Abstract class in c#

• In c#, Abstract class is a class that can not be instantiated on its own
• It is typically used as a base class for other class.
• Abstract class provides a way to achieve abstraction, because there you can just declare the methods (abstract methods) and implement them later.
• It can contain both abstract methods (methods without implementation details) and non-abstract methods (method with implementation details).
• Similar goal can be achieved with interface, but in abstract class you can also define non-abstract method. These methods are needed when you need to share some common functionality.

public abstract class Shape
{
   // it is the basic syntax of abstract class
}

Example:

abstract class Shape
{
    public abstract void Draw(); // Abstract method without implementation

    public void Display()  //non abstract method
    {
        Console.WriteLine("Displaying shape...");
    }


}

// concrete class
class Circle : Shape
{
    public override void Draw()
    {
        Console.WriteLine("Drawing a circle");
    }
}

// concrete class
class Square : Shape
{
    public override void Draw()
    {
        Console.WriteLine("Drawing a square");
    }
}

class Program
{
    static void Main()
    {
       Shape circle = new Circle();
        circle.Draw();     // Drawing a circle
        circle.Display(); //Displaying shape...

        Shape square = new Square();
        square.Draw();  // Drawing a square
        square.Display(); //Displaying shape...

    }
}

• Shape is an abstract class, which contains an abstract method Draw() and non-abstract method Display().
• We are defining high level overview of any shape. Whatever shape we will have, it must have a feature Draw. Since we don’t know the details of the shape yet (it may be circle, pentagon, rectangle) , we can not define it it advance. Circle/Rectangle/Pentagon will have it’s own draw method. That’s why Draw() method is marked as abstract-method. It’s implementation will be defined later.
• Now we have two class Circle and Square. Each class will inherit the Shape class and override the Draw() method.

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