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

Array.FindIndex<T> Method (T[], 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 entire Array.

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

public static int FindIndex<T>(
	T[] array,
	Predicate<T> match


Type: T[]

The one-dimensional, zero-based Array 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.

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

ThePredicate<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 the Length of array.

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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