How to use C# string IndexOf

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

int string.IndexOf(string str)
  1. str - The parameter string to check its occurrences.
  1. 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.

  1. System.ArgumentNullException: If the Argument is null.


  1. "This is a test".IndexOf("Test") returns 10
  2. "This is a test".IndexOf("vb") returns -1
Full Source C#
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.