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IndexOf Method (T)
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List.IndexOf Method (T)

Searches for the specified object and returns the zero-based index of the first occurrence within the entire List.

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

public int IndexOf (
	T item
public final int IndexOf (
	T item
public final function IndexOf (
	item : T
) : int
Not applicable.



The object to locate in the List. The value can be a null reference (Nothing in Visual Basic) for reference types.

Return Value

The zero-based index of the first occurrence of item within the entire List, if found; otherwise, –1.

The List is searched forward starting at the first element and ending at the last element.

This method determines equality using the default equality comparer EqualityComparer.Default for T, the type of values in the list.

This method performs a linear search; therefore, this method is an O(n) operation, where n is Count.

The following code example demonstrates all three overloads of the IndexOf method. A List of strings is created, with one entry that appears twice, at index location 0 and index location 5. The IndexOf(T) method overload searches the list from the beginning, and finds the first occurrence of the string. The IndexOf(T,Int32) method overload is used to search the list beginning with index location 3 and continuing to the end of the list, and finds the second occurrence of the string. Finally, the IndexOf(T,Int32,Int32) method overload is used to search a range of two entries, beginning at index location two; it returns –1 because there are no instances of the search string in that range.

using System;
using System.Collections.Generic;

public class Example
    public static void Main()
        List<string> dinosaurs = new List<string>();


        foreach(string dinosaur in dinosaurs)

        Console.WriteLine("\nIndexOf(\"Tyrannosaurus\"): {0}", 

        Console.WriteLine("\nIndexOf(\"Tyrannosaurus\", 3): {0}", 
            dinosaurs.IndexOf("Tyrannosaurus", 3));

        Console.WriteLine("\nIndexOf(\"Tyrannosaurus\", 2, 2): {0}", 
            dinosaurs.IndexOf("Tyrannosaurus", 2, 2));

/* This code example produces the following output:


IndexOf("Tyrannosaurus"): 0

IndexOf("Tyrannosaurus", 3): 5

IndexOf("Tyrannosaurus", 2, 2): -1

Windows 98, Windows Server 2000 SP4, Windows CE, Windows Millennium Edition, Windows Mobile for Pocket PC, Windows Mobile for Smartphone, Windows Server 2003, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP SP2, Windows XP Starter Edition

The Microsoft .NET Framework 3.0 is supported on Windows Vista, Microsoft Windows XP SP2, and Windows Server 2003 SP1.

.NET Framework

Supported in: 3.0, 2.0

.NET Compact Framework

Supported in: 2.0

XNA Framework

Supported in: 1.0

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