Form cannot refer to itself through its default instance on shared call

Question

Consider the following code, in a brand new WinForms .NET 4.0 application, with default settings:

Public Class Form1
  Private Sub AAA()
    Form1.AAA(Nothing) 'cannot refer to itself through its default instance; use 'Me' instead.
  End Sub

  Private Shared Sub AAA(str As String)
  End Sub
End Class

I am getting this error:

{FORM_CLASS_NAME} cannot refer to itself through its default instance; use 'Me' instead.

I also get this warning at the same line:

Access of shared member, constant member, enum member or nested type through an instance; qualifying expression will not be evaluated.

Assuming default instance is meant here, it ends up in an infinite loop - VS suggests to change Me.AAA() to Form1.AAA(), and then back. AAA() works in both.

Converting Private Sub AAA() to Shared solves the error. It seems like from Microsoft's point of view, all overloads must be shared, if at least one is. Or you get this default instance confusion. Why?

To clarify, I do not want to use default instance here, just do a shared call.

If anyone encountered the same situation, please advise.

Answer 1

Creating a variable alias that has the same name as the type of the Form class is without a doubt the single most disastrous VB.NET problem. But it was necessary to give VB6 developers a fighting chance to move to VB.NET.

The workaround is to stop trying to be explicit about what method you want to call. This compiles fine and is unambiguous, at least in your snippet:

  Private Sub AAA()
       AAA(Nothing)       '' fine
  End Sub

If that really, really hurts then simply swapping the two methods removes the ambiguity:

Private Shared Sub AAA(str As String)
End Sub

Private Sub AAA()
    Form1.AAA(Nothing)    '' fine
End Sub

Answer 2

Can you get away with this? Your usage will be very similar Form1.AAA() vs. code.AAA().

Public Class Form1

    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        code.AAA()
    End Sub

    Private Class code
        Public Shared Sub AAA()
        End Sub
    End Class

End Class

Answer 3

EDIT

Given the new information in the OP - another solution to your issue may be to use optional parameters -- ie :

Private Shared Sub AAA(Optional ByVal str As String = Nothing)

Also - the resolution works out in the "right" way if you simply change the ordering of the declarations -- this avoids the compiler error:

Private Shared Sub AAA(ByVal str As String)
End Sub

Private Sub AAA()
    Form1.AAA(Nothing)
End Sub

--

Keeping this below because it can be helpful in other circumstances

Perhaps your larger application did something like this - VB is full of messes like this you can get yourself into. This will compile but it will crash :

Public Class Form1

    Private Shared Sub AAA()
        Form1.Text = "this"
    End Sub

    Private Sub Label1_TextChanged(sender As System.Object,  _
                                             e As System.EventArgs) _
                                             Handles Label1.TextChanged
        Form1.AAA()
    End Sub

End Class

Just the same, this actually is "fine" (I use the term loosely)...

Public Class Form1
    Private Shared dont As Boolean = True

    Private Sub Form1_Load(sender As System.Object, e As System.EventArgs) _
                                                      Handles MyBase.Load
        dont = False
    End Sub

    Private Shared Sub AAA()
        If Not dont Then Form1.Text = "this"
    End Sub

    Private Sub Label1_TextChanged(sender As System.Object, _
                                               e As System.EventArgs) _
                                               Handles Label1.TextChanged
        Form1.AAA()
    End Sub

End Class

This is because the text changed handler will fire before Form1 completes loading (ie : during InitializeComponent()!) and will refer to the default instance which is not yet finished being created - so VB tries to create a new one for you so that you can call the shared method which spins you down the infinite loop.

Oddly, the Load handler is "fine" (again, loosely) to call Form1.AAA() in - as in your opening code - because the default instance (Form1 the instance of Form1 the Class) is finished creation at that point and another won't be created to satisfy the call. Any other code path, however, that starts in the shared call and ultimately ends up touching any instance data, no matter how torturous the path, will loop around and crash.

See also : Why is there a default instance of every form in VB.Net but not in C#?

Answer 4

Unclear what you are trying to accomplish overall. In the OP Form1.AAA should be just AAA.

Private Sub AAA()
    AAA(Nothing)
End Sub
Private Sub AAA(str As String)
    If str IsNot Nothing Then MsgBox(str) ' else ???
End Sub
Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click
    AAA()
    AAA("hello")
End Sub