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EqualityComparer(Of T) Class

Updated: January 2010

Provides a base class for implementations of the IEqualityComparer(Of T) generic interface.

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

'Declaration
<SerializableAttribute> _
Public MustInherit Class EqualityComparer(Of T) _
	Implements IEqualityComparer, IEqualityComparer(Of T)
'Usage
Dim instance As EqualityComparer(Of T)

Type Parameters

T

The type of objects to compare.

Derive from this class to provide a custom implementation of the IEqualityComparer(Of T) generic interface for use with collection classes such as the Dictionary(Of TKey, TValue) generic class, or with methods such as List(Of T).Sort.

The Default property checks whether type T implements the System.IEquatable(Of T) generic interface and, if so, returns an EqualityComparer(Of T) that contains the implantation of the IEquitableEquals method. Otherwise, it returns an EqualityComparer(Of T), as provided by T.

We recommend that you derive from the EqualityComparer(Of T) class instead of implementing the IEqualityComparer(Of T) interface, because the EqualityComparer(Of T) class tests for equality using the IEquatable(Of T).Equals method instead of the Object.Equals method. This is consistent with the Contains, IndexOf, LastIndexOf, and Remove methods of the Dictionary(Of TKey, TValue) class and other generic collections.

The following example creates a dictionary collection of objects of type Box with an equality comparer. Two boxes are considered equal if their dimensions are the same. It then adds the boxes to the collection.

The dictionary is recreated with an equality comparer that defines equality in a different way: Two boxes are considered equal if their volumes are the same.

Imports System
Imports System.Collections
Imports System.Collections.Generic

Class Program
    Shared boxes As Dictionary(Of Box, [String])

    Public Shared Sub Main(ByVal args As String())

        Dim boxDim As New BoxSameDimensions()
        boxes = New Dictionary(Of Box, String)(boxDim)

        Console.WriteLine("Boxes equality by dimensions:")
        Dim redBox As New Box(8, 4, 8)
        Dim greenBox As New Box(8, 6, 8)
        Dim blueBox As New Box(8, 4, 8)
        Dim yellowBox As New Box(8, 8, 8)
        AddBox(redBox, "red")
        AddBox(greenBox, "green")
        AddBox(blueBox, "blue")
        AddBox(yellowBox, "yellow")

        Console.WriteLine()
        Console.WriteLine("Boxes equality by volume:")

        Dim boxVolume As New BoxSameVolume()
        boxes = New Dictionary(Of Box, String)(boxVolume)
        Dim pinkBox As New Box(8, 4, 8)
        Dim orangeBox As New Box(8, 6, 8)
        Dim purpleBox As New Box(4, 8, 8)
        Dim brownBox As New Box(8, 8, 4)
        AddBox(pinkBox, "pink")
        AddBox(orangeBox, "orange")
        AddBox(purpleBox, "purple")
        AddBox(brownBox, "brown")
    End Sub 

    Public Shared Sub AddBox(ByVal bx As Box, ByVal name As String)
        Try
            boxes.Add(bx, name)
            Console.WriteLine("Added {0}, Count = {1}, HashCode = {2}", _
                              name, boxes.Count.ToString(), bx.GetHashCode())
        Catch generatedExceptionName As ArgumentException
            Console.WriteLine("A box equal to {0} is already in the collection.", name)
        End Try 
    End Sub 
End Class 
Public Class Box
    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 
End Class 


Class BoxSameDimensions
    Inherits EqualityComparer(Of Box)

    Public Overloads Overrides Function Equals(ByVal b1 As Box, _
                                      ByVal b2 As Box) As Boolean 
        If b1.Height = b2.Height And b1.Length = b2.Length _
                                    And b1.Width = b2.Width Then 
            Return True 
        Else 
            Return False 
        End If 
    End Function 


    Public Overloads Overrides Function GetHashCode(ByVal bx As Box) As Integer 
        Dim hCode As Integer = bx.Height Xor bx.Length Xor bx.Width
        Return hCode.GetHashCode()
    End Function 

End Class 

Class BoxSameVolume
    Inherits EqualityComparer(Of Box)

    Public Overloads Overrides Function Equals(ByVal b1 As Box, _
                                          ByVal b2 As Box) As Boolean 
        If b1.Height * b1.Width * b1.Length = _
                                b2.Height * b2.Width * b2.Length Then 
            Return True 
        Else 
            Return False 
        End If 
    End Function 


    Public Overloads Overrides Function GetHashCode(ByVal bx As Box) As Integer 
        Dim hCode As Integer = bx.Height Xor bx.Length Xor bx.Width
        Return hCode.GetHashCode()
    End Function 

End Class 
' This example produces the following output: 
' *  
' Boxes equality by dimensions: 
' Added red, Count = 1, HashCode = 46104728 
' Added green, Count = 2, HashCode = 12289376 
' A box equal to blue is already in the collection. 
' Added yellow, Count = 3, HashCode = 55530882 

' Boxes equality by volume: 
' Added pink, Count = 1, HashCode = 30015890 
' Added orange, Count = 2, HashCode = 1707556 
' A box equal to purple is already in the collection. 
' A box equal to brown is already in the collection. 
' *  
'

System.Object
  System.Collections.Generic.EqualityComparer(Of T)

Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune

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

.NET Framework

Supported in: 3.5, 3.0, 2.0

.NET Compact Framework

Supported in: 3.5, 2.0

XNA Framework

Supported in: 3.0, 2.0, 1.0

Date

History

Reason

January 2010

Updated example.

Information enhancement.

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