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List(Of T).Find Method

Searches for an element that matches the conditions defined by the specified predicate, and returns the first occurrence within the entire List(Of T).

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

'Declaration
Public Function Find ( _
	match As Predicate(Of T) _
) As T

Parameters

match
Type: System.Predicate(Of T)

The Predicate(Of T) delegate that defines the conditions of the element to search for.

Return Value

Type: T
The first element that matches the conditions defined by the specified predicate, if found; otherwise, the default value for type T.

ExceptionCondition
ArgumentNullException

match is Nothing.

The Predicate(Of 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 the current List(Of T) are individually passed to the Predicate(Of T) delegate, moving forward in the List(Of T), starting with the first element and ending with the last element. Processing is stopped when a match is found.

Important noteImportant

When searching a list containing value types, make sure the default value for the type does not satisfy the search predicate. Otherwise, there is no way to distinguish between a default value indicating that no match was found and a list element that happens to have the default value for the type. If the default value satisfies the search predicate, use the FindIndex method instead.

This method performs a linear search; therefore, this method is an O(n) operation, where n is Count.

The following example demonstrates the Find method on a List(Of T) that contains a simple complex object.

Imports System.Collections.Generic
' Simple business object. A PartId is used to identify a part  
' but the part name can change.  
Public Class Part
    Implements IEquatable(Of Part)
    Public Property PartName() As String 
        Get 
            Return m_PartName
        End Get 
        Set(value As String)
            m_PartName = Value
        End Set 
    End Property 
    Private m_PartName As String 
    Public Property PartId() As Integer 
        Get 
            Return m_PartId
        End Get 
        Set(value As Integer)
            m_PartId = Value
        End Set 
    End Property 
    Private m_PartId As Integer 

    Public Overrides Function ToString() As String 
        Return Convert.ToString("ID: " & PartId & "   Name: ") & PartName
    End Function 
    Public Overrides Function Equals(obj As Object) As Boolean 
        If obj Is Nothing Then 
            Return False 
        End If 
        Dim objAsPart As Part = TryCast(obj, Part)
        If objAsPart Is Nothing Then 
            Return False 
        Else 
            Return Equals(objAsPart)
        End If 
    End Function 
    Public Overrides Function GetHashCode() As Integer 
        Return PartId
    End Function 
    Public Overloads Function Equals(other As Part) As Boolean _
        Implements IEquatable(Of Part).Equals
        If other Is Nothing Then 
            Return False 
        End If 
        Return (Me.PartId.Equals(other.PartId))
    End Function 
    ' Should also override == and != operators. 
End Class 
Public Class Example
    Public Shared Sub Main()
        ' Create a list of parts. 
        Dim parts As New List(Of Part)()

        ' Add parts to the list.
        parts.Add(New Part() With { _
             .PartName = "crank arm", _
             .PartId = 1234 _
        })
        parts.Add(New Part() With { _
             .PartName = "chain ring", _
             .PartId = 1334 _
        })
        parts.Add(New Part() With { _
             .PartName = "regular seat", _
             .PartId = 1434 _
        })
        parts.Add(New Part() With { _
             .PartName = "banana seat", _
             .PartId = 1444 _
        })
        parts.Add(New Part() With { _
             .PartName = "cassette", _
             .PartId = 1534 _
        })
        parts.Add(New Part() With { _
             .PartName = "shift lever", _
             .PartId = 1634 _
        })



        ' Write out the parts in the list. This will call the overridden ToString method 
        ' in the Part class.
        Console.WriteLine()
        For Each aPart As Part In parts
            Console.WriteLine(aPart)
        Next 


        ' Check the list for part #1734. This calls the IEquitable.Equals method 
        ' of the Part class, which checks the PartId for equality.
        Console.WriteLine(vbLf & "Contains: Part with Id=1734: {0}",
                          parts.Contains(New Part() With { _
             .PartId = 1734, _
             .PartName = "" _
        }))

