asp.net - Resolving "Validation (): Element ‘xxxx’ is not supported" warning in Visual Studio 2005/2008

Question

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.

Answer 1

Delete the files from “C:Documents and Settings[Username]Application DataMicrosoftVisualStudio9.0ReflectedSchemas” folder (or “…VisualStudio8.0…” if running Visual Studio 2005) in Windows XP. In Windows 7 it is under "C:Users{User Profile}AppDataRoamingMicrosoft...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: http://forums.asp.net/t/1205528.aspx http://blogs.msdn.com/mikhailarkhipov/archive/2005/04/21/410557.aspx

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.