Display Message box from ASP.NET Webform

I am looking to find some code that will allow me to display a message box when called from an asp.net webform.  I would like to be able have the code in a class module where i have all of my other generic functions.  Then from whereever i what to display the message I just call the function and pass the text that i want to appear as part of the message.

I have found one example that does what i want but when i call it the current web page is blank while the message box is displayed.

I also found the following code in an article here in the experts exchange but i cant seem to get it to work when i call it from a class module.  It would if the function is part of the webform but when i try and call it from a class module it get the following error:

"Object reference not set to an instance of an object"

Here is the code that i am using:

Public Sub Display_Message(ByVal strMsg As String)
 
  Dim strScript As String
  strScript = "<script language=JavaScript>"
  strScript &= "alert(strMsg);"
  strScript &= "</script>"
  If (Not Page.IsStartupScriptRegistered("clientScript")) Then
      Page.RegisterStartupScript("clientScript", strScript)
  End If

Again, this works if the function is part of the page but i want to be able to call it from a class module.

Does any body have any good examples of displaying a message box or any ideas why the above code does not work?