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