Thx all for the reactions. I must say it's difficult for a beginning devloper to choose the good model. But as I said, I don't have a choice in this case. All things mentioned by you I indeed read in those links, but thanks for the help.
Gonzalo, my updatepanel is declared and use in my PageBase (a class library inheriting Page) in the WebUtilities Project, not in a page in the Web App project.
Originally I indeed tried to acces my custom classes in the Old_App_Code dir. And indeed it's just a dir like any other dir, the problem is that for some reason I can't acces those classes. Why? I don't know. So I wanted to put those classes in a seperate WebUtilities project, but that failes to its seems.
My method in this WebUtility class is:
public void CreateMessageAlertInUpdatePanel(UpdatePanel updatePanel, string message, string scriptKey)
System.Web.UI.ScriptManager.RegisterStartupScript(updatePanel, this.GetType(), scriptKey, "alert('" + message + "');", true);
And the compiler does not recognise this UpdatePanel, I asume because it's not a WebSite or WebApp with AJAX enabled but a simple library class project.
Thx for al response in advance!