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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)


  • str – The parameter string to check its occurrences.


  • Integer – If the parameter String occurred as a substring in the specified String.

It returns the position of the first character of the substring.

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


  • System.ArgumentNullException: If the Argument is null.


  • “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()

        private void button1_Click(object sender, EventArgs e)
            string str = null;
            str = "CSharp TOP 10 BOOKS";

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 position 14.

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