There's also an issue with the ShadowCache (used for rendering designer) getting out of sync with your XAML in VS2012. I just found this out, as I was having a horrible time with the following error with derived user controls:

The local property XXXX can only be applied to types that are derived from YYYYY.

The shadow cache is located in your user profile directory:

app data\Local\Microsoft\VisualStudio\11.0\Designer\ShadowCache

You can delete everything in there and restart VS2012. After that, things worked much better (for at least a while!)

Delete everything in ShadowCache

C:\Users\username\AppData\Local\Microsoft\VisualStudio\11.0\Designer\ShadowCache\

Copy-pasteable paths

Just copy/paste one of these paths into Windows Explorer. %LOCALAPPDATA% is a Windows Environment Variable it will be replaced with the corresponding value when entered into Explorer (also works in Command Prompt)

VS2012

%LOCALAPPDATA%\Microsoft\VisualStudio\11.0\Designer\ShadowCache\

VS2013

%LOCALAPPDATA%\Microsoft\VisualStudio\12.0\Designer\ShadowCache\

VS2015

%LOCALAPPDATA%\Microsoft\VisualStudio\14.0\Designer\ShadowCache\

ASP.NET

If you develop ASP.NET also delete

%LOCALAPPDATA%\Microsoft\WebsiteCache

%LOCALAPPDATA%\Temp\VWDWebCache

I'm not sure this will fix all cases but this is what worked for me (all the time now).

I have VS2012, using the Blend designer in the IDE, building an x64 application, when I would define a new ICommand and add the Command attribute, I would get the designer isn't supported in x64 error (I forget the exact warning). If I switch to x86, I'd get invalid XAML markup errors complaining the the ICommand wasn't recognized or was not accessible, no matter how many times I rebuilt in x64. If I switch to x86, build, then switch back to x64, voila, not problems with the designer.

Now I recall similar issues with VS2008 and Blend version 3 and 4. From what I've read in other posts, using Any CPU might also resolve the issue.