Sandbox Solutions and the Update Panel

There are already several articles on the web​ relating to the use of the ASP.NET UpdatePanel in sandbox solutions, so to add to the mix, here's another one.

Sandbox solutions and the UpdatePanel: the truth of it is that you cannot use the UpdatePanel in sandbox solutions. If you try to use it you get an error stating "The control with ID 'UpdatePanel1' requests a ScriptManager on the page". Using the ScriptManager.GetCurrent returns null. If you try to add your own you get an error as you can only have one ScriptManager instance at a time. You can add your own, however, by adding it as the first control in the Form.Controls control tree, but even this will most likely break the page unless you duplicate the exact scripts you may be adding to the page in code - the symptom of this is the ScriptResource.axd that would normally contains your scripts returns with a 404.

That aside, the reason the ScriptManager.GetCurrent doesn't return the actual ScriptManager you have added to your page or master page is to do with the way sandbox solutions replace some objects with its own object types. For instance, the System.Web.UI.Page object is replaced by the SPUserCodePage object.

The question for me is why is the ScriptManager not available? Why not allow the use of the UpdatePanel in a sandbox?