When “Could not load file or assembly” happens

So I was getting an ” Could not Load File or Assembly” error, even thou I was pretty certain I was referencing the right dlls in the correct folders. What could this error possibly mean?
The actual error I had was:
System.IO.FileLoadException: Could not load file or assembly ‘Moq, Version=2.6.1014.1, Culture=neutral, PublicKeyToken=69f491c39445e920’ or one of its dependencies.
Some things I checked:
  • Maybe  I dont have the moq.dll assembly, but it was there.
  • Then I checked  the assembly version, my moq.dll was 4.x (the error stated 2.6.x was required), so there is a difference in the expected versions, why?
  • Checked all projects in the solution, all the ones using Moq.dll where explicitly pointing at 4.x
  • Reflector show time. I was using moq.contrib.dll on the project that was failing, and since it makes sense that such library would depend on moq, it was worth checking it out, this is what I saw

Reflector Screenshot

This means that moq.contrib.dll did indeed depend on moq, so I just needed to find a version of it that pointed to the right assembly, I was lucky because I had a set of dlls that work, if you are not so lucky, get latest from the moq.contrib google code and simply run the Build.cmd and you are all set to go (you can replace the moq.dll dependecy if they have moved on).
On the way I found some other interesting things
  • Like this thread, basically it said that this was possibly a compiler bug. I thought damn a dead end, but it wasn’t :).
  • A tool to find out how the dll binding happen callled FusLogw.exe
Also known as the Assembly Binder Log Viewer, awesome right? 😀
Where is it?  I searched online and the tool seems to move a lot in differnt versions of Windows, In xp
it could be here: C:\Program Files\Microsoft SDKs\Windows\v6.0A\bin\fuslogvw.exe
In my windows 7 install I found it here: C:\Program Files (x86)\Microsoft SDKs\Windows\v7.0A\bin>FUSLOGVW.exe
Oh yeah and you need to run it from the command line as an Administrator
It might seem obvious, but you need to refresh after you run something that fails , it is, after all a log viewer

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