Class Modifiers and their uses in C# .Net

Class/Interface (Base class)

Inheritance (Inherited class)

Implementation (Interface)

(Child)

(Instance)

Is initiated??

General

Abstract

Sealed

Static*

General

Yes

Yes

Yes

No

No

Yes

Abstract

Yes

Yes

Yes

No

No

No

Sealed

No

No

No

No

No

Yes

Static

No

No

No

No

No

No

Interface

Yes

Yes

Yes

No

Yes

No

 

 

 


 

 

 

 

 

This table is for showing relationship between a base class and child class

In this table I want to show the accessibility among the various types of classes and interface. From this table you also can know what type of class can be initiated. This is one example that helps you to understand the table.

If you use Abstract class as base class and take a general class as child and want to inherit, this will possible …here you can see..

Base: Abstract (ASadi)

Child: Interface (Sadi)

Accessibility: Yes

abstract class Sadi : ASadi

{

#region ASadi Members

public int getint() {

throw new Exception(“The method or operation is not implemented.”); }

#endregion

}

If you execute this code, it executes properly

From the first column of the table we can say that general class inherites both general and abstract class and also implements an interface. These classes cannot inherites sealed as well as static class. Only General and Sealed classes can be initiated.

From the table and above discussion we can conclude that,

Abstract Class

A class defined as abstract is used as a base class. Such a class is used for the purpose of inheritance only i.e. other classes are derived from this class. We cannot create an object of an abstract class. An abstract class may contain methods and properties. The classes derived from the abstract class inherit these methods and properties. The abstract class may also contain abstract methods and properties. Abstract method and properties do not have any functionality. The derived class defines their full functionality.

Here is an example of an abstract class:

abstract class MyAbstract { public abstract void AbMethod(); }

Sealed Class

Classes can be declared as sealed. This is accomplished by putting the sealed keyword before the keyword class in the class definition. For example:

public sealed class classSealed { // Class members here. public string ID; public double Price; }

A sealed class cannot be used as a base class. For this reason, it cannot also be an abstract class. Sealed classes are primarily used to prevent derivation. Because they can never be used as a base class, some run-time optimizations can make calling sealed class members slightly faster. Sealing a class means one cannot derive from it. Sealing a method means one cannot override it. In C# structs are implicitly sealed; therefore, they cannot be inherited. If we try to inherit from a sealed class in another class we will get compile time error about Inconsistent accessibility (code is shown in following code listing). In C# a method cannot be declared as sealed. However when we override a method in a derived class, we can declare the overridden method as sealed as shown below. By declaring it as sealed, we can avoid further overriding of this method.

Static Class

Static classes are classes that contain only static members. Following is an example of static class. public static MyClass { …..}

A class can be declared static, which indicates that it contains only static members. It is not possible to use the new keyword to create instances of a static class. Static classes are loaded automatically by the .NET Framework common language runtime (CLR) when the program or namespace that contains the class is loaded.

Use a static class to contain methods that are not associated with a particular object. For example, it is a common requirement to create a set of methods that do not act on instance data and are not associated to a specific object in your code. You could use a static class to hold those methods.

Following are the main features of a static class:

·         They only contain static members.

·         They cannot be instantiated.

·         They are sealed.

·         They cannot contain Instance Constructors (C# Programming Guide).

Creating a static class is therefore basically the same as creating a class that contains only static members and a private constructor. A private constructor prevents the class from being instantiated.

The advantage of using a static class is that the compiler can check to make sure that no instance members are accidentally added. The compiler will guarantee that instances of this class cannot be created.

Static classes are sealed and therefore cannot be inherited. They cannot inherit from any class except Object. Static classes cannot contain an instance constructor; however, they can have a static constructor. For more information, see Static Constructors (C# Programming Guide).

 

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