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List.FindIndex Method (Generic Predicate)

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 List.

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

public int FindIndex (
	Predicate<T> match
)
public int FindIndex (
	Predicate<T> match
)
public function FindIndex (
	match : Predicate<T>
) : int
Not applicable.

Parameters

match

The Predicate delegate that defines the conditions of the element to search for.

Return Value

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

Exception typeCondition

ArgumentNullException

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

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

The Predicate 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 the current List are individually passed to the Predicate delegate.

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 FindIndex method. A List 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(Generic Predicate) method overload traverses the list 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.

NoteNote:

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(Int32,Generic Predicate) method overload is used to search the list beginning at position 2 and continuing to the end of the list. It finds the element at position 5. Finally, the FindIndex(Int32,Int32,Generic Predicate) 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;
using System.Collections.Generic;

public class Example
{
    public static void Main()
    {
        List<string> dinosaurs = new List<string>();

        dinosaurs.Add("Compsognathus");
        dinosaurs.Add("Amargasaurus");
        dinosaurs.Add("Oviraptor");
        dinosaurs.Add("Velociraptor");
        dinosaurs.Add("Deinonychus");
        dinosaurs.Add("Dilophosaurus");
        dinosaurs.Add("Gallimimus");
        dinosaurs.Add("Triceratops");

        Console.WriteLine();
        foreach(string dinosaur in dinosaurs)
        {
            Console.WriteLine(dinosaur);
        }

        Console.WriteLine("\nFindIndex(EndsWithSaurus): {0}", 
            dinosaurs.FindIndex(EndsWithSaurus));

        Console.WriteLine("\nFindIndex(2, EndsWithSaurus): {0}",
            dinosaurs.FindIndex(2, EndsWithSaurus));

        Console.WriteLine("\nFindIndex(2, 3, EndsWithSaurus): {0}",
            dinosaurs.FindIndex(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;
        }
        else
        {
            return false;
        }
    }
}

/* This code example produces the following output:

Compsognathus
Amargasaurus
Oviraptor
Velociraptor
Deinonychus
Dilophosaurus
Gallimimus
Triceratops

FindIndex(EndsWithSaurus): 1

FindIndex(2, EndsWithSaurus): 5

FindIndex(2, 3, EndsWithSaurus): -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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