Abstract class

What is Abstract class?.

The main Purpose of used abstract class is to provide default functionality of its subclassses.

when be declare abstract method in a base class the ever derived class must be declare own its definition.

Points to remember.

• * it is madatory to override abstract method in a derived class.
• * we can not create instance of abstrac classs.
• * but it can be used as parameter in method.
• Abstract classes are declared using the abstract keyword.
• We cannot create an object of an abstract class.
• If you want to use it then it must be inherited in a subclass.
• An Abstract class contains both abstract and non-abstract methods.
• The methods inside the abstract class can either have an implementation or no implementation.
• We can inherit two abstract classes; in this case the base class method implementation is optional. 
• .An Abstract class has only one subclass.
• Methods inside the abstract class cannot be private(virtual member & abstract class can not be private)
• If there is at least one method abstract in a class then the class must be abstract.
• we can create Constructor or property in abstract class. 

Why can't create instance of an abstract class in C# 

No, you cannot create an instance of an abstract class because it does not have a complete implementation. 

 The purpose of an abstract class is to function as a base for subclasses. 

 It acts like a template, or an empty or partially empty structure,

 you should extend it and build on it before you can use it.

PERFORMENCE: abstract classes with virtual methods have better performance than interface implementation in the .NET Framework 4.0.

Note:1. Virtual and abstract member can not be private.

         2.  An abstract class cannot be a sealed class

         3 .Declaration of abstract methods are only allowed in abstract classes.

         4 .An abstract class can have abstract members as well non abstract members.

         5. We can create property in abstract class.

Example:

public abstract class AbsA
    {
        public abstract void MethodTest();
    }

    class TestB:AbsA {

        public override void MethodTest() {
            Console.WriteLine("TestA");
        }

    }