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String.IndexOf Method (String)

Reports the zero-based index of the first occurrence of the specified string in this instance.

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

public int IndexOf(
	string value


Type: System.String
The string to seek.

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 0.


value is null.

Index numbering starts from zero.

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

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 within a string instance by using the comparison rules of the current culture, call the IndexOf(String, StringComparison) method overload with a value of StringComparison.CurrentCulture for its comparisonType parameter.

The following example uses the IndexOf method to determine the starting position of an animal name in a sentence. It then uses this position to insert an adjective that describes the animal into the sentence.

using System;

public class InsertTest {
    public static void Main() 
        string animal1 = "fox";
        string animal2 = "dog";

        string strTarget = String.Format("The {0} jumped over the {1}.", animal1, animal2);

        Console.WriteLine("The original string is:{0}{1}{0}", Environment.NewLine, strTarget);

        Console.Write("Enter an adjective (or group of adjectives) to describe the {0}: ==> ", animal1);
        string adj1 = Console.ReadLine();

        Console.Write("Enter an adjective (or group of adjectives) to describe the {0}: ==> ", animal2);    
        string adj2 = Console.ReadLine();

        adj1 = adj1.Trim() + " ";
        adj2 = adj2.Trim() + " ";

        strTarget = strTarget.Insert(strTarget.IndexOf(animal1), adj1);
        strTarget = strTarget.Insert(strTarget.IndexOf(animal2), adj2);

        Console.WriteLine("{0}The final string is:{0}{1}", Environment.NewLine, strTarget);
// Output from the example might appear as follows:
//       The original string is:
//       The fox jumped over the dog.
//       Enter an adjective (or group of adjectives) to describe the fox: ==> bold
//       Enter an adjective (or group of adjectives) to describe the dog: ==> lazy
//       The final string is:
//       The bold fox jumped over the lazy dog.

.NET Framework

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

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

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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