List(Of T).Contains Method (T)
Determines whether an element is in the List(Of T).
Assembly: mscorlib (in mscorlib.dll)
Parameters
- item
-
Type:
T
The object to locate in the List(Of T). The value can be null for reference types.
Implements
ICollection(Of T).Contains(T)This method determines equality by using the default equality comparer, as defined by the object's implementation of the IEquatable(Of T).Equals method for T (the type of values in the list).
This method performs a linear search; therefore, this method is an O(n) operation, where n is Count.
The following example demonstrates the Contains and Exists methods on a List(Of T) that contains a simple business object that implements Equals.
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 IEquatable.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 contains a list of complex objects of type Cube. The Cube class implements the IEquatable(Of T).Equals method so that two cubes are considered equal if their dimensions are the same. In this example, the Contains method returns true, because a cube that has the specified dimensions is already in the collection.
Imports System Imports System.Collections.Generic Class Program Public Shared Sub Main(ByVal args As String()) Dim cubes As New List(Of Cube)() cubes.Add(New Cube(8, 8, 4)) cubes.Add(New Cube(8, 4, 8)) cubes.Add(New Cube(8, 6, 4)) If cubes.Contains(New Cube(8, 6, 4)) Then Console.WriteLine("An equal cube is already in the collection.") Else Console.WriteLine("Cube can be added.") End If 'Outputs "An equal cube is already in the collection." End Sub End Class Public Class Cube Implements IEquatable(Of Cube) Public Sub New(ByVal h As Integer, ByVal l As Integer, ByVal w As Integer) Me.Height = h Me.Length = l Me.Width = w End Sub Private _Height As Integer Public Property Height() As Integer Get Return _Height End Get Set(ByVal value As Integer) _Height = value End Set End Property Private _Length As Integer Public Property Length() As Integer Get Return _Length End Get Set(ByVal value As Integer) _Length = value End Set End Property Private _Width As Integer Public Property Width() As Integer Get Return _Width End Get Set(ByVal value As Integer) _Width = value End Set End Property Public Overloads Function Equals(ByVal other As Cube) _ As Boolean Implements IEquatable(Of Cube).Equals If Me.Height = other.Height And Me.Length = other.Length _ And Me.Width = other.Width Then Return True Else Return False End If End Function End Class
Available since 8
.NET Framework
Available since 2.0
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1