Visual-studio – Element is not a known element warning in Visual Studio when using User controls

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I have a Visual Studio 2008 project that is showing the following warning when using User Controls, and I haven’t been able to find a solution anywhere.

Element <element>is not a known element

How can I fix this?

Best Solution

This sounds like a classic re-build your solution and "close and re-open Visual Studio" problem.

It's possible it may also be related to a similar problem I had which I answered at Resolving "Validation (): Element ‘xxxx’ is not supported" warning in Visual Studio 2005/2008.