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IEqualityComparer(Of T) Interface

Updated: September 2009

Defines methods to support the comparison of objects for equality.

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

'Declaration
Public Interface IEqualityComparer(Of T)
'Usage
Dim instance As IEqualityComparer(Of T)

Type Parameters

T

The type of objects to compare.

This interface allows the implementation of customized equality comparison for collections. That is, you can create your own definition of equality for type T, and specify that this definition be used with a collection type that accepts the IEqualityComparer(Of T) generic interface. In the .NET Framework, constructors of the Dictionary(Of TKey, TValue) generic collection type accept this interface.

A default implementation of this interface is provided by the Default property of the EqualityComparer(Of T) generic class. The StringComparer class implements IEqualityComparer(Of T) of type String.

This interface supports only equality comparisons. Customization of comparisons for sorting and ordering is provided by the IComparer(Of T) generic interface.

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 adds custom Box objects to a dictionary collection. The Box objects are considered equal if their dimensions are the same.

Imports System
Imports System.Collections.Generic
Class Example
    Public Shared Sub Main()
        Try 

            Dim boxEqC As New BoxEqualityComparer()

            Dim boxes As Dictionary(Of Box, [String]) = _
                        New Dictionary(Of Box, String)(boxEqC)

            Dim redBox As New Box(4, 3, 4)
            Dim blueBox As New Box(4, 3, 4)

            boxes.Add(redBox, "red")
            boxes.Add(blueBox, "blue")

        Catch argEx As ArgumentException
            Console.WriteLine(argEx.Message)
        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 BoxEqualityComparer
    Implements IEqualityComparer(Of Box)

    Public Overloads Function Equals(ByVal b1 As Box, ByVal b2 As Box) _
                   As Boolean Implements IEqualityComparer(Of Box).Equals

        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 Function GetHashCode(ByVal bx As Box) _
                As Integer Implements IEqualityComparer(Of Box).GetHashCode
        Dim hCode As Integer = bx.Height Xor bx.Length Xor bx.Width
        Return hCode.GetHashCode()
    End Function 

End Class

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

September 2009

Added example and revised remarks.

Information enhancement.

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