List.IndexOf Method (T, Int32, Int32)

Searches for the specified object and returns the zero-based index of the first occurrence within the range of elements in the List that starts at the specified index and contains the specified number of elements.

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

public int IndexOf (
	T item,
	int index,
	int count
public int IndexOf (
	T item, 
	int index, 
	int count
public function IndexOf (
	item : T, 
	index : int, 
	count : int
) : int
Not applicable.



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


The zero-based starting index of the search.


The number of elements in the section to search.

Return Value

The zero-based index of the first occurrence of item within the range of elements in the List that starts at index and contains count number of elements, if found; otherwise, –1.

Exception typeCondition


index is outside the range of valid indexes for the List.


count is less than 0.


index and count do not specify a valid section in the List.

The List is searched forward starting at index and ending at index plus count minus 1, if count is greater than 0.

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