MSTest unit test passes by itself, fails when other tests are run

Question

I'm having trouble with some MSTest unit tests that pass when I run them individually but fail when I run the entire unit test class. The tests test some code that SLaks helped me with earlier, and he warned me what I was doing wasn't thread-safe. However, now my code is more complicated and I don't know how to go about making it thread-safe. Here's what I have:

public static class DLLConfig
{
    private static string _domain;

    public static string Domain
    {
        get
        {
            return _domain = AlwaysReadFromFile
                ? readCredentialFromFile(DOMAIN_TAG)
                : _domain ?? readCredentialFromFile(DOMAIN_TAG);
        }
    }
}

And my test is simple:

string expected = "the value I know exists in the file";
string actual = DLLConfig.Domain;
Assert.AreEqual(expected, actual);

When I run this test by itself, it passes. When I run it alongside all the other tests in the test class (which perform similar checks on different properties), actual is null and the test fails. I note this is not a problem with a property whose type is a custom Enum type; maybe I'm having this problem with the Domain property because it is a string? Or maybe it's a multi-threaded issue with how MSTest works?

Answer 1

I suspect the other tests are modifying some value in the DLLConfig class that is causing the result of the getter to change. Unit tests should always be run from a known initial state, so you should either set this up in the test method itself, or in a method marked with the TestInitialize attribute which is run before each test.