oriented concept called Encapsulation. The idea is that any programs that make use of this object won’t have direct access to the behaviors or data-but rather those programs must make use of our object’s interface.

There are three main parts of Object:

1. Interface

2. Implementation or Behavior

3. Member or Instance variables

Interface

The interface is defined as a set of methods (Sub and Function routines), properties (Property routines), events, and fields (variables or attributes) that are declared Public in scope.

Implementation or Behavior

The code inside of a method is called the implementation. Sometimes it is also called behavior since it is this code that actually makes the object do useful work.

Client applications can use our object even if we change the implementation-as long as we don’t change the interface. As long as our method name and its parameter list and return data type remain unchanged, we can change the implementation all we want.

So Method Signature depends on:

·         Method name

·         Data types of parameters

·         Either Parameter is passed ByVal or ByRef.

·         Return type of method 

It is important to keep in mind that encapsulation is a syntactic tool-it allows our code to continue to run without change. However, it is not semantic-meaning that, just because our code continues to run, doesn’t mean it continues to do what we actually wanted it to do.

Member or Instance Variables

The third key part of an object is its data, or state. Every instance of a class is absolutely identical in terms of its interface and its implementation-the only thing that can vary at all is the data contained within that particular object.

Member variables are those declared so that they are available to all code within our class. Typically member variables are Private in scope-available only to the code in our class itself. They are also sometimes referred to as instance variables or as attributes. The .NET Framework also refers to them as fields.

We shouldn’t confuse instance variables with properties. A Property is a type of method that is geared around retrieving and setting values, while an instance variable is a variable within the class that may hold the value exposed by a Property.

Interface looks like a class, but has no implementation.

The only thing it contains is definitions of events, indexers, methods and/or properties. The reason interfaces only provide definitions is because they are inherited by classes and structs, which must provide an implementation for each interface member defined.

Defining an Interface: MyInterface.cs

interface IMyInterface

{

void MethodToImplement();

}

Above listing shows defines an interface named IMyInterface.

All the methods of Interface are public by default and no access modifiers (like private, public) are allowed with any method of Interface.

Using an Interface: InterfaceImplementer.cs

class InterfaceImplementer : IMyInterface

{

public void MethodToImplement()

{

Console.WriteLine(“MethodToImplement() called.”);

}

}

The InterfaceImplementer class in above listing implements the IMyInterface interface. Indicating that a class inherits an interface is the same as inheriting a class. In this case, the following syntax is used:

class InterfaceImplementer : IMyInterface

Note that this class inherits the IMyInterface interface; it must implement its all members. While implementing interface methods all those needs to be declared public only. It does this by implementing the MethodToImplement() method. Notice that this method implementation has the exact same signature, parameters and method name, as defined in the IMyInterface interface. Any difference will cause a compiler error.

Inheritance is the idea that one class, called a subclass, can be based on another class, called a base class. Inheritance provides a mechanism for creating hierarchies of objects.

Inheritance is an important object-oriented concept. It allows you to build a hierarchy of related classes, and to reuse functionality defined in existing classes.

Inheritance is the ability to apply another class’s interface and code to your own class.

Normal base classes may be instantiated themselves, or inherited. Derived classes can inherit base class members marked with protected or greater access. The derived class is specialized to provide more functionality, in addition to what its base class provides. Inheriting base class members in derived class is not mandatory.

C# supports two types of Inheritance mechanisms: -

1) Implementation Inheritance

2) Interface Inheritance

What is Implementation Inheritance?

- When a class (type) is derived from another class(type) such that it inherits all the members of the base type it is Implementation Inheritance

What is Interface Inheritance?

- When a type (class or a struct) inherits only the signatures of the functions from another type it is Interface Inheritance

In general, Classes can be derived from another class, hence support Implementation inheritance At the same time Classes can also be derived from one or more interfaces Hence they support Interface inheritance Structs can derive from one more interface, hence support Interface Inheritance Structs cannot be derived from another class they are always derived from SystemValueType

Types of Inheritance

1.    Single Inheritance

2.    Multilevel Inheritance

3.    Multiple Inheritance        (Implementation is possible through Interface)

4.    Hierarchical Inheritance

Multilevel Inheritance

Class B

Class B

Single Inheritance

Class A

Class D

Class C

Class A

Hierarchical Inheritance

Class A

Class B

Class C

Class A

Class B

Class C

Multiple Inheritance

Example: -

Single Inheritance: -                           Multilevel Inheritance: -                   Hierarchical Inheritance: -             
public class A                                       public class A                                       public class A     
{   }                                                          {   }                                                          {    }
public class B : A                                 public class B : A                                 public class B : A
{   }                                                          {   }                                                          {    }        

                                                                public class C : B                                 public class C : A

Multiple Inheritance: -              {   }                                                          {    }

public class A                                                                                                       public class D : A

{    }                                                                                                                         {    }

public class B

{   }

public class C : A, B

{   }

Polymorphism 

Polymorphism is the ability to define a method or property in a set of derived classes with matching method signatures but provide different implementations and then distinguish the objects’ matching interface from one another at runtime when you call the method on the base class.

It is a feature to use one name in many forms. It can be achieved in following ways: -

·         Method Overloading

·         Method Overriding

·         Method Hiding

Method overriding and hiding makes use of the following three method keywords –

1.    new

2.    virtual

3.    override

1. When a derived class inherits from a base class, it gains all the methods, fields, properties and events of the base class. To change the data and behavior of a base class, you have two choices: you can replace the base member with a new derived member, or you can override a virtual base member.

Replacing a member of a base class with a new derived member requires the new keyword. If a base class defines a method, field, or property, the new keyword is used to create a new definition of that method, field, or property on a derived class. The new keyword is placed before the return type of a class member that is being replaced. For example:

public class BaseClass

{

    public void DoWork() { }

    public int WorkField;

    public int WorkProperty

    {

        get { return 0; }

    }

}

public class DerivedClass : BaseClass

{

    public new void DoWork() { }

    public new int WorkField;

    public new int WorkProperty

    {

        get { return 0; }

    }

}

DerivedClass B = new DerivedClass();

B.DoWork();  // Calls the new method.

BaseClass A = (BaseClass)B;

A.DoWork();  // Calls the old method.

