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Nov 06, 2008 08:33 PM|LINK
Are you adding your user control to the Page's control collection after loading it? If you don't do that, then it won't be rendered and won't have a ClientID or a UniqueID. Alternatively, you can try setting the TargetControID property with the user control
panel's ID and then add the CollapsiblePanelExtender and the user control to the Page Controls collection and see if it works.