pop up window in class file(.cs)

I have a webform project with a  classe file in the project.
 In  classe file  I want to have a popup to appear if a catch statement occurs, however none of the two below code solutions work.
 Catch ex As Exception
   MessageBox.Show(ex.Message, "Name Error", MessageBoxButtons.OK, MessageBoxIcon.Stop)
   Page.RegisterStartupScript("startupScript", "<script language=JavaScript>alert('No Name listed. Check spelling and try again.');</script>")
End Try
 or  
System.Windows.Forms.MessageBox.Show("Here's a message!");
 Thanks for all help.