2,3. In order for an instance of a derived class to completely take over a class member from a base class, the base class has to declare that member as virtual. This is accomplished by adding the virtual keyword before the return type of the member. A derived class then has the option of using theoverride keyword, instead of new, to replace the base class implementation with its own. For example:

public class BaseClass

{

    public virtual void DoWork() { }

    public virtual int WorkProperty

    {

        get { return 0; }

    }

}

public class DerivedClass : BaseClass

{

    public override void DoWork() { }

    public override int WorkProperty

    {

        get { return 0; }

    }

}

DerivedClass B = new DerivedClass();

B.DoWork();  // Calls the new method.

BaseClass A = (BaseClass)B;

A.DoWork();  // Also calls the new method.

Remarks about Virtual

·         When a virtual method is invoked, the run-time type of the object is checked for an overriding member. The overriding member in the most derived class is called, which might be the original member, if no derived class has overridden the member.

·         By default, methods are non-virtual. You cannot override a non-virtual method.

·         You cannot use the virtual modifier with the static, abstract, private or override modifiers.

·         Virtual properties behave like abstract methods, except for the differences in declaration and invocation syntax.

·         It is an error to use the virtual modifier on a static property.

·         A virtual inherited property can be overridden in a derived class by including a property declaration that uses the override modifier.

Remarks about Override

·         The override modifier is required to extend or modify the abstract or virtual implementation of an inherited method, property, indexer, or event.

·         An override method provides a new implementation of a member inherited from a base class. The method overridden by an override declaration is known as the overridden base method. The overridden base method must have the same signature as the override method.

·         You cannot override a non-virtual or static method. The overridden base method must be virtual, abstract, or override.

·         An override declaration cannot change the accessibility of the virtual method. Both the override method and the virtual method must have the same access level modifier.

·         You cannot use the modifiers new, static, virtual, or abstract to modify an override method.

·         An overriding property declaration must specify the exact same access modifier, type, and name as the inherited property, and the overridden property must be virtual, abstract, or override.

Class and Objects

Classes

A class is a construct that enables you to create your own custom types by grouping together variables of other types, methods and events. A class is like a blueprint. It defines the data and behavior of a type. If the class is not declared as static, client code can use it by creating objects or instances which are assigned to a variable. The variable remains in memory until all references to it go out of scope. At that time, the CLR marks it as eligible for garbage collection. If the class is declared as static, then only one copy exists in memory and client code can only access it through the class itself, not an instance variable.

Declaring Class

public class Customer

{

    //Fields, properties, methods and events go here…

}

The class keyword is preceded by the access level. Because public is used in this case, anyone can create objects from this class. The name of the class follows the class keyword.

Objects

An object is basically a block of memory that has been allocated and configured according to the blueprint. A program may create many objects of the same class. Objects are also called instances, and they can be stored in either a named variable or in an array or collection.

Creating Objects

A class and an object are different things. A class defines a type of object, but it is not an object itself. An object is a concrete entity based on a class, and is sometimes referred to as an instance of a class.

Objects can be created by using the new keyword followed by the name of the class that the object will be based on, like this:

Customer object1 = new Customer();

When an instance of a class is created, a reference to the object is passed back to the programmer. In the previous example, object1 is a reference to an object that is based on Customer.

Class Modifiers

A class-declaration can optionally include a sequence of class modifiers:

class-modifiers:

class-modifier

class-modifiers   class-modifier

class-modifier:

new, public, protected, internal, private, abstract, sealed

The new modifier is permitted on nested classes. The new modifier can be used to modify a nested type if the nested type is hiding another type.

The public, protected, internal, and private modifiers control the accessibility of the class. Depending on the context in which the class declaration occurs, some of these modifiers may not be permitted

The abstract modifier is used to indicate that a class is incomplete and that it is intended to be used only as a base class. An abstract class differs from a non-abstract class in the following ways:

·         An abstract class cannot be instantiated directly, and it is a compile-time error to use the new operator on an abstract class. While it is possible to have variables and values whose compile-time types are abstract, such variables and values will necessarily either be null or contain references to instances of non-abstract classes derived from the abstract types.

·         An abstract class is permitted (but not required) to contain abstract methods and members.

·         An abstract class cannot be sealed.

Features of Abstract Methods:

·         An abstract method is implicitly a virtual method.

·         Abstract method declarations are only permitted in abstract classes.

·         Because an abstract method declaration provides no actual implementation, there is no method body; the method declaration simply ends with a semicolon and there are no braces ({ }) following the signature. For example:

Copypublic abstract void MyMethod();

·         The implementation is provided by an overriding method, which is a member of a non-abstract class.

·         It is an error to use the static or virtual modifiers in an abstract method declaration.

Abstract properties behave like abstract methods, except for the differences in declaration and invocation syntax.

·         It is an error to use the abstract modifier on a static property.

·         An abstract inherited property can be overridden in a derived class by including a property declaration that uses the override modifier.

An abstract class must provide implementation for all interface members.

Example: -

// abstract_keyword.cs

// Abstract Classes

using System;

abstract class MyBaseC   // Abstract class

{

   protected int x = 100;

   protected int y = 150;

   public abstract void MyMethod();   // Abstract method

   public abstract int GetX   // Abstract property

   {

      get;

   }

   public abstract int GetY   // Abstract property

   {

      get;

   }

}

class MyDerivedC: MyBaseC

{

   public override void MyMethod()

   {

      x++;

      y++;  

   }  

   public override int GetX   // overriding property

   {

      get

      {

         return x+10;

      }

   }

   public override int GetY   // overriding property

   {

      get

      {

         return y+10;

      }

   }

   public static void Main()

   {

      MyDerivedC mC = new MyDerivedC();

      mC.MyMethod();

      Console.WriteLine(“x = {0}, y = {1}”, mC.GetX, mC.GetY);   

   }

}

The sealed modifier is used to prevent derivation from a class. A compile-time error occurs if a sealed class is specified as the base class of another class.

A sealed class cannot also be an abstract class.

The sealed modifier is primarily used to prevent unintended derivation, but it also enables certain run-time optimizations. In particular, because a sealed class is known to never have any derived classes, it is possible to transform virtual function member invocations on sealed class instances into non-virtual invocations.

Example: use of Sealed modifier

// cs_sealed_keyword.cs

// Sealed classes

using System;

sealed class MyClass

{

   public int x;

   public int y;

}

class MainClass

{

   public static void Main()

   {

      MyClass mC = new MyClass();

      mC.x = 110;

      mC.y = 150;

      Console.WriteLine(“x = {0}, y = {1}”, mC.x, mC.y);

   }

}

Output: x=110, y=150

Constructors

Whenever a class or struct is created, its constructor is called. A class or struct may have multiple constructors that take different arguments.

