How To Define A User Defined Interface And Its Implementation In Class Using C#

using System;
//Interface definition
public interface Shape {
    void area();
}
public class Circle: Shape //implementing user defined interface
    {
        public void area() {
            Console.WriteLine("*** Calculating Area of Circle ***");
            Console.Write("Enter the Radius:");
            float r = float.Parse(Console.ReadLine());
            Console.WriteLine("Area of Circle = " + (3.142 * r * r));
        }
    }
public class Square: Shape {
    public void area() {
        Console.WriteLine("*** Calculating Area of Square ***");
        Console.Write("Enter the Length:");
        float side = float.Parse(Console.ReadLine());
        Console.WriteLine("Area of Square = " + (side * side));
    }
}
class arrinterfacetest {
    static void Main(string[] args) {
        Console.WriteLine("*** Array of Interface Test ***");
        Shape[] s = new Shape[2];
        s[0] = new Circle();
        s[1] = new Square();
        for (int i = 0; i < s.Length; i++) {
            s[i].area();
            Console.ReadKey();
        }
    }
}