VS 2010 Test Runner error "The agent process was stopped while the test was running."

This message is caused by an exception on a thread different from the executing test thread. All answers so far boil down to this simple explanation. It is a known bug in Visual Studio not to display any sensible information in that case.

Visual Studio’s test runner totally chokes if a thread other than the executing test thread throws an exception: It gets swallowed and there’s no output, no chance to intercept and debug and no nothing except a burned-down smoldering mess that was supposed to be your unit test.

I've just experienced the similar problem: some tests fail and they are different in different test runs. I don't know exactly the reason why it happens, but it began to occur when I added a finalizer to one of my classes. When I disable the finalizer - the problem disappears. When I turn the finalizer on - the problem comes back.

Right now I don't know how to overcome this.

I was having this problem, and it turned out to be a problem in my code which the Test Framework wasn't catching properly. A little accidental refactoring had left me with this code:

public void GetThingy()
{
    this.GetThingy();
}

This is of course an infinite recursion, and caused a StackOverflowException (I guess). What this caused was the dreaded: "The agent process was stopped while the test was running."

A quick code inspection showed me the problem, and my tests are now running fine. Hope this helps - might be worth inspecting the code looking for issues, or maybe extracting a bit into a console app and checking it works properly there.