Constructors allow the programmer to set default values, limit instantiation, and write code that is flexible and easy to read.

·         Constructor is used to initialize an object (instance) of a class.

·         Constructor is a like a method without any return type.

·         Constructor has same name as class name.

·         Constructor follows the access scope (Can be private, protected, public, Internal and external).

·         Constructor can be overloaded.

Constructors generally following types:

·         Default Constructor

·         Parameterized constructor

·         Private Constructor

·         Static Constructor

·         Copy Constructor

Default Constructor

A constructor that takes no parameters is called a default constructor.

When a class is initiated default constructor is called which provides default values to different data members of the class.

You need not to define default constructor it is implicitly defined.

Example: -

class Program

    {

        class C1

        {

            int a, b;

            public C1()

            {

                this.a = 10;

                this.b = 20;

            }

            public void display()

            {

                Console.WriteLine(“Value of a: {0}”, a);

                Console.WriteLine(“Value of b: {0}”, b);

            }

        }

        static void Main(string[] args)

        {

            C1 ob1 = new C1();

            ob1.display();

            Console.ReadLine();

        }

    }

Output: – Value of a: 10

              Value of b: 20

Parameterized constructor

Constructor that accepts arguments is known as parameterized constructor. There may be situations, where it is necessary to initialize various data members of different objects with different values when they are created. Parameterized constructors help in doing that task.

class Program

    {

        class C1

        {

            int a, b;

            public C1(int x, int y)

            {

                this.a = x;

                this.b = y;

            }

            public void display()

            {

                Console.WriteLine(“Value of a: {0}”, a);

                Console.WriteLine(“Value of b: {0}”, b);

            }

        }

        static void Main(string[] args)

        { // Here when you create instance of the class

   //  parameterized constructor will be called

 C1 ob1 = new C1(10,20);

            ob1.display();

            Console.ReadLine();

        }

    }

Output: – Value of a: 10

              Value of b: 20

Private Constructor

Private constructors are used to restrict the instantiation of object using ‘new’ operator.  A private constructor is a special instance constructor. It is commonly used in classes that contain static members only.

·         If you don’t want the class to be inherited we declare its constructor private.

·         We can’t initialize the class outside the class or the instance of class can’t be created outside if its constructor is declared private.

·         We have to take help of nested class (Inner Class) or static method to initialize a class having private constructor.

Example: -

    class Program

    {

        class C1

        {

            int a, b;

            public C1(int x, int y)

            {

                this.a = x;

                this.b = y;

}

            public static C1 create_instance()

            { return new C1(12, 20); }

            public void display()

            {

                Console.WriteLine(“Value of a: {0}”, a);

                Console.WriteLine(“Value of b: {0}”, b);

                int z = a + b;

                Console.WriteLine(z);

            }

        }

        static void Main(string[] args)

        { // Here the class is initiated using a static method of the

class than only you can use private constructor

C1 ob1 = C1.create_instance();

            ob1.display();

            Console.ReadLine();

        }

    }

Static Constructors

C# supports two types of constructor, a class constructor static constructor and an instance constructor (non-static constructor).

Static constructors might be convenient, but they are slow. The runtime is not smart enough to optimize them in the same way it can optimize inline assignments. Non-static constructors are inline and are faster.

Static constructors are used to initializing class static data members.

Point to be remembered while creating static constructor:

1. There can be only one static constructor in the class.

2. The static constructor should be without parameters.

3. It can only access the static members of the class.

4. There should be no access modifier in static constructor definition.

Static members are preloaded in the memory. While instance members are post loaded into memory.

Static methods can only use static data members.

Example:

class Program

    {

        public class test

        {

            static string name;

            static int age;

            static test()

            {

 Console.WriteLine(“Using static constructor to initialize  

                    static data members”);

                name = “John Sena”;

                age = 23;

            }

            public static void display()

            {

                Console.WriteLine(“Using static function”);

                Console.WriteLine(name);

                Console.WriteLine(age);

            }

        }

        static void Main(string[] args)

        {

            test.display();

            Console.ReadLine();

        }}

Output:

Using static constructor to initialize static data members

Using static function

John Sena

23

Copy Constructor

If you create a new object and want to copy the values from an existing object, you use copy constructor.

This constructor takes a single argument: a reference to the object to be copied.

Example:

class Program

    {

        class c1

        {

            int a, b;

            public c1(int x, int y)

            {

                this.a = x;

                this.b = y;

            }

            // Copy construtor

            public c1(c1 a)

            {

                this.a = a.a;

                this.b = a.b;

            }

            public void display()

            {

                int z = a + b;

                Console.WriteLine(z);

            }

        }

        static void Main(string[] args)

        {

            c1 ob1 = new c1(10, 20);

            ob1.display();

      // Here we are using copy constructor. Copy constructor is

using the values already defined with ob1

            c1 ob2 = new c1(ob1);

            ob2.display();

            Console.ReadLine();

        }

    }

Output:

30

30

Destructors 

The .NET framework has an in built mechanism called Garbage Collection to de-allocate memory occupied by the un-used objects. The destructor implements the statements to be executed during the garbage collection process. A destructor is a function with the same name as the name of the class but starting with the character ~.

Example:

class Complex

{

public Complex()

{

// constructor

}

~Complex()

{

// Destructor

}

}

·         Remember that a destructor can’t have any modifiers like private, public etc. If we declare a destructor with a modifier, the compiler will show an error.

·         Also destructor will come in only one form, without any arguments.

·         There is no parameterized destructor in C#. 

Destructors are invoked automatically and can’t be invoked explicitly. An object becomes eligible for garbage collection, when it is no longer used by the active part of the program. Execution of destructor may occur at any time after the instance or object becomes eligible for destruction.

Operator Overloading

Operator overloading permits user-defined operator implementations to be specified for operations where one or both of the operands are of a user-defined class or struct type.

In another way, Operator overloading is a concept in which operator can define to work with the user defined data types such as structs and classes in the same way as the pre-defined data types.

