Invalid Markup XAML Designer When Inheriting From Base Class in Separate Assembly I'm trying to create a series of WpfUserControls in Visual Studio.  I have a base class called BindableBase that takes care of all the boiler plate INotifyPropertyChanged stuff that resides in a "Core" library.  All my ViewModels inherit from BindableBase so when I attempt to set the data context in XAML:
<UserControl.DataContext>
    <local:MyViewModel />
</UserControl.DataContext>

I get the Designer "Invalid Markup" error.  When I inspect my code I'm being told that The name "MyViewModel" does not exist in the namespace [UI Namespace].  I know this isn't true because I can build and run the project just fine with no problems, it's just the design time features that I am missing.
I've tracked the issue down to my BindableBase class that is in a separate assembly.  If I move it to the assembly with my ViewModel and View everything works great, but when I move it back to the "Core" namespace design-time features break.
Is there a way to get around this issue?  Moving my BindableBase class is not an option because we have multiple UI assemblies that all have to reference that class.

I have this problem too sometimes. I generally try Cleaning and Rebuilding my solution after restarting and reopening it. If this fails, I delete the problematic code, save and then restart Visual Studio and reopen the solution and retype the code, followed by a Clean and Rebuild if necessary.