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String.IndexOf Method (String, Int32)

[ This article is for Windows Phone 8 developers. If you’re developing for Windows 10, see the latest documentation. ]

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 String.Empty, the return value is startIndex.


value is null.


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

Index numbering starts from zero. startIndex 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.

Notes to Callers

We recommend that you avoid calling string comparison methods that substitute default values. Instead, call methods that require parameters to be explicitly specified. To find the first index of a substring that occurs after a particular character position by using the comparison rules of the current culture, call the IndexOf method overload with a value of StringComparison.CurrentCulture for its comparisonType parameter.

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

using System;

public class Example
   public static void Demo(System.Windows.Controls.TextBlock outputBlock)

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

      outputBlock.Text += String.Format("The search string is:{0}\"{1}\"{0}", "\n", strSource) + "\n";

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

         outputBlock.Text += "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;

         outputBlock.Text += String.Format("{0}The search parameter '{1}' was found {2} times.{0}",
                 "\n", strTarget, totFinds) + "\n";

         totFinds = 0;

      } while (true);

Windows Phone OS

Supported in: 8.1, 8.0, 7.1, 7.0

Windows Phone