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).