Sunday, May 17, 2009

Cast null values in tests

by Patrik Hägne via Legend and truth on 5/17/2009 10:45:00 PM

My first tests of classes are often tests that asserts that the constructor throws ArgumentNullException for null arguments, something like this:

[Test]
public void constructor_throws_when_fileSystem_is_null()
{
    Assert.Throws<ArgumentNullException>(() =>
        {
            new TransactionalFileSystem((IFileSystem)null, this.transactionManager);
        });
}

As you see I explicitly cast the null value to the type of the parameter in the constructor signature, this is benefitial for two reasons:

1. It reads better, the reader know what it is that is NOT passed in.
2. Several times in the past I have refactored and reordered the parameters in the constructor with the result that the tests test the wrong parameters this is avoided this way.