Explain abstract class in c# with example

An abstract class in C# is a class that cannot be instantiated on its own and is meant to serve as a base or template for other classes. It may contain abstract (unimplemented) methods and properties that must be implemented by derived classes. Here's a simple example:

```csharp
using System;

// Define an abstract class
abstract class Shape
{
    // Abstract method to calculate area (must be implemented by derived classes)
    public abstract double CalculateArea();
    
    // Regular method
    public void Display()
    {
        Console.WriteLine("This is a shape.");
    }
}

// Derived class 1: Circle
class Circle : Shape
{
    public double Radius { get; set; }

    public Circle(double radius)
    {
        Radius = radius;
    }

    public override double CalculateArea()
    {
        return Math.PI * Math.Pow(Radius, 2);
    }
}

// Derived class 2: Rectangle
class Rectangle : Shape
{
    public double Width { get; set; }
    public double Height { get; set; }

    public Rectangle(double width, double height)
    {
        Width = width;
        Height = height;
    }

    public override double CalculateArea()
    {
        return Width * Height;
    }
}

class Program
{
    static void Main()
    {
        Circle circle = new Circle(5);
        Rectangle rectangle = new Rectangle(4, 6);

        circle.Display();
        Console.WriteLine("Circle Area: " + circle.CalculateArea());

        rectangle.Display();
        Console.WriteLine("Rectangle Area: " + rectangle.CalculateArea());
    }
}
```

In this example, the `Shape` class is abstract and defines an abstract method `CalculateArea()`. The `Circle` and `Rectangle` classes inherit from `Shape` and provide their own implementations of the `CalculateArea()` method. The `Display()` method is a regular method that can be shared among all derived classes.

You cannot create an instance of the `Shape` class, but you can create instances of its derived classes (`Circle` and `Rectangle`) and call their methods. This allows you to define a common interface for shapes while providing flexibility for individual shape types to implement their specific behaviors.

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