2009-10-27

Almost equal

Had another interesting problem today. A test that really should have worked started failing most of the time. The test was designed to make sure that a certain DateTime value did not change on update under a special circumstance. It was designed something like this:

[TestMethod]
public void Updating_Foo_Will_Not_Update_ChangeDate()
{
    //Create a Foo to work with
    Foo foo = new Foo()
    {
        Name = "Update test foo",
    };

    foo = repository.SaveFoo( foo );
    DateTime lastChanged = foo.Changed.Value;

    foo.Bar = new Bar { Id = 1 };
    repository.SaveFoo( foo );

    var updatedFoo = repository.GetAllFoo().WithId( foo.Id );

    Assert.AreEqual( lastChanged, updatedFoo.Changed.Value );
}

This assertion, as you might imagine, was not fulfilled. Stepping through the code showed that the SaveFoo method in the repository did not change the value of foo.Changed, as is was not supposed do. It did, however save the foo to the database. On a first inspection, the two dates also seemed to be exactly the same, so the assertion should have been correct.

Or so I thought. When I looked closer at the dates, it turned out they weren’t exactly the same. Close, but not exactly. More specifically, the Ticks value of them differed. The value of lastChanged was 633922004866809617, while the value of updatedFoo.Changed.Value (that had made a round trip to the database) was 633922004866800000. Not a huge difference, less than 1 ms, but enough to make the test fail. How stupid I was, assuming that the precision of DateTime in .NET, and DateTime in SQL Server was the same! Further reading revealed that SQL Server has a DateTime precision of about 3.33 ms, while .NET has a precision of 100 ns. So everytime I saved the time to the database, it would change it!

The solution? Well, since I didn’t really care about differences of a few milliseconds, I decided to extend the DateTime struct with a brilliant new comparison method:

public static bool IsAlmostEqualTo( this DateTime dateTime1, DateTime dateTime2 )
{
    var diff = dateTime1 - dateTime2;
    return Math.Abs( diff.TotalMilliseconds ) <= 5;
}