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

int string.IndexOf(string str)

Parameters:

str - The parameter string to check its occurrences

Returns:

Integer - If the parameter String occurred as a substring in the specified String

it returns position of the first character of the substring .

If it does not occur as a substring, -1 is returned.

Exceptions:

System.ArgumentNullException: If the Argument is null.

example:

"This is a test".IndexOf("Test") returns 10

"This is a test".IndexOf("vb") returns -1

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()); 
        }
    }
}