How to use Substring function c#.net

Substring :

"Substring in C# string Class returns a new string that is a substring of this string. The substring begins at the specified given index and extended up to the given length.."

How to Use :
  string  string.Substring(strat index,length)

Input Parameter :
         start index = (Int)
         length = (Int)

Output Parameter :
       string

Example  :: 
/////////////////////////  CODE   /////////////////////

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace Substring
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            string str = null;
            string retString = null;
            str = "This is Nirav M. patel";
            retString = str.Substring(6, 9);
            MessageBox.Show(retString);
        }
    }
}
//////////////////////////////  OUTPUT   /////////////////////////
s Nirav M

///////////////////////////////////////////////////////////////////////////

--
/\/ir@\/  <(.'.)>