Why does this unit test fail when testing DateTime equality?

Question

Using NUnit 2.2 on .NET 3.5, the following test fails when using DateTime.Equals. Why?

[TestFixture]
public class AttributeValueModelTest
{
    public class HasDate
    {
        public DateTime? DateValue
        {
            get
            {
                DateTime value;
                return DateTime.TryParse(ObjectValue.ToString(), out value) ? value : new DateTime?();
            }
        }

        public object ObjectValue { get; set; }
    }

    [Test]
    public void TwoDates()
    {
        DateTime actual = DateTime.Now;
        var date = new HasDate {ObjectValue = actual};
        Assert.IsTrue(date.DateValue.Value.Equals(actual));
    }
}

Answer 1

The dates aren't equal. TryParse drops some ticks. Compare the Tick values.

For one test run:

Console.WriteLine(date.DateValue.Value.Ticks);
Console.WriteLine(actual.Ticks);

Yields:

633646934930000000
633646934936763185

Answer 2

The problem isn't really TryParse, but actually ToString().

A DateTime object starts with precision (if not accuracy) down to millionth of seconds. ToString() convertsit into a string, with precision only to a second.

TryParse is doing the best it can with what it is given.

If you add a format specifier (along the lines of "yyyy-MM-dd HH:mm:ss.ffffff"), it should work.

Answer 3

To specify a format that includes all the precision, you can use the String.Format() method. The example that James gives would look like this:

String.Format("{0:yyyy-MM-dd HH:mm:ss.ffffff}", ObjectValue);

I don't know what that will do when you pass it something that's not a date.

Perhaps a simpler approach is to add a special case when you've already got a date object:

    public DateTime? DateValue
    {
        get
        {
            DateTime value = ObjectValue as DateTime;
            if (value != null) return value;
            return DateTime.TryParse(ObjectValue.ToString(), out value) ? value : new DateTime?();
        }
    }