EqualityComparer(Of T) Class
Updated: January 2010
Provides a base class for implementations of the IEqualityComparer(Of T) generic interface.
Assembly: mscorlib (in mscorlib.dll)
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. ' * '
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.