IEqualityComparer(Of T) Interface
Updated: September 2009
Defines methods to support the comparison of objects for equality.
Assembly: mscorlib (in mscorlib.dll)
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
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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.