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String.IndexOf Method (String, Int32)

Reports the zero-based index of the first occurrence of the specified String in this instance. The search starts at a specified character position.

Namespace:  System
Assembly:  mscorlib (in mscorlib.dll)

public int IndexOf(
	string value,
	int startIndex


Type: System.String

The string to seek.

Type: System.Int32

The search starting position.

Return Value

Type: System.Int32
The zero-based index position of value if that string is found, or -1 if it is not. If value is Empty, the return value is startIndex.


value is null.


startIndex is negative.


startIndex is less than zero or greater than the length of this string.

Index numbering starts from 0 (zero). The startIndex parameter can range from 0 to the length of the string instance. If startIndex equals the length of the string instance, the method returns -1.

This method performs a word (case-sensitive and culture-sensitive) search using the current culture. The search begins at the startIndex character position of this instance and continues until the last character position.

The following example searches for all occurrences of a specified string within a target string.

using System;

public class IndexOfTest {
    public static void Main() {

        string strSource = "This is the string which we will perform the search on";

        Console.WriteLine("The search string is:{0}\"{1}\"{0}", Environment.NewLine, strSource);

        string strTarget = "";
        int found = 0;
        int totFinds = 0;

        do {
            Console.Write("Please enter a search value to look for in the above string (hit Enter to exit) ==> ");

            strTarget = Console.ReadLine();

            if (strTarget != "") {

                for (int i = 0; i < strSource.Length; i++) {

                    found = strSource.IndexOf(strTarget, i);

                    if (found > 0) {
                        i = found;

            Console.WriteLine("{0}The search parameter '{1}' was found {2} times.{0}",
                    Environment.NewLine, strTarget, totFinds);

            totFinds = 0;

        } while ( true );

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5, 3.0, 2.0, 1.1, 1.0

.NET Compact Framework

Supported in: 3.5, 2.0, 1.0

XNA Framework

Supported in: 3.0, 2.0, 1.0




Clarified the relationship between string length and the startIndex parameter.

Content bug fix.

Noted that an exception is not thrown for an empty string if the index parameter is invalid.

Content bug fix.