I have a biztalk map which uses a custom XSLT file rather than the graphical mapper. This map makes use of methods held in a custom dll, accessing via a custom extension XML file.
I created this map a while ago, came back to it today (on a new vm) and found that I was unable to run a successful test - right click the btm in solution explorer and hit 'test map'. On trying this I was presented with a lengthy error message - the important bit being:
Value cannot be null.
Parameter name: extension
My first though was that the custom dll probably hadn't been gac'd on the new VM, so I remedied this. Still the problem persisted. The solution was to simply restart Visual Studio - it seems that VS must keep a cache of the gac'd dlls.