MSTest – ExpectedException and exception message.

Since this is my first blog post ill start out small enough.

Recently we have migrated our unit test from NUnit into MSTest. Although by large the transition was quite smooth, one of the more annoying issues is the different behavior of the ExpectedException attribute.

An intended design or an oversight by MS?

I would (wishfully) think the latter is the case. But in any case I would say its a Mistake.

In any case, since MsTest does not support asserting on the exception message, we had to revise some of the tests. The revision itself is quite simple, just need to wrap the test code with try catch clause. Something like:

[TestMethod]
public void SomeTest()
{
    try
    {
        //The test code:
        //...
        //...
        Assert.Fail("We should not get here");
    }
    catch(SomeExceptionType ex)
    {
        Assert.AreEqual("Expected Message",
            ex.Message);  
    }
}

However after doing this for a couple of tests we have managed to come up with a slightly better pattern:

[TestMethod]
[ExpectedException(typeof(SomeExceptionType))]
public void SomeTest()
{
    try
    {
        //The test code:
        //...
        //...
     
    }
    catch(SomeExceptionType ex)
    {
        Assert.AreEqual("Expected Message",
            ex.Message);
        throw;
    }
}

Although the difference is quite minor, still we found the second version much more expressive and readable.

To conclude, there’s a lesson to be learnt here. When trying to replace a well used framework its best to make sure that the transition will be as easy as possible (Unless you are MS and can get away with this).

Posted in: Team System,Unit Test Patterns