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IEqualityComparer(Of T) Interface

Defines methods to support the comparison of objects for equality.

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

'Declaration
Public Interface IEqualityComparer(Of In T)

Type Parameters

In T

The type of objects to compare.

This type parameter is contravariant. That is, you can use either the type you specified or any type that is less derived. For more information about covariance and contravariance, see Covariance and Contravariance in Generics.

The IEqualityComparer(Of T) type exposes the following members.

  NameDescription
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryEqualsDetermines whether the specified objects are equal.
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryGetHashCodeReturns a hash code for the specified object.
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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


.NET Framework

Supported in: 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

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

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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