There are many operators which can not be overloaded, which are listed below: -

Conditional Operator     &&, ||

Compound Assignment +=, -=, *=, /=, %=

Other Operators            [], ( ), =, ?:, ->, new, sizeof, typesof.

   public class Item

    {

        public int i;

        public Item(int j)

        { i = j; }

        public static Item operator +(Item x, Item y)

        {

            Console.WriteLine(“OPerator +” + x.i + “” + y.i);

            Item z = new Item(x.i + y.i);

            return z;

        }

    }

    class Program

    {

        static void Main(string[] args)

        {

            Item a = new Item(10);

            Item b = new Item(5);

            Item c;

            c = a + b;

            Console.WriteLine(c.i);

            Console.Read();

        }

    }

Output: Operator + 10 5

15

·         In C#, a special function called operator function is used for overloading purpose.

·         These special function or method must be public and static.

·         They can take only value arguments.

·         The ref and out parameters are not allowed as arguments to operator functions.

The general form of an operator function is as follows.

public static return_type operator op (argument list)

Where the op is the operator to be overloaded and operator is the required keyword.

Example: Overloading of Unary operator

class Complex

{

    private int x;

    private int y;

    public Complex()

    {}

    public Complex(int i, int j)

    {

        x = i;

        y = j;

    }

    public void ShowXY()

    {

        Console.WriteLine(“{0}\t{1}”,x,y);

    }

    public static Complex operator -(Complex c) //PASSING OBJECT

    {

        Complex temp = new Complex();

        temp.x = -c.x;

        temp.y = -c.y;

        return temp;

    }

}

    class Program

    {

        static void Main(string[] args)

        {

            Complex c1 = new Complex(10, 20);

            c1.ShowXY(); // displays 10 & 20

            Complex c2 = new Complex();

            c2.ShowXY(); // displays 0 & 0

            c2 = -c1;   //overloading unary operator

            c2.ShowXY(); // diapls -10 & -20

            Console.Read();

        }

    }

Output:

10 20

0   0

-10 -20

Namespace:

Namespaces are C# program elements designed to help you organize your programs. They also provide assistance in avoiding name clashes between two sets of code.

In Microsoft .Net, Namespace is like containers of objects. They may contain unions, classes, structures, interfaces, enumerators and delegates. Main goal of using namespace in .Net is for creating a hierarchical organization of program. In this case, you need not to worry about the naming conflicts of classes, functions, variables etc., inside a project.

 In Microsoft .Net, every program is created with a default namespace. This default namespace is called as global namespace. But the program itself can declare any number of namespaces, each of them with a unique name. The advantage is that every namespace can contain any number of classes, functions, variables and also namespaces etc., whose names are unique only inside the namespace. The members with the same name can be created in some other namespace without any compiler complaints from Microsoft .Net.

To declare namespace C# .Net has a reserved keyword namespace. If a new project is created in Visual Studio .NET it automatically adds some global namespaces. These namespaces can be different in different projects. But each of them should be placed under the base namespace System. The names space must be added and used through the using operator, if used in a different project.

A namespace has the following properties:

·         They organize large code projects.

·         They are delimited with the . operator.

·         The using directive means you do not need to specify the name of the namespace for every class.

·         The global namespace is the “root” namespace: global::system will always refer to the .NET Framework namespace System.

Now have a look at the example of declaring some namespace:

namespace SampleNamespace

{

    class SampleClass{}

    interface SampleInterface{}

    struct SampleStruct{}

    enum SampleEnum{a,b}

    delegate void SampleDelegate(int i);

    namespace SampleNamespace.Nested

    {

       class SampleClass2{}

    }

}

Within a namespace, you can declare one or more of the following types:

·         another namespace

·         class

·         interface

·         struct

·         enum

·         delegate

Namespaces implicitly have public access and this is not modifiable.

It is possible to define a namespace in two or more declarations. For example, the following example defines two classes as part of the MyCompany namespace:

namespace MyCompany.Proj1

{

    class MyClass

    {

    }

}

namespace MyCompany.Proj1

{

    class MyClass1

    {

    }

}

Example: The following example shows how to call a static method in a nested namespace:

using System;

namespace SomeNameSpace

{

    public class MyClass

    {

        static void Main()

        {

            Nested.NestedNameSpaceClass.SayHello();

        }

    }

    // a nested namespace

    namespace Nested  

    {

        public class NestedNameSpaceClass

        {

            public static void SayHello()

            {

                Console.WriteLine(“Hello”);

            }

        }

    }

}

Output

Hello

Example:  Calling Nested Namespace Members

// Namespace Declaration

using System;

namespace Ex_nestedNamespace

{

    namespace tutorial

    {

        class example

        {

            public static void MyPrint1()

            {

                Console.WriteLine(“First Example of calling another namespace member.”);

            }

        }

    }

    namespace Ex_NameSpace

    {

        class Program

        {

            static void Main(string[] args)

            {

                tutorial.example.MyPrint1();

                tutorial.example1.MyPrint2();

                Console.Read();

            }

        }

    }

}

namespace Ex_nestedNamespace.tutorial

{

    class example1

    {

        public static void MyPrint2()

        {

            Console.WriteLine(“Second Example of calling another namespace member.”);

        }

    }

}

Output:

First Example of calling another namespace member.

Second Example of calling another namespace member.

Interface

An Interface is a reference type and it contains only abstract members. Interface’s members can be Events, Methods, Properties and Indexers. But the interface contains only declaration for its members. Any implementation must be placed in class that realizes them. The interface can not contain constants, data fields, constructors, destructors and static members. All the member declarations inside interface are implicitly public and they cannot include any access modifiers.

An interface has the following properties:

·         An interface is like an abstract base class: any non-abstract type that implements the interface must implement all its members.

·         An interface cannot be instantiated directly.

·         Interfaces can contain events, indexers, methods, and properties.

·         Interfaces contain no implementation of methods.

·         Classes and structs can implement more than one interface.

