Create a shared IEqualityComparer - vb.net

Question

I'm doing some LINQ which requires a custom comparer, so I created a new class implementing IEqualityComparer. However, when I use it, I have to create an instance of it each time.
Dim oldListOnly = oldList.Except(newList, New MyEqualityComparer)
Dim newListOnly = newList.Except(oldList, New MyEqualityComparer)
I may be misunderstanding how .NET works, but it seems wasteful to create a new comparer each time. I really just want one instance (the equivalent of static in C++/C#).
So I tried creating a "static" class, which in vb.net is a module. But got an 'Implements' not valid in Modules error.
I then tried making the Equals and GetHashCode function shared methods on my class, but got this error: Methods that implement interface members cannot be declared 'Shared'.
Any ideas how to accomplish my goal here? Or am I simply misunderstanding what's going behind the scenes?

Answer 1

Your understanding is correct, although the waste is unlikely to be noticeable. For your situation, you could use the singleton pattern, which usually goes something like this:
Public Class MyEqualityComparer
Implements IEqualityComparer(Of whatever)
Private Sub New()
'no outsider creation
End Sub
Private Shared ReadOnly _instance As New MyEqualityComparer()
Public Shared ReadOnly Property Instance As MyEqualityComparer
Get
Return _instance
End Get
End Property
'other code
End Class

Answer 2

Why not simply do
Dim comparer = New MyEqualityComparer
Dim oldListOnly = oldList.Except(newList, comparer )
Dim newListOnly = newList.Except(oldList, comparer )

Answer 3

There needs to be an instance of a concrete type that implements IEqualityComparer. What you can do with a module, however, is define a public instance which is initialized to "New EqualityComparer". You can then pass that default instance to the Except method.
Something like:
Public Module MyComparer
Public acmeComparer As acmeCompareType
Public Class acmeCompareType
Implements IEqualityComparer(Of System.Drawing.Point)
Public Function Equals1(x As System.Drawing.Point, y As System.Drawing.Point) As Boolean Implements System.Collections.Generic.IEqualityComparer(Of System.Drawing.Point).Equals
Return Math.Abs(x.X) = Math.Abs(y.X) AndAlso Math.Abs(x.Y) = Math.Abs(y.Y)
End Function
Public Function GetHashCode1(obj As System.Drawing.Point) As Integer Implements System.Collections.Generic.IEqualityComparer(Of System.Drawing.Point).GetHashCode
' Note that obj is a struct passed by value, so we can safely modify it here
' (without affecting the caller's instance)
obj.X = Math.Abs(obj.X)
obj.Y = Math.Abs(obj.Y)
Return obj.GetHashCode
End Function
End Class
End Module