mosessaur

public abstract class Shape
{
    //...Class implementation

    public abstract void Draw(int x, int y)
    {
        //this method mustn't be implemented here.
        //If we do implement it, the result is a Syntax Error.
    } 
}


public abstract class Shape2D : Shape
{
    //...Class implementation
    //...you do not have to implement the the method Draw(int x, int y)
}

public class Cricle : Shape2D
{
    //here we should provide an implemetation for Draw(int x, int y)
    public override void Draw(int x, int y)
    {
        //must do some work here
    }

}

Shape m_MyShape = new Shape(); //it is Wrong to that.

Shape m_MyShape = new Circle(); // True

Shape m_MyShape; 
/*

declare refrences only, and the refrences can refer to intances of
any concrete classes derived from abstract class
*/

Circle m_MyCircle = new Circle();
m_MyShape  = m_MyCircle; // Also True