Using "this" keyword

using System;
class Rectangle
{
    double height, width;
    public Rectangle()
    {
        height = 0;
        width = 0;
    }
    public Rectangle(double height, double width)
    {
        this.height = height;
        this.width = width;
    }
    public Rectangle(double len)
    {
        height = len;
        width = len;
    }
    public double area()
    {
        return width * height;
    }
}

using System;
class Program
{
    public static void Main(String[] args)
    {
        Rectangle rec = new Rectangle();
        Rectangle rec2 = new Rectangle(3.0, 7.5);
        Rectangle rec3 = new Rectangle(5.0);
        Console.WriteLine("The area of the Rectangle is: " + rec.area());
        Console.WriteLine("The area of the Rectangle is: " + rec2.area());
        Console.WriteLine("The area of the Rectangle is: " + rec3.area());
    }
}