We were having a problem with a BizTalk assembly (containing only maps) having a “project” reference to a standard C# library.  As soon as the project reference was added, we were unable to open the maps – errors along the lines of “unable to open source schema” and “unexpected error encountered…vsee\internal\inc\vscomptr.inl”.  See here for a good forum discussion on the issue that we came across.

It turns out this issue crops up if you have a VSTS unit testing project in the same solution…How is that for a strange product interaction?  Possible workarounds include removing the unit testing project (and putting it in a separate solution) or using a file reference instead of a project reference.  (If you go the latter route, go to the properties of your solution and set up the project build dependencies manually to account for this!)

We opted for file references rather than removing our unit test assembly, at least for the time being.  Hopefully this will be addressed soon…