Have you tried
Private Delegate Function MessageBoxHandler(ByVal Message As String, ByVal Style As MessageBoxIcon) As DialogResult
Private Function ShowMessageBox(ByVal Message As String, ByVal Style As MessageBoxIcon) As DialogResult
Return MessageBox.Show(Message, My.Application.Info.AssemblyName, MessageBoxButtons.YesNoCancel, Style)
End Function
...
aa = Me.ShowMessageBox("a", MessageBoxIcon.Question)