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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
)

Parameters

value
Type: System.String

The string to seek.

startIndex
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.

ExceptionCondition
ArgumentNullException

value is null.

ArgumentOutOfRangeException

startIndex is less than 0 (zero) or greater than the length of this string.

Index numbering starts from 0. 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.

Character sets include ignorable characters, which are characters that are not considered when performing a linguistic or culture-sensitive comparison. In a culture-sensitive search, if value contains an ignorable character, the result is equivalent to searching with that character removed. If value consists only of one or more ignorable characters, the IndexOf(String, Int32) method always returns startIndex, which is the character position at which the search begins. In the following example, the IndexOf(String, Int32) method is used to find the position of a soft hyphen (U+00AD) followed by an "m" in two strings. Only one of the strings contains the required substring. If the example is run on the .NET Framework 4 or later, in both cases, because the soft hyphen is an ignorable character, the method returns the index of "m" in the string. Note that in the case of the first string, which includes the soft hyphen followed by an "m", the method fails to return the index of the soft hyphen but instead returns the index of the "m".

using System;

public class Example
{
   public static void Main()
   {
      string searchString = "\u00ADm";
      string s1 = "ani\u00ADmal" ;
      string s2 = "animal";

      Console.WriteLine(s1.IndexOf(searchString, 2));
      Console.WriteLine(s2.IndexOf(searchString, 2));
   }
}
// The example displays the following output: 
//       4 
//       3

Notes to Callers

As explained in Best Practices for Using Strings in the .NET Framework, we recommend that you avoid calling string comparison methods that substitute default values and 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(String, Int32, StringComparison) 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 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) {
                        totFinds++;
                        i = found;
                    }
                    else 
                        break;
                }
            }
            else 
                return;

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

            totFinds = 0;

        } while ( true );
    }
}

.NET Framework

Supported in: 4.5, 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

.NET for Windows Store apps

Supported in: Windows 8

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone Silverlight 8.1, Windows Phone Silverlight 8

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

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

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