·         An interface itself can inherit from multiple interfaces.

interface IPoint

{

    int x

    { get; set; }

    int y

    { get; set; }

}

namespace Ex_Interface

{

    class MyPoint:IPoint

    {

        private int myX;

        private int myY;

        public MyPoint(int x, int y)

        {

            myX= x;

            myY=y;

        }

        public int x

        {

            get

            {

                return myX;

            }

            set

            {

                myX=value;

            }

        }

        public int y

        {

            get

            {

                return myY;

            }

            set

            {

                myY=value;

            }

        }

    }

    class Program

    {

        private static void PrintPoint(IPoint P)

        {

            Console.WriteLine(“x={0}, y={1}”,P.x,P.y);

        }

        static void Main(string[] args)

        {

            MyPoint P = new MyPoint(2, 3);

            Console.Write(“My Point::”);

            PrintPoint(P);

            Console.Read();

        }

    }

}

Output:

My Point::x=2, y=3

Another Example of Interface by Casting Interface methods:

interface add

{ int sum();}

interface Multiply

{ int mul();}

class Calculate : add, Multiply

{

    int a, b;

    public Calculate(int x, int y)

    {

        a = x;

        b = y;

    }

    public int sum()

    { return (a + b);}

    public int mul()

    { return a * b; }

}

namespace Ex_MultipleInterface

{

    class Program

    {

        static void Main(string[] args)

        {

            Calculate cal = new Calculate(5, 10);

            add A = (add)cal;

            Console.WriteLine(“Sum::” + A.sum());

            Multiply M = (Multiply)cal;

            Console.WriteLine(“Multiplication::” + M.mul());

            Console.Read();

        }

    }

}

Output:

Sum::15

Multiplication::50

Delegates

In .NET, you use delegates to call event procedure. Delegates are objects that you use to call the methods of other objects. Delegates are said to be object-oriented function pointers since they allow a function to be invoked indirectly by using a reference to the function.

However, unlike function pointers, the delegates in .NET are reference types, based on the class System.Delegate. In addition, delegates in .NET can reference both shared and instance methods.

In another way, a delegate can be defined as a type safe function pointer. You use delegates to call the methods of other objects. They are object-oriented function pointers since they allow a function to be invoked indirectly by using a reference to the function.

Where are Delegates used?

The most common example of using delegates is in events.

You define a method that contains code for performing various tasks when an event (such as a mouse click) takes place.

This method needs to be invoked by the runtime when the event occurs. Hence this method, that you defined, is passed as a parameter to a delegate.

Starting Threads/Parallel Processing:

You defined several methods and you wish to execute them simultaneously and in parallel to whatever else

the application is doing. This can be achieved by starting new threads. To start a new thread for your method you pass your method details to a delegate.

Generic Classes: Delegates are also used for generic class libraries which have generic functionality defined. However the generic class may need to call certain functions defined by the end user implementing the generic class. This can be done by passing the user defined functions to delegates.

Creating and Using Delegates:

Using delegates is a two step process-
……….1) Define the delegate to be used
……….2) Create one or more instances of the delegate

Syntax for defining a delegate:

delegate string reviewStatusofARegion();

to define a delegate we use a key word delegate followed by the method signature the delegate represents. In the above example string reviewStatusofARegion(); represents any method that returns a string and takes no parameters.

Syntax for creating an instance of the delegate:

reviewStatusofARegion = new reviewStatusofARegion(myClass.getEurope);

private string getEurope()
{
return “Doing Great in Europe”;
}

To create an instance of the delegate you call its constructor. The delegate constructor takes one parameter which is the method name.

The method signature should exactly match the original definition of the delegate. If it does not match the compiler would raise an Error.

C# provides support for Delegates through the class called Delegate in the System namespace. Delegates are of two types.

·         Single-cast delegates

·         Multi-cast delegates

A Single-cast delegate is one that can refer to a single method whereas a Multi-cast delegate can refer to and eventually fire off multiple methods that have the same signature.

The signature of a delegate type comprises are the following.

·         The name of the delegate

·         The arguments that the delegate would accept as parameters

·         The return type of the delegate

A delegate is either public or internal if no specifier is included in its signature. Further, you should instantiate a delegate prior to using the same.

The following is an example of how a delegate is declared.

Listing 1: Declaring a delegate

public delegate void TestDelegate(string message);

The return type of the delegate shown in the above example is “void” and it accepts a string argument. Note that the keyword “delegate” identifies the above declaration as a delegate to a method. This delegate can refer to and eventually invoke a method that can accept a string argument and has a return type of void, i.e., it does not return any value.

Listing 2: Instantiating a delegate

TestDelegate t = new TestDelegate(Display);

Implementing Delegates in C#

This section illustrates how we can implement and use delegates in C#.This section illustrate how we can implement and use delegates in C#.

Example 1: Single Cast Delegate

namespace Ex_Delegate

{

    delegate int Operation(int x, int y); //declaration

    class Metaphor

    {

        public static int Add(int a, int b)

        { return a + b; }

        public static int Sub(int a, int b)

        { return a – b; }

        public static int Mul(int a, int b)

        { return a * b; }

    }

    class Program

    {

        static void Main(string[] args)

        {   // Delegate instances

            Operation opr1 = new Operation(Metaphor.Add);

            Operation opr2 = new Operation(Metaphor.Sub);

            Operation opr3 = new Operation(Metaphor.Mul);

            //invoking of delegates

            int ans1 = opr1(200, 100);

            int ans2 = opr2(200, 100);

            int ans3 = opr3(20, 10);

            Console.WriteLine(“\n Addition:” + ans1);

            Console.WriteLine(“\n Subtract:” + ans2);

            Console.WriteLine(“\n Multiplication:” + ans3);

            Console.Read();

        } }}

Example 2: Single Cast Delegate

namespace Ex_SingleCastDelegate

{           //Declare the delegate

    public delegate void TestDelegate(string message);

    class Program

    {

        public static void Display(string message)

        {Console.WriteLine(“The string entered is : ” + message);}

        static void Main(string[] args)

        { //Initiate the delegate

TestDelegate t = new TestDelegate(Display);           

Console.WriteLine(“Please enter a string::”);

            string message = Console.ReadLine();

            t(message);

            Console.ReadLine();

        }}}

Multicast Delegate

A multi-cast delegate is basically a list of delegates or a list of methods with the same signature. A multi-cast delegate can call a collection of methods instead of only a single method.

Example: Multicast Delegate

namespace Ex_MulticastDelegate

{

    public delegate void TestDelegate();

    class Program

    {

        public static void Display1()

        {

            Console.WriteLine(“This is first method”);

        }

        public static void Display2()

        {

            Console.WriteLine(“This is second method”);      

        }

        static void Main(string[] args)

        {

            TestDelegate t1 = new TestDelegate(Display1);

            TestDelegate t2 = new TestDelegate(Display2);

            t1 = t1 + t2;   // Make t1 a multi-cast delegate

            t1();   //Invoke delegate

            Console.Read();

        } } }

In another way, You can also assign the references of multiple methods to a delegate and use it to invoke multiple methods. Such a delegate is called a multi-cast delegate as multiple method references are cast to it and then the delegate is used to invoke these methods.

