Today I created a simple Silver Light 4 application and copied a XAML and couple of classes from my existing project. Changed all the references of the old name spaces to new name space and compile it and I got this error. It took me 10 minutes to figure out what was the problem. Simple, I missed one namespace which was still pointing to the old namespace. So if you run into this error message, most probabily you missed one or more name space references to old name space. It seems some other people run into the same problem, if you do not have ‘CompileXAML’ option not set in the MSBuild.