I have a UserControl -called MFD- which gives me a good size window, complete with areas, TreeView, DataGrid, and lots of buttons. So, I can provide the user with a consistent, repetitive UI, And it works very well, When it works. which is actually a random experience, likely to be unsucessful half of the time. Any change to the code (C# or XAML) is likely to make the error come and go at will (not mine). The error is that the name "MFD" is not recognized in the context (sometimes the namespace is mentioned, sometimes, not.) This is the line which defines the namespace: xmlns:UserControl="clr-namespace:CNV_V2_PresentationLayer" This is the line which loads the MFD user control. When the xaml parser remembers to. Since the MFD UC is in a folder beneath CNV_V2_PresentationLayer, should the UC folder be included in the namespace path? I have tried both and it the IDE did not care. Vexing. Error codes are: XDG0008 and MC3074. They both describe the same situation.