If you are doing any multi-threading and are using MSTest, you will probably run across this error. For some reason, MSTest by default runs in STA threading mode. WTF, Microsoft! Why so stuck in the old COM world? When I run the same test using NUnit, I don't have this problem. Unfortunately, my company has chosen MSTest, so I have a lot of testing problems. NUnit is so much better, IMO.
After determining that I wasn't referencing any unmanaged code that would flip the thread into STA, which can also cause this error, the only thing left was the testing suite I was using. I dug around a little and found this obscure setting for the Test Run Config settings file that you can't set using its interface. You have to open it up as a text file and add the following setting. This is for VS2008.
<ExecutionThread apartmentState="MTA" />
This didn't break any other tests, so I'm not sure why STA is the default, or why there is nothing in the test run configuration app to change this setting.
Here is the code I was testing:
public void ProcessTest(ProcessInfo infos)
WaitHandle waits = new WaitHandle[infos.Length];
int i = 0;
foreach (ProcessInfo info in infos)
AutoResetEvent are = new AutoResetEvent(false);
info.Are = are;
waits[i++] = are;
Processor pr = new Processor();
WaitCallback callback = pr.Process;