Abstrat Classes and Members C#

Abstract Classes and Class members:
#The abstract keyword enables you to create classes and class members that are incomplete and must be implemented in a derived class.
#Classes can be declared as abstract by putting the keyword abstract before the class definition. For example:

 public abstract class A
{
    // Class members here.
}

#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. For example, a class library may define an abstract class
#Abstract classes may also define abstract methods. This is accomplished by adding the keyword abstract before the return type of the method. For example:
public abstract class A
{
    public abstract void DoWork(int i);
}

#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:
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.
    }
}

Note:

#If a virtual method is declared abstract, it is still virtual to any class inheriting from the abstract class.
#A class inheriting an abstract method cannot access the original implementation of the method—in the previous example, DoWork on class F cannot call DoWork on class D. In this way, an abstract class can force derived classes to provide new method implementations for virtual methods.