IndexOf Method (T[], T, Int32)

Array.IndexOf Generic Method (T[], T, Int32)

Searches for the specified object and returns the index of the first occurrence within the range of elements in the Array that extends from the specified index to the last element.

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

public static int IndexOf<T> (
	T[] array,
	T value,
	int startIndex
J# supports the use of generic types and methods, but not the declaration of new ones.
JScript does not support generic types and methods.
Not applicable.

Type Parameters


The type of the elements of the array.



The one-dimensional, zero-based Array to search.


The object to locate in array.


The zero-based starting index of the search.

Return Value

The zero-based index of the first occurrence of value within the range of elements in array that extends from startIndex to the last element, if found; otherwise, –1.

Exception typeCondition


array is a null reference (Nothing in Visual Basic).


startIndex is outside the range of valid indexes for array.

The Array is searched forward starting at startIndex and ending at the last element.

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 of the array as the startindex will result in a return value of -1, while values greater than Length will raise an ArgumentOutOfRangeException.

This method is an O(n) operation, where n is the number of elements from startIndex to the end of array.

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(J[],J) method overload searches the array from the beginning, and finds the first occurrence of the string. The IndexOf(J[],J,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(J[],J,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

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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