Over the last couple of days I started getting the following validation warning (green squiggly line at design time) in a Visual Studio 2008 ASP.NET WebForms project:
Validation (): Element ‘xxxx’ is not supported.
Where 'xxxx' was a standard ASP.NET server control (asp:panel, asp:container, asp:textbox)
The application would still compile and run but the warnings were an annoyance.
See my answer for the resolution I found.
Delete the files from the C:\Documents and Settings\[Username]\Application Data\Microsoft\VisualStudio\9.0\ReflectedSchemas folder (or …\VisualStudio\8.0\… if running Visual Studio 2005) in Windows XP. In Windows 7 it is under C:\Users\{User Profile}\AppData\Roaming\Microsoft\...etc. Remember also the "VisualStudio" part of the path will be different depending on the version installed.
I closed Visual Studio (always a good idea for changes that will affect the IDE), deleted the files then re-opened the project. The warnings were gone.
I found references to this solution at:
A quick Google search found the solution. FYI, the search term I used in Google was “element is not supported”.
I don't know why this happens but I do know there are some flaky domain profile things happening in the network environment.