Odd, VB or C#? On build with C# I get :
Unknown build error, ' 'clr-namespace:Whatever; assembly=Junk' mapping URI is not valid. Line 3 Position 9.'
Also, stupidly enough, if I remove the assembly reference from the project the compiler is smart enough to flag up an additional error: "Assembly 'Junk' was not found" which implies that it *does* know we're looking for that assembly in the XAML after all :-{

That said, and although I feel your pain, I must say I've probably added hundreds of xmlns: attributes without hitting that problem - but I'm the lazy type who waits for the dropdown to be sure I'm selecting the right namespace....

>>I get a compile error with VS2010 / .NET 4. What version are you using?
>
>VS2010 / .NET 4. No compile errors whatsoever.
>> .
>>>Although I would have much rather been at the beachfront bonfire party with my peeps, I had to get a relatively simple Silverlight 4.0 LOB project done. So I stayed home, unhappily typing XAML like this:
>>>
>>>xmlns:df="clr-namespace:System.Windows.Controls; assembly=System.Windows.Controls.Data.DataForm.Toolkit"
>>>
>>>I build the project. No errors! Great!
>>>
>>>And then I run it. Winding...winding...winding... Ahh!!! Now what? I get a XamlParseException:
>>>
>>>The type 'DataForm' was not found because 'clr-namespace:System.Windows.Controls; assembly=System.Windows.Controls.Data.DataForm.Toolkit' is an unknown namespace. [Line: 15 Position: 34]
>>>
>>>Since my VS help doesn't work, like, AT ALL, I go to Google (which I would probably do anyway, given the relatively unhelpful ways of Microsoft Help.) However, after 10 minutes of googling I still don't find any good answers to this particular problem.
>>>
>>>But the error is clearly in that line. Postion 34 starts with "ols:"
>>>
>>>What's wrong with "ols"? What's wrong with me? What's wrong with Microsoft? What's WRONG with this line:
>>>
>>>xmlns:df="clr-namespace:System.Windows.Controls; assembly=System.Windows.Controls.Data.DataForm.Toolkit"
>>>
>>>For an answer, see my next message.