        ' Find items where name contains "seat".
        Console.WriteLine(vbLf & "Find: Part where name contains ""seat"": {0}",
                          parts.Find(Function(x) x.PartName.Contains("seat")))

        ' Check if an item with Id 1444 exists.
        Console.WriteLine(vbLf & "Exists: Part with Id=1444: {0}",
                          parts.Exists(Function(x) x.PartId = 1444))

        'This code example produces the following output: 
        '         
        '        ID: 1234   Name: crank arm 
        '        ID: 1334   Name: chain ring 
        '        ID: 1434   Name: regular seat 
        '        ID: 1444   Name: banana seat 
        '        ID: 1534   Name: cassette 
        '        ID: 1634   Name: shift lever 
        
        '        Contains: Part with Id=1734: False 
        
        '        Find: Part where name contains "seat": ID: 1434   Name: regular seat 
        
        '        Exists: Part with Id=1444: True  
        '          

    End Sub 
End Class

The following example demonstrates the find methods for the List(Of T) class. The example for the List(Of T) class contains book objects, of class Book, using the data from the Sample XML File: Books (LINQ to XML). The FillList method in the example uses LINQ to XML to parse the values from the XML to property values of the book objects.

The following table describes the examples provided for the find methods.

Method

Example

Find(Predicate(Of T))

Finds a book by an ID using the IDToFind predicate delegate.

C# example uses an anonymous delegate.

FindAll(Predicate(Of T))

Find all books that whose Genre property is "Computer" using the FindComputer predicate delegate.

FindLast(Predicate(Of T))

Finds the last book in the collection that has a publish date before 2001, using the PubBefore2001 predicate delegate.

C# example uses an anonymous delegate.

FindIndex(Predicate(Of T))

Finds the index of first computer book using the FindComputer predicate delegate.

FindLastIndex(Predicate(Of T))

Finds the index of the last computer book using the FindComputer predicate delegate.

FindIndex(Int32, Int32, Predicate(Of T))

Finds the index of first computer book in the second half of the collection, using the FindComputer predicate delegate.

FindLastIndex(Int32, Int32, Predicate(Of T))

Finds the index of last computer book in the second half of the collection, using the FindComputer predicate delegate.

Imports System.Collections.Generic
Imports System.Linq
Imports System.Xml.Linq
Module Module1

    Private IDToFind As String = "bk109" 

    Public Books As New List(Of Book)


    Sub Main()

        FillList()

        ' Find a book by its ID. 
        Dim result As Book = Books.Find(AddressOf FindID)
        If result IsNot Nothing Then
            DisplayResult(result, "Find by ID: " & IDToFind)
        Else

            Console.WriteLine(vbCrLf & "Not found: " & IDToFind)
        End If
        Console.WriteLine()

        ' Find last book in collection that has a publish date before 2001.
        result = Books.FindLast(AddressOf PubBefore2001)
        If result IsNot Nothing Then
            DisplayResult(result, "Last book in collection published before 2001:")
        Else
            Console.WriteLine(vbCrLf & "Not found: " & IDToFind)
        End If
        Console.WriteLine()

        ' Find all computer books. 
        Dim results As List(Of Book) = Books.FindAll(AddressOf FindComputer)
        If results.Count <> 0 Then
            DisplayResults(results, "All computer books:")
        Else
            Console.WriteLine(vbCrLf & "No books found.")
        End If
        Console.WriteLine()


        ' Find all books under $10.00.
        results = Books.FindAll(AddressOf FindUnderTen)
        If results.Count <> 0 Then
            DisplayResults(results, "Books under $10:")
        Else
            Console.WriteLine(vbCrLf & "No books found.")
        End If
        Console.WriteLine()

        ' Find index values.
        Console.WriteLine()
        Dim ndx As Integer = Books.FindIndex(AddressOf FindComputer)
        Console.WriteLine("Index of first computer book: " & ndx)
        ndx = Books.FindLastIndex(AddressOf FindComputer)
        Console.WriteLine("Index of last computer book: " & ndx)

