The tag 'XXX' does not exist in XML namespace 'clr-namespace:YYY'

Question

I have implemented a converter to convert Int32 to String to be able to binding a property to a textBox.

I implement this converter in the namespace MyApp.Converters and

Answer 1

There is one very obscure case when you get this error - if you're using Microsoft.mshtml.dll and you upgrade to Windows 10 anniversary edition, or as it turns out the Creator edition too.

Even if this isn't what happens to you make sure the library containing the component that cannot be found can be fully compiled. Look in the error list (not the grid but the Output console) for any clues.

In my case I was using Microsoft.mshtml.dll as part of an HTML editor and it was unregistered somehow (in the GAC).

The following article explains with screenshots:

https://techninotes.blogspot.com/2016/08/fixing-cannot-find-wrapper-assembly-for.html#comment-form

In short I had to run a Visual Studio command prompt (as admin) run these commands:

cd C:\Windows\assembly\GAC\Microsoft.mshtml\7.0.3300.0__b03f5f7f11d50a3a
regasm Microsoft.mshtml.dll

I did say obscure :-) The important point is to read the errors that may be buried amongst all your warnings because there might be an important clue.

This can be especially confusing if an Operating system update suddenly breaks your project.