What are Attributes?

An Attribute is a declarative tag which can be used to provide information to the compiler about the behaviour of the C# elements such as classes and assemblies.

C# provides convenient technique that will handle tasks such as performing compile time operations , changing the behaviour of a method at runtime or maybe even handle unmanaged code.

C# Provides many Built-in Attributes.

Some Popular ones are

·         Obsolete

·         DllImport

·         Conditional

·         WebMethod

It is also possible to create new ones by extending the System.Attribute class.

For example:

using System;

[CLSCompliant(true)]

Public class myClass

{ // class code }

Web services also make use of attributes. The attribute [WebMethod] is used to specify that a particular method is to be exposed as a web service.

Why Attributes ?

The reason attributes are necessary is because many of the services they provide would be very difficult to accomplish with normal code. You see, attributes add what is called metadata to your programs. When your C# program is compiled, it creates a file called an assembly, which is normally an executable or DLL library. Assemblies are self-describing because they have metadata written to them when they are compiled. Via a process known as reflection, a program’s attributes can be retrieved from its assembly metadata. Attributes are classes that can be written in C# and used to decorate your code with declarative information. This is a very powerful concept because it means that you can extend your language by creating customized declarative syntax with attributes.

How it is used in C#?

Attributes are elements that allow you to add declarative information to your programs. This declarative information is used for various purposes during runtime and can be used at design time by application development tools. For example, there are attributes such as DllImportAttribute that allow a program to communicate with the Win32 libraries. Another attribute, ObsoleteAttribute, causes a compile-time warning to appear, letting the developer know that a method should no longer be used. When building Windows forms applications, there are several attributes that allow visual components to be drag-n-dropped onto a visual form builder and have their information appear in the properties grid. Attributes are also used extensively in securing .NET assemblies, forcing calling code to be evaluated against pre-defined security constraints. These are just a few descriptions of how attributes are used in C# programs.

Predefined .NET Attribute	Valid Targets	Description
AttributeUsage	Class	Specifies the valid usage of another attribute class.
CLSCompliant	All	Indicates whether a program element is compliant with the Common Language Specification (CLS).
DllImport	Method	Specifies the DLL location that contains the implementation of an external method.
MTAThread	Method (Main)	Indicates that the default threading model for an application is multithreaded apartment (MTA).
NonSerialized	Field	Applies to fields of a class flagged as Serializable; specifies that these fields won’t be serialized.
Obsolete	All except Assembly, Module, Parameter, and Return	Marks an element obsolete—in other words, it informs the user that the element will be removed in future versions of the product.
ParamArray	Parameter	Allows a single parameter to be implicitly treated as a params (array) parameter.
Serializable	Class, struct, enum, delegate	Specifies that all public and private fields of this type can be serialized.
STAThread	Method (Main)	Indicates that the default threading model for an application is STA.
ThreadStatic	Field (static)	Implements thread-local storage (TLS)—in other words, the given static field isn’t shared across multiple threads and each thread has its own copy of the static field.

Predefined attributes

Example:

Pre-defined attributes are used to store external information into metadata. For example, consider the following piece of code:

public class testAttribute {

[DllImport("sampleDLL.dll")]

public static extern sampleFunction(int sampleNo, string sampleString );

public static void Main( ) {

string strVar;

sampleFunction(10, “Test Attribute”);

}

}

Using the example code above, you can import a method calledsampleFunction from sampleDLL.dll and use it in your program as if it’s your own method. This is achieved using the pre-defined attribute “DllImport”.

Multi-Threading

Multithreading forms a subset of multitasking. Instead of having switch between programs this feature switches between different parts of the same program. For example when you are writing words in Ms-word then spell checking is going on background.

Thread – A thread (or “thread of execution”) is a sort of context in which code is running. Any one thread follows program flow for wherever it is in the code, in the obvious way.

A thread is a unit of processing, and multitasking is the simultaneous execution of multiple threads. Multitasking comes in two flavors: cooperative and preemptive. Very early versions of Microsoft Windows supported cooperative multitasking, which meant that each thread was responsible for relinquishing control to the processor so that it could process other threads.

However, Microsoft Windows NT-and, later, Windows 95, Windows 98, and Windows 2000-support the same preemptive multitasking that OS/2 does. With preemptive multitasking, the processor is responsible for giving each thread a certain amount of time in which to execute-a timeslice. The processor then switches among the different threads, giving each its timeslice, and the programmer doesn’t have to worry about how and when to relinquish control so that other threads can run. .NET will only work only on preemptive multitasking operating systems.

1. Starting Thread

Object thread is obtained from System.Threading namespace. With the use object of this class we can create a new thread, delete, pause, and resume threads. Simple a new thread is created by Thread class and started by Thread.Start().

eg. Thread th = new Thread (new ThreadStart (somedata));

th.Start();

2. Pausing Thread

Some time the requirement to pause a thread for certain time of interval; you can attain the same by using Sleep (n) method. This method takes an integer value to determine how long a thread should pause or Sleep.

eg. th.Sleep(2000);

Note:

To pause or sleep a thread for an in determine time, just call the sleep () method as: [make sure you have added System.Threading namespace] Thread.Sleep(TimeOut.Infinite).

To Resume or interrupt this call : Thread.Interrupt () method.

3. Suspending Thread

Of course, there is a Suspend () method which suspends the thread. It is suspended until a Resume () method called.

eg. if (th.ThreadState = = ThreadState.Running)

th.Suspended();

4. Resuming Thread

To Resume a suspended thread, there is a Resume () method, thread resumes if earlier suspended if not so then there is no effect of Resume () method on the thread.

eg. if (th.ThreadState = = ThreadState.Suspended)

th.Resume();

5. Killing Thread

You can call Abort () method to kill a thread, before calling the same method, make sure thread is alive.

eg. if (th.IsAlive)

th.Abort();

Suspend and Resume in Threading

It is similar to sleep and Interrupt. Suspend allows you to block a thread until another thread calls Thread.Resume ().The difference between sleep and suspend is that the later does no immediately place a thread in the wait state. The thread does not suspend until the .Net runtime determines that it is in a safe place to suspend it. Sleep will immediately place a thread in a wait state.

