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

I don't know if this is the same in .NET, but in Java the equals often will only compare if the instances are the same, not if the values are the same. You'd instead want to use compareTo.

Answer 2

public DateTime? DateValue
        {
            get
            {
                DateTime value;
                bool isDate = DateTime.TryParse(ObjectValue.ToString(), out value); 
                return isDate ? new DateTime?(value) : new DateTime?();
            }
        }

Answer 3

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 4

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 5

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?();
        }
    }