VB.net による Interface の最小構成

Public Class Form1

    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        Dim objClass As IntShwoFile
        Static Counter As Integer
        Select Case Counter
            Case 2
                Counter = 3
                objClass = New ClsB
            Case 3
                Counter = 1
                objClass = New ClsC
            Case Else
                Counter = 2
                objClass = New ClsA
            End Select
            objClass.ShowMessage("abc")
    End Sub

End Class

Public Interface IntShwoFile

    Sub ShowMessage(MessageString As String)

End Interface

Public Class ClsA
    Implements IntShwoFile

    Private Sub IntShwoFile_ShowMessage(MessageString As String) Implements

        IntShwoFile.ShowMessage
        MessageBox.Show("A:" & MessageString)
    End Sub
End Class

Public Class ClsB
    Implements IntShwoFile

    Private Sub IntShwoFile_ShowMessage(MessageString As String) Implements

        IntShwoFile.ShowMessage
        MessageBox.Show("B:" & MessageString)
    End Sub
End Class

Public Class ClsC
    Implements IntShwoFile

    Private Sub IntShwoFile_ShowMessage(MessageString As String) Implements

        IntShwoFile.ShowMessage
        MessageBox.Show("C:" & MessageString)
    End Sub
End Class