Referring to another project in XAML: Undefined CLR namespace

I know that this may be a silly question, but have you included a reference to the other project in your visual studio project ?

Note that each project (C#, VB, etc.) creates its own assembly.

If it's in another assembly, you need to specify the assembly name:

xmlns:me="clr-namespace:MarkdownEditor;assembly=MarkdownEditor"

Note that generally each project in a solution creates a distinct assembly. And note that a project's name can be different than the assembly name. You can see the assembly name in the project's properties (in the application tab).

Also, be aware that a project's name can be different than the assembly name. You can see the assembly name in the project's properties (in the application tab).