How To Get Classes To Expose The Same Shared/Static Method

Question

I have a base class with several derived classes. I want all of my derived classes to have the same Public Shared (static) method with their own implementation. How do I do this? Is it even possible?

Answer 1

No. You could use a singleton, but there may be a better design.

Answer 2

By definition, a static method cannot be declared as virtual. To achieve this without static methods you would do something like the following:

public abstract class BaseFoo
{
    protected virtual void Bar()
    {
    }
}

public class Foo : BaseFoo
{
    protected override void Bar()
    {
        base.Bar();
    }
}

If you wanted to achieve something similar to having it as static you could use the singleton pattern so that you share a single instance of the object.

Edit:

I realized your original question talked about VB syntax, so here's the same thing in VB:

Public MustInherit Class BaseFoo

    Protected Overridable Sub Bar()
    End Sub

End Class

Public Class Foo
    Inherits BaseFoo

    Protected Overrides Sub Bar()
        MyBase.Bar()
    End Sub

End Class

Answer 3

I know this seems like a useful feature "at design time" but imagine how this could be used:

If GetForms is Shared (static in C#) you don't have an object with a type to distinguish which method to use, i.e. you can't say BaseForm.GetForms() in some way that it can be determined which of ChildFormTypeA.GetForms() and ChildFormTypeB.GetForms() is actually called.

The best you can do is document this is the intention if you just want explicit calls to ChildFormTypeA.GetForms() and ChildFormTypeB.GetForms(), or if you really need the concept make it a factory method e.g.

MustInherit Class BaseForm

  MustOverride Sub GetForms()

End Class

Class MyForm
   Inherits BaseForm

   Public Sub New()
   End Sub

   Overrides Sub GetForms()
   End Sub   
End Class

Now you just need

Dim f as BaseForm = New MyForm
f.GetForms

Alternatively GetForms might be Protected and called from the Sub New.