Abstract class is used to create a base class that can be shared by multiple derived classes.

• It cannot be instantiated.
• It may define abstract methods, which have no implementation.
• All the abstract methods must be implemented by its derived classes.
• It is created by using the abstract keyword.
• It can contain both abstract and non-abstract methods.

Example - Abstract class with abstract method

public abstract class MyAbstractClass
{
   public abstract void MyAbstractMethod(int number);
}

Abstract class can change the original implementation of a base class it inherits.

Example

public class MyBaseClass
{
   public virtual void MyMethod(int number)
   {
       // Original implementation.
   }
}
 
public abstract class MyAbstractClass : MyBaseClass
{
   public abstract override void MyMethod(int number);
}
 
public class MyDerivedClass : MyAbstractClass
{
   public override void MyMethod(int number)
   {
       // New implementation code.
   }
}