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Array.IndexOf<T> Method (T[], T, Int32, Int32)

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

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

public static int IndexOf<T>(
	T[] array,
	T value,
	int startIndex,
	int count

Type Parameters


The type of the elements of the array.


Type: T[]
The one-dimensional, zero-based Array to search.
Type: T
The object to locate in array.
Type: System.Int32
The zero-based starting index of the search. 0 (zero) is valid in an empty array.
Type: System.Int32
The number of elements in the section to search.

Return Value

Type: System.Int32
The zero-based index of the first occurrence of value within the range of elements in array that starts at startIndex and contains the number of elements specified in count, if found; otherwise, –1.


array is null.


startIndex is outside the range of valid indexes for array.


count is less than zero.


startIndex and count do not specify a valid section in array.

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

The elements are compared to the specified value using the Object.Equals method. If the element type is a nonintrinsic (user-defined) type, the Equals implementation of that type is used.

Passing the Length property of the array as the startindex parameter results in a return value of -1; passing values greater than Length raises an ArgumentOutOfRangeException.

This method is an O(n) operation, where n is count.

The following code example demonstrates all three generic overloads of the IndexOf method. An array of strings is created, with one entry that appears twice, at index location 0 and index location 5. The IndexOf<T>(T[], T) method overload searches the array from the beginning, and finds the first occurrence of the string. The IndexOf<T>(T[], T, Int32) method overload is used to search the array beginning with index location 3 and continuing to the end of the array, and finds the second occurrence of the string. Finally, the IndexOf<T>(T[], 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;

public class Example
    public static void Main()
        string[] dinosaurs = { "Tyrannosaurus",
            "Compsognathus" };

        foreach(string dinosaur in dinosaurs)

            "\nArray.IndexOf(dinosaurs, \"Tyrannosaurus\"): {0}", 
            Array.IndexOf(dinosaurs, "Tyrannosaurus"));

            "\nArray.IndexOf(dinosaurs, \"Tyrannosaurus\", 3): {0}", 
            Array.IndexOf(dinosaurs, "Tyrannosaurus", 3));

            "\nArray.IndexOf(dinosaurs, \"Tyrannosaurus\", 2, 2): {0}", 
            Array.IndexOf(dinosaurs, "Tyrannosaurus", 2, 2));

/* This code example produces the following output:


Array.IndexOf(dinosaurs, "Tyrannosaurus"): 0

Array.IndexOf(dinosaurs, "Tyrannosaurus", 3): 5

Array.IndexOf(dinosaurs, "Tyrannosaurus", 2, 2): -1

.NET Framework

Supported in: 4, 3.5, 3.0, 2.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

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.