How to use C# string IndexOf

using System;
using System.Windows.Forms;

namespace WindowsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            string str = null;
            str = "CSharp TOP 10 BOOKS";
            MessageBox.Show(str.IndexOf("BOOKS").ToString()); 
        }
    }
}

When you execute this C# program you will get the number 14 in the message box. 

That means the substring "BOOKS" occurred and start in the position 14.