Abstract Class

An abstract class cannot be instantiated. The purpose of an abstract class is to provide a common definition of a base class that multiple derived classes can share.

Abstract classes may also define abstract methods. This is accomplished by adding the keyword abstract before the return type of the method.
Abstract methods have no implementation, so the method definition is followed by a semicolon instead of a normal method block. Derived classes of the abstract class must implement all abstract methods. When an abstract class inherits a virtual method from a base class, the abstract class can override the virtual method with an abstract method. For example:

VB

C#

C++

F#

JScript

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// compile with: /target:library
  
  public class D
  {
      public virtual void DoWork(int i)
      {
          // Original implementation.
  
      }
  }
  
  public abstract class E : D
  {
      public abstract override void DoWork(int i);
  }
  
  public class F : E
  {
      public override void DoWork(int i)
      {
          // New implementation.
  
      }
  }

Define Abstract Properties 

 An abstract property declaration does not provide an implementation of the 
property accessors -- it declares that the class supports properties, but leaves 
the accessor implementation to derived classes. 

public abstract double Area
      {
          get;
      }