Important:

You can change thread priority for that just supply : th.Priority = ThreadPriority.Highest. [th – Thread name]. Priority sets the sequence of thread in which they are running. You can set the following priority to thread(s):

1. ThreadPriority.Highest

2. ThreadPriority.AboveNormal

3. ThreadPriority.Normal

4. ThreadPriority.BelowNormal

5. ThreadPriority.Lowest

Code Example of Multithreading

using System.Threading;

namespace Ex_ThreadExample

{

    class SimpleThread

    {

        private Thread thread1;

        private Thread thread2;

        private void Method1()

        {

            for (int i =0; i<10;i++)

            {

                Console.WriteLine (“i = ” +i);

                Thread.Sleep (400); // 200 miliseconds pause

            }

        }

        private void Method2()

        {

            for (int i =0;i<10;i++)

            {

                   Console.WriteLine (“i = ” + 100 * i);

                   Thread.Sleep (100); // 100 miliseconds pause

            }

        }

        static void Main(string[] args)

        {

            SimpleThread app = new SimpleThread ();

            app.thread1 = new Thread (new ThreadStart (app.Method1));   // thread start Delegate Method

            app.thread2 = new Thread (new ThreadStart (app.Method2));

            app.thread1.Start ();

            app.thread2.Start ();

            Console.WriteLine ();

            Console.ReadLine();

        }

    }

}

————-x————–x————–x—————x———-

Socket Programming in C#

Network programming in windows is possible with sockets. A socket is like a handle to a file. Socket programming resembles the file IO as does the Serial Communication. You can use sockets programming to have two applications communicate with each other. The application are typically on the different computers but they can be on same computer. For the two applications to talk to each either on the same or different computers using sockets one application is generally a server that keeps listening to the incoming requests and the other application acts as a client and makes the connection to the server application.

The server application can either accept or reject the connection. If the  server accepts the connection, a dialog can begin with between the client and the server.  Once the client is done with whatever it needs to do it can close the connection with the server. Connections are expensive in the sense that servers allow finite connections to occur.  During the time client has an active connection it can send the data to the server and/or receive the data.

Socket programming in .NET is made possible by Socket class present inside the System.Net. Sockets namespace.

Socket class  has several method and properties and a constructor.

·         The first step is to create an object of this class. Since there is only one constructor we have no choice but to use it.

Here is how to create the socket:

m_socListener = new Socket(AddressFamily.InterNetwork,SocketType.Stream,ProtocolType.IP);

AddressFamily is an enum defined in Sockets namespace.

·         Next we need to specify socket type: and we would use reliable two way connection-based sockets (stream) instead of un-reliable Connectionless sockets ( datagrams) . So we obviously specify stream as the socket type and finally we are using TCP/IP so we would specify protocol type as TCP.

·         Once we have created a Socket we need to make a connection to the server since we are using connection-based communication.

·         To connect to the remote computer  we need to know the IP Address and port at which to connect.

·         In .NET there is a class under System.Net namespace called IPEndPoint which represents a network computer  as an IP address and a port number.

·         The IPEndPoint has two  constructors – one that takes a IP Address and Port number  and one that takes long and port number. Since we have computer IP address we would use the former

public IPEndPoint(System.Net.IPAddress address, int port);

·         As you can see the first parameter takes a IPAddress object. If you examine the IPAddress class you will see that it has a static method called Parse that returns IPAddress given a string ( of dot notation ) and second parameter will be the port number. Once we have endpoint ready we can use Connect method of Socket class to connect to the end point  ( remote server computer ).

Here is the code:

System.Net.IPAddress ipAdd = System.Net.IPAddress.Parse(“10.10.101.200″);

System.Net.IPEndPoint remoteEP = new IPEndPoint (ipAdd,8221);

m_socClient.Connect (remoteEP);

Description

These three lines of code will make a connection to the remote host running on computer with IP 10.10.101.200 and listening at port 8221. If the Server is running and started ( listening ), the connection will succeed. If however the server is not running an exception called SocketException will be thrown. If you catch the exception and check the Message property of the exception in this case you see following text:

“No connection could be made because the target machine actively refused it.”

Similarly if you already have made a connection and the server somehow dies, you will get following exception if you try to send data.

“An existing connection was forcibly closed by the remote host”

Assuming that the connection is made, you can send data to other side using the Send method of the Socket class.

Send method has several overloads. All of them take a byte array. For example if you want to send “Hello There” to host you can use following call:

try

{

String szData = “Hello There”;

byte[] byData = System.Text.Encoding.ASCII.GetBytes(szData);

m_socClient.Send(byData);

}

catch (SocketException se)

{

MessageBox.Show ( se.Message );

}

Note that the Send method is blocking. What it means the call will block till the data has been sent or an exception has been thrown. There is a non-blocking version of the send which we will discuss in the next part of this article.

Similar to send there is a Receive method on the Socket class. You can receive data using following call:

byte [] buffer = new byte[1024];

int iRx = m_socClient.Receive (buffer);

The Receive method again is blocking. It means that if there is no data available the call will block until some data arrives or an exception   is thrown.

Non-blocking version of Receive method is more useful than the non-blocking version of Send because if we opt for block Receive, we are effectively doing polling. There is no event about data arrival. This model does not work well for serious applications. But all that is the subject of our next part of this article. For now we will settle with the blocking version.

Server Side Code:  

using System;

using System.Net.Sockets;

public class AsynchIOServer

{

public static void Main()

{

TCPListener tcpListener = new TCPListener(10);

tcpListener.Start();

Socket socketForClient = tcpListener.Accept();

if (socketForClient.Connected)

{

Console.WriteLine(“Client connected”);

NetworkStream networkStream = new NetworkStream(socketForClient);

System.IO.StreamWriter streamWriter = new System.IO.StreamWriter(networkStream);

System.IO.StreamReader streamReader = new System.IO.StreamReader(networkStream);

string theString = “Sending”;

streamWriter.WriteLine(theString);

Console.WriteLine(theString);

streamWriter.Flush();

theString = streamReader.ReadLine();

Console.WriteLine(theString);

streamReader.Close();

networkStream.Close();

streamWriter.Close();

}

socketForClient.Close();

Console.WriteLine(“Exiting…”);

}

}

Client Code:

using System;

using System.Net.Sockets;

public class Client

{

static public void Main( string[] Args )

{

TCPClient socketForServer;

try

{

socketForServer = new TCPClient(“localHost”, 10);

}

catch

{

Console.WriteLine(

“Failed to connect to server at {0}:999″, “localhost”);

return;

