List.FindIndex Method (Int32, Int32, 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 range of elements in the List that starts at the specified index and contains the specified number of elements.

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

Public Function FindIndex ( _
	startIndex As Integer, _
	count As Integer, _
	match As Predicate(Of T) _
) As Integer
Dim instance As List(Of T)
Dim startIndex As Integer
Dim count As Integer
Dim match As Predicate(Of T)
Dim returnValue As Integer

returnValue = instance.FindIndex(startIndex, count, match)
public int FindIndex (
	int startIndex, 
	int count, 
	Predicate<T> match
public function FindIndex (
	startIndex : int, 
	count : int, 
	match : Predicate<T>
) : int
Not applicable.



The zero-based starting index of the search.


The number of elements in the section to search.


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


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


startIndex is outside the range of valid indexes for the List.


count is less than 0.


startIndex and count do not specify a valid section in the List.

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

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.


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".

Imports System
Imports System.Collections.Generic

Public Class Example

    Public Shared Sub Main()

        Dim dinosaurs As New List(Of String)


        For Each dinosaur As String In dinosaurs

        Console.WriteLine(vbLf & _
            "FindIndex(AddressOf EndsWithSaurus): {0}", _
            dinosaurs.FindIndex(AddressOf EndsWithSaurus))

        Console.WriteLine(vbLf & _
            "FindIndex(2, AddressOf EndsWithSaurus): {0}", _
            dinosaurs.FindIndex(2, AddressOf EndsWithSaurus))

        Console.WriteLine(vbLf & _
            "FindIndex(2, 3, AddressOf EndsWithSaurus): {0}", _
            dinosaurs.FindIndex(2, 3, AddressOf EndsWithSaurus))

    End Sub

    ' Search predicate returns true if a string ends in "saurus".
    Private Shared Function EndsWithSaurus(ByVal s As String) _
        As Boolean

        ' AndAlso prevents evaluation of the second Boolean
        ' expression if the string is so short that an error
        ' would occur.
        If (s.Length > 5) AndAlso _
            (s.Substring(s.Length - 6).ToLower() = "saurus") Then
            Return True
            Return False
        End If
    End Function
End Class

' This code example produces the following output:
'FindIndex(AddressOf EndsWithSaurus): 1
'FindIndex(2, AddressOf EndsWithSaurus): 5
'FindIndex(2, 3, AddressOf 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