tag does not exist in XML namespace

This may not be the solution for your problem, but I see the error you are asking about quite often, and the issue is generally caused by a problem loading the assembly.

If you check your assemble, and it has a yellow triangle attached to it, then it is likely not compatible with your current project setup, due do different versions of .NET, or something of the sort.

This will usually fail silently, or only cause a warning; when you try to reference the assemble in your XAML you will get an error like you described.

According to this article, you just do as follow:

FROM:

xmlns:ZZZ="clr-namespace:YYY;assembly=YYY"

TO:

xmlns:ZZZ="clr-namespace:YYY;assembly="

leave empty value for assembly=

This is the solution that works for me.

Looks like it was mostly a user error. But this could happen to others. When I copied the xmlns:moduleviewmodel definition from its original file I had to add the assembly= portion on my own. Like I said I both did it myself as well as use the autocomplete the follows from typing 'xmlns:moduleviewmodel=' . Right before we found the error we tried the autocomplete again because we found that there was one of the 7 namespaces not generating the error. It was then that I noticed that there was a letter in assembly path that was not capitalized that should be. The weird thing is the autocomplete actually inserts this error on its own. While we were compiling that I noticed the erroneous letter. The weirder thing is that after I fixed all the paths manually we tried the autocomplete again and it spelled it correctly.

I have no idea the cause of the errored autocomplete but with the fixed letter it compiles just fine.

Now I just wonder if anyone will believe the autocomplete was changing up on me!

Check Warnings in error list, in my case there where messages about unresolved .Net 4.5.1 references, while my project target framework was 4.5