FindIndex(T) Method (T[], Int32, Int32, Predicate(T))

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


Searches for an element that matches the conditions defined by the specified predicate, and returns the zero-based 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 FindIndex<T>(
	T[] array,
	int startIndex,
	int count,
	Predicate<T> match


Type: T[]

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

Type: System.Int32

The zero-based starting index of the search.

Type: System.Int32

The number of elements in the section to search.

Type: System.Predicate<T>

The Predicate<T> that defines the conditions of the element to search for.

Return Value

Type: System.Int32

The zero-based index of the first occurrence of an element that matches the conditions defined by match, if found; otherwise, –1.

Type Parameters


The type of the elements of the array.

Exception Condition

array is null.


match 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 Predicate<T> is a delegate to a method that returns true if the object passed to it matches the conditions defined in the delegate. The elements of array are individually passed to the Predicate<T>.

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

The following code example demonstrates all three overloads of the FindIndex generic method. An array of strings is created, containing 8 dinosaur names, two of which (at positions 1 and 5) end with "saurus". The code example also defines a search predicate method named EndsWithSaurus, which accepts a string parameter and returns a Boolean value indicating whether the input string ends in "saurus".

The FindIndex<T>(T[], Predicate<T>) method overload traverses the array from the beginning, passing each element in turn to the EndsWithSaurus method. The search stops when the EndsWithSaurus method returns true for the element at position 1.


In C# and Visual Basic, it is not necessary to create the Predicate<string> delegate (Predicate(Of String) in Visual Basic) explicitly. These languages infer the correct delegate from context and create it automatically.

The FindIndex<T>(T[], Int32, Predicate<T>) method overload is used to search the array beginning at position 2 and continuing to the end of the array. It finds the element at position 5. Finally, the FindIndex<T>(T[], Int32, Int32, Predicate<T>) method overload is used to search the range of three elements beginning at position 2. It returns –1 because there are no dinosaur names in that range that end with "saurus".

using System;

public class Example
    public static void Main()
        string[] dinosaurs = { "Compsognathus", 
            "Amargasaurus",   "Oviraptor",      "Velociraptor", 
            "Deinonychus",    "Dilophosaurus",  "Gallimimus", 
            "Triceratops" };

        foreach(string dinosaur in dinosaurs)

            "\nArray.FindIndex(dinosaurs, EndsWithSaurus): {0}", 
            Array.FindIndex(dinosaurs, EndsWithSaurus));

            "\nArray.FindIndex(dinosaurs, 2, EndsWithSaurus): {0}",
            Array.FindIndex(dinosaurs, 2, EndsWithSaurus));

            "\nArray.FindIndex(dinosaurs, 2, 3, EndsWithSaurus): {0}",
            Array.FindIndex(dinosaurs, 2, 3, EndsWithSaurus));

    // Search predicate returns true if a string ends in "saurus".
    private static bool EndsWithSaurus(String s)
        if ((s.Length > 5) && 
            (s.Substring(s.Length - 6).ToLower() == "saurus"))
            return true;
            return false;

/* This code example produces the following output:


Array.FindIndex(dinosaurs, EndsWithSaurus): 1

Array.FindIndex(dinosaurs, 2, EndsWithSaurus): 5

Array.FindIndex(dinosaurs, 2, 3, EndsWithSaurus): -1

Universal Windows Platform
Available since 4.5
.NET Framework
Available since 2.0
Portable Class Library
Supported in: portable .NET platforms
Windows Phone Silverlight
Available since 8.0
Windows Phone
Available since 8.1
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