Visual Studio 2015 InvalidProgramException in Unit Test With Fakes

Question

I am using Visual Studio 2015 Enterprise RTM to write unit tests for a project that uses Unity Container.

I have discovered that the simple act of adding a fakes asse

Answer 1

The only general solution is to make sure that all parts match the version of CLR you are using very closely and that VS has the latest updates.

There is no magic bullet for this problem. You need to know (dig out) the exact CLR version compatibility of all parts that are connected in your project when you inject fakes. Mind you that "compatibility" might be just the matter of manifest but more frequently they are the matter of nuances of how was/is the final code generated and for which version of the virtual machine.

These things normally don't matter for running and debugging since there are several layers that make sure that minor version differences either don't matter or you get a silent switch to whatever your code is declared to be compatible with.

But, when you use Fakes, the "system" injects raw code into your (which includes 3rd party libraries involved) and that means that it skips most checks - couldn't work otherwise. But, when the time comes to actually run the code, the engine (virtual machine) has to do some checks for it's own safety/integrity and it tends to get paranoid and bail out if it looks like declarations don't match closely enough.

This is the reason why someone asked whether assemblies involved are strongnamed or signed. That's the only level of guarantee that "system" will really trust. Without that it will do a degree of guessing and while it mostly doesn't matter for normal runs if matters a lot for code injection.

I'm still not talking possible real problems - this is all assuming that actual code is fine and just declarations are confused. You could try to play with that but it would take a lot of time and effort. Much easier to go checking if you can get versions of assemblies that are better matching.

The fact that errors vanished when you switched your flavor back to 4.5 tells you that either some of the assemblies involved are not "close enough" for 4.6 or there can be some glitches with code injection that were fixed by updates you haven't taken in yet.

Yes it involves a lot of pain but that's the price of wanting to be at the frontier.