        Dim mid As Integer = Books.Count / 2
        ndx = Books.FindIndex(mid, mid, AddressOf FindComputer)
        Console.WriteLine("Index of first computer book in the second half of the collection: " & ndx)



        ndx = Books.FindLastIndex(Books.Count - 1, mid, AddressOf FindComputer)
        Console.WriteLine("Index of last computer book in the second half of the collection: " & ndx)


    End Sub 



    Private Sub FillList()

        ' Create XML elements from a source file. 
        Dim xTree As XElement = XElement.Load("c:\temp\books.xml")

        ' Create an enumerable collection of the elements. 
        Dim elements As IEnumerable(Of XElement) = xTree.Elements

        ' Evaluate each element and set values in the book object. 
        For Each el As XElement In elements
            Dim Book As New Book()
            Book.ID = el.Attribute("id").Value
            Dim props As IEnumerable(Of XElement) = el.Elements
            For Each p As XElement In props
                If p.Name.ToString.ToLower = "author" Then
                    Book.Author = p.Value
                End If 
                If p.Name.ToString.ToLower = "title" Then
                    Book.Title = p.Value
                End If 
                If p.Name.ToString.ToLower = "genre" Then
                    Book.Genre = p.Value
                End If 
                If p.Name.ToString.ToLower = "price" Then
                    Book.Price = Convert.ToDouble(p.Value)
                End If 
                If p.Name.ToString.ToLower = "publish_date" Then
                    Book.Publish_date = Convert.ToDateTime(p.Value)
                End If 
                If p.Name.ToString.ToLower = "description" Then
                    Book.Description = p.Value
                End If 
            Next
            Books.Add(Book)
        Next

        DisplayResults(Books, "All books:")
        Console.WriteLine()

    End Sub 

    ' Predicate delegates for 
    ' Find and FindAll methods. 
    Private Function FindID(ByVal bk As Book) As Boolean 
        If bk.ID = IDToFind Then 
            Return True 
        Else 
            Return False 
        End If 
    End Function 
    Private Function FindComputer(ByVal bk As Book) As Boolean 
        If bk.Genre = "Computer" Then 
            Return True 
        Else 
            Return False 
        End If 
    End Function 
    Private Function FindUnderTen(ByVal bk As Book) As Boolean 
        Dim tendollars As Double = 10.0
        If bk.Price < tendollars Then 
            Return True 
        Else 
            Return False 
        End If 
    End Function 
    Private Function PubBefore2001(ByVal bk As Book) As Boolean 
        Dim year2001 As DateTime = New DateTime(2001, 1, 1)
        Return bk.Publish_date < year2001
    End Function 
    Private Sub DisplayResult(ByVal result As Book, ByVal title As String)
        Console.WriteLine()
        Console.WriteLine(title)
        Console.WriteLine(vbLf & result.ID & vbTab & result.Author & _
                          vbTab & result.Title & vbTab & result.Genre & _
                          vbTab & result.Publish_date & vbTab & result.Price)
        Console.WriteLine()
    End Sub 
    Private Sub DisplayResults(ByVal results As List(Of Book), ByVal title As String)
        Console.WriteLine()
        Console.WriteLine(title)
        For Each b As Book In results
            Console.Write(vbLf & b.ID & vbTab & b.Author & _
                              vbTab & b.Title & vbTab & b.Genre & _
                              vbTab & b.Publish_date & vbTab & b.Price)
        Next
        Console.WriteLine()
    End Sub 

    Public Class Book
        Public ID As String 
        Public Author As String 
        Public Title As String 
        Public Genre As String 
        Public Price As Double 
        Public Publish_date As DateTime
        Public Description As String 
    End Class 


End Module

.NET Framework

Supported in: 4.5, 4, 3.5, 3.0, 2.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

.NET for Windows Store apps

Supported in: Windows 8

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone Silverlight 8.1, Windows Phone Silverlight 8

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

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