使用IEqualityComparer时未调用Custom Equals函数

Question

我为代表数据库中对象的类创建了一个IEqualityComparer，我想重写这些类的Equals方法，以便在调用Equals、Contains等时比较对象的Id。问题是我的Equals函数从未被调用，有人能理解为什么测试总是返回false吗？

谢谢！

DatabaseEntityEqualityComparer.vb

Imports System.Collections.Generic
Public Class DatabaseEntityEqualityComparer
    Implements IEqualityComparer(Of IDatabaseEntity)
    Public Overloads Function Equals(x As IDatabaseEntity, y As IDatabaseEntity) As Boolean Implements IEqualityComparer(Of IDatabaseEntity).Equals
        'check whether the compared objects reference the same data
        If x Is y Then Return True
        'check whether any of the compared objects is null
        If x Is Nothing OrElse y Is Nothing Then Return False
        'check whether the IDatabaseEntity properties are equal, and are of the same type
        Return (x.Id = y.Id) AndAlso (x.GetType() = y.GetType())
    End Function
    Public Overloads Function GetHashCode(obj As IDatabaseEntity) As Integer Implements IEqualityComparer(Of IDatabaseEntity).GetHashCode
        If obj Is Nothing Then
            Return 0
        End If
        Return obj.Id.ToString().GetHashCode()
    End Function
End Class

IDatabaseEntity.vb

Public Interface IDatabaseEntity
    Property Id As Integer
End Interface

Person.vb

Public Class Person
    Implements IDatabaseEntity
    Public Property Id As Integer Implements IDatabaseEntity.Id
End Class

运行测试

Dim p1 As New Person()
p1.Id = 1
Dim p2 As New Person()
p2.Id = 1
If p1.Equals(p2) Then
    Response.Write("Equals")            
Else
    Response.Write("Not Equals")
End If
Response.End()

Answer 1

IEqualityComparer是一个允许创建自定义euqality比较器的接口。例如，您可以在许多LINQ方法中使用它，如Intersect、Except、GroupBy、Join等。但您必须将比较器的实例传递给这些方法。

如果要更改覆盖Equals和GetHashCode的一般情况：

Public Class Person
    Implements IDatabaseEntity
    Public Property Id As Integer Implements IDatabaseEntity.Id
    Public Overrides Function Equals(obj As Object) As Boolean
        If obj Is Nothing Then Return False
        If Object.ReferenceEquals(Me, obj) Then Return True
        Dim Person2 As Person = TryCast(obj, Person)
        If Person2 Is Nothing Then Return False
        Return Person2.Id = Id
    End Function
    Public Overrides Function GetHashCode() As Integer
        Return Id
    End Function
End Class

如果你想使用比较器，你可以这样使用它的Equals：

Dim comparer = New DatabaseEntityEqualityComparer()
If comparer.Equals(p1, p2) Then
    Response.Write("Equals")   
End If