Todays gotcha: Windows 7 assembly blocking

Yesterday I was playing with our new build server, trying to get it to run my fully-functional unit tests with code coverage analysis. Why that didn’t work is a different story, but in the progress of changing project settings and checking in updates something strange happened. Suddenly my local web site stopped working.

The error message

My local copy of the site with the web root set to my working folder for the project suddenly refused to start, prompting me with the yellow screen of death and this cryptic error message:

(In the example above, I removed all the lines except the one where it indicated failure, which was the first line of my AutoMapper bootstrapper.)

Troubleshooting

I started troubleshooting blindly, rolling back the project, cleaning, rebuilding, checking that all dependencies were copied on build etc. etc., but all looked fine. I then checked the trust settings of IIS in case they had been messed up. No luck there either.

After some time googling I decided to follow a vague tip about checking the properties of all assemblies that are references or other dependences. BOOM – at the very bottom of the properties dialog of the AutoMapper assembly was this message:

This file came from another computer and might be blocked to help protect this computer.

Why on earth Windows had suddenly decided that the assembly came from another computer (and what does that mean, don’t most assemblies come from other computers?) I had no Idea about. But luckily it left me the option to unblock the assembly. After doing so, and after deleting the copy in the bin folder letting the rebuild take a fresh copy the unblocked assembly and finally restarting IIS, the site came back to life.