VerificationException During Coverage? Check Your Security Attributes

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Ran into this yesterday and took a little bit to figure out:

I can run unit tests through NUnit or TestDriven.NET just fine… but if I run those same tests throughNCover, I get a System.Security.VerificationException - “Operation could destabilize the runtime.”

I searched around and found that the exception comes up basically when the JIT can’t verify type safety of an assembly or when it tries to run something in medium trust that should be running in full trust. I also found another person who ran into something similar.

Turns out I had marked my assembly with AllowPartiallyTrustedCallersAttribute (for various reasons I won’t get into) and that was causing problems when NCover tried to instrument it. Removing the attribute fixed the issue and I could get coverage running.

Long term that’s probably not the best solution and I need to learn more about the new .NET 4 security model to figure out what really needs to happen, but if you’re seeing a VerificationException when you run tests under coverage but you don’t see that exception when you run without coverage… check your security attributes.