}

NetworkStream networkStream = socketForServer.GetStream();

System.IO.StreamReader streamReader = new System.IO.StreamReader(networkStream);

System.IO.StreamWriter streamWriter = new System.IO.StreamWriter(networkStream);

try

{

string outputString;

// read the data from the host and display it

{

outputString = streamReader.ReadLine();

Console.WriteLine(outputString);

streamWriter.WriteLine(“Client Message”);

Console.WriteLine(“Client Message”);

streamWriter.Flush();

}

}

catch

{

Console.WriteLine(“Exception reading from Server”);

}

// tidy up

networkStream.Close();

}

}

———–x—————-x————————–x————————-x—————-x—————x———-

Error Handling

Overview of Exception Handling

Exceptions are error conditions that arise when the normal flow of a code path-that is, a series of method calls on the call stack-is impractical. Exception handling is an in built mechanism in .NET framework to detect and handle run time errors. The exceptions are anomalies that occur during the execution of a program. They can be because of user, logic or system errors. If a user (programmer) do not provide a mechanism to handle these anomalies, the .NET run time environment provide a default mechanism, which terminates the program execution. C# provides three keywords try, catch and finally to do exception handling. The try encloses the statements that might throw an exception whereas catch handles an exception if one exists. The finally can be used for doing any clean up process.

The general form try-catch-finally in C# is shown below:

try

{

// Statement which can cause an exception.

}

catch(Type x)

{

// Statements for handling the exception

}

finally

{

//Any cleanup code

}

If any exception occurs inside the try block, the control transfers to the appropriate catch block and later to the finally block.

But in C#, both catch and finally blocks are optional. The try block can exist either with one or more catch blocks or a finally block or with both catch and finally blocks. 

If there is no exception occurred inside the try block, the control directly transfers to finally block. We can say that the statements inside the finally block is executed always. Note that it is an error to transfer control out of a finally block by using break, continue, return or goto. 

In C#, exceptions are nothing but objects of the type Exception. The Exception is the ultimate base class for any exceptions in C#. The C# itself provides couple of standard exceptions. Or even the user can create their own exception classes, provided that this should inherit from either Exception class or one of the standard derived classes of Exception class like DivideByZeroExcpetion ot ArgumentException etc.

The modified form of the above program with exception handling mechanism is as follows: -

//C#: Exception Handling

using System;

class MyClient

{

public static void Main()

{

int x = 0; int div = 0;

try

{           div = 100/x;

Console.WriteLine(“Not executed line”);

}

catch(DivideByZeroException de)

{ Console.WriteLine(“Exception occured”); }

finally

{ Console.WriteLine(“Finally Block”); }

Console.WriteLine(“Result is {0}”,div);

}

}

Multiple Catch Blocks 

A try block can throw multiple exceptions, which can handle by using multiple catch blocks. Remember that more specialized catch block should come before a generalized one. Otherwise the compiler will show a compilation error. 

//C#: Exception Handling: Multiple catch

using System;

class MyClient

{

public static void Main()

{

int x = 0;

int div = 0;

try

{

div = 100/x;

Console.WriteLine(“Not executed line”);

}

catch(DivideByZeroException de)

{ Console.WriteLine(“DivideByZeroException” ); }

catch(Exception ee)

{ Console.WriteLine(“Exception” ); }

finally

{ Console.WriteLine(“Finally Block”); }

Console.WriteLine(“Result is {0}”,div);

}

}

Catching All Exception

By providing a catch block without a brackets or arguments, we can catch all exceptions occurred inside a try block. Even we can use a catch block with an Exception type parameter to catch all exceptions happened inside the try block since in C#, all exceptions are directly or indirectly inherited from the Exception class.  

//C#: Exception Handling: Handling all exceptions

using System;

class MyClient

{

public static void Main()

{

int x = 0;

int div = 0;

try

{           div = 100/x;

Console.WriteLine(“Not executed line”);

}

catch

{ Console.WriteLine(“oException” );}

Console.WriteLine(“Result is {0}”,div);

}

}

The following program handles all exception with Exception object.

//C#: Exception Handling: Handling all exceptions

using System;

class MyClient

{

public static void Main()

{

int x = 0;

int div = 0;

try

{

div = 100/x;

Console.WriteLine(“Not executed line”);

}

catch(Exception e)

{ Console.WriteLine(“oException” );}

Console.WriteLine(“Result is {0}”,div);

}

}

Throwing an Exception

In C#, it is possible to throw an exception programmatically. The ‘throw’ keyword is used for this purpose. The general form of throwing an exception is as follows.

throw exception_obj;

For example the following statement throws an ArgumentException explicitly.

throw new ArgumentException(“Exception”);

Example

//C#: Exception Handling:

using System;

class MyClient

{

public static void Main()

{

try

{           throw new DivideByZeroException(“Invalid Division”);}

catch(DivideByZeroException e)

{ Console.WriteLine(“Exception” ); }

Console.WriteLine(“LAST STATEMENT”);

}

}

Standard Exceptions

There are two types of exceptions: exceptions generated by an executing program and exceptions generated by the common language runtime. System.Exception is the base class for all exceptions in C#. Several exception classes inherit from this class including ApplicationException and SystemException. These two classes form the basis for most other runtime exceptions. Other exceptions that derive directly from System.Exception include IOException, WebException etc.

The common language runtime throws SystemException. The ApplicationException is thrown by a user program rather than the runtime. The SystemException includes the ExecutionEngineException, StaclOverFlowException etc. It is not recommended that we catch SystemExceptions nor is it good programming practice to throw SystemExceptions in our applications.

·         System.OutOfMemoryException

·         System.NullReferenceException

·         Syste.InvalidCastException

·         Syste.ArrayTypeMismatchException

·         System.IndexOutOfRangeException        

·         System.ArithmeticException

·         System.DevideByZeroException

·         System.OverFlowException

User-Defined Exceptions

In C#, it is possible to create our own exception class. But Exception must be the ultimate base class for all exceptions in C#. So the user-defined exception classes must inherit from either Exception class or one of its standard derived classes.

//C#: Exception Handling: User defined exceptions

using System;

class MyException : Exception

{

public MyException(string str)

{

 Console.WriteLine(“User defined exception”);}

}

class MyClient

{

public static void Main()

{

try

{ throw new MyException(“RAJESH”); }

catch(Exception e)

{Console.WriteLine(“Exception cau