>Thanks a lot, I hope I see it. So even if I create several constructors, ":this()" makes the parametherless constructor to be called.
>But when or why can it be needed?

Because you do not want to duplicate code - look what happens here

	public static class qqq
	{
		static void Main()
		{
			A a = new A(2, 3);

			Console.ReadLine();

		}
	}


	class A
	{
		int Prop1;
		int Prop2;
		int Prop3;

		public A()
		{
			Prop1 = 1;
			Console.WriteLine("prop1");
		}

		public A(int prop2)
			: this()
		{
			Prop2 = prop2;
			Console.WriteLine("prop2");
		}

		public A(int prop2, int prop3)
			: this(prop2)
		{
			Prop3 = prop3;
			Console.WriteLine("prop3");
		}
	}