How to use IndexOf function c#.net

Substring :

"The IndexOf method in string Class in C# returns the index of the first occurrence of the specified substring.."

How to Use :
 int number = string.IndexOf(string str)

Input Parameter :
         str = (string)

Output Parameter :
       int number

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

        private void button1_Click(object sender, EventArgs e)
        {
            string str = null;
            str = "driver driveing a car";
            MessageBox.Show(str.IndexOf("driveing").ToString());
        }
    }
}

//////////////////////////////  OUTPUT   /////////////////////////
7

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

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