Where Should Documentation Go? Or “Is DateTime Broken?”

“Where should my documentation go?” may seem like an odd question to ask, but where you put your documentation defines how it is consumed and where you train your Developers and Users to look for it when troubleshooting problems.

In the VisualStudio & .NET world, I find more often than not, our team goes to Intellisense first for documentation (and I suspect that is the most common practice in other teams as well).

We’ll put summary tags explaining parameters, expected behavior, etc. over commonly used key functions for the convenience of new developers and maintainers, and later generate XML documentation from the tags for storage and distribution.

Here’s an example of one of the more detailed summary tags we have for our extension method, DistinctBy:

/// <summary>
/// Returns distinct elements from a sequence using the provided function to compare values.
/// </summary>
/// <typeparam name="TSource">The type of the elements of source.</typeparam>
/// <typeparam name="TKey">The type of the key used to compare elements.</typeparam>
/// <param name="source">The sequence to remove duplicate elements from.</param>
/// <param name="keySelector">A function to select the key for determining equality between elements.</param>
/// <returns>An IEnumerable&lt;T&gt; that contains distinct elements from
/// the source sequence.</returns>
public static IEnumerable<TSource> DistinctBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)

Now that we’ve established where our team (and I suspect other developers) go first for documentation, I’d like to present a small program I wrote that caused the team a lot of grief two weeks ago:

static void Main(string[] args)
    var now = DateTime.MinValue;
    var nowPlusASecond = now.AddSeconds(1);
    var nowPlusOneSecondAndABit = now.AddSeconds(1.0001);
    if (nowPlusASecond.Ticks == nowPlusOneSecondAndABit.Ticks)
        Console.WriteLine("Do you think we will get here?");
        Console.WriteLine("Apparently these DateTimes refer to the same instant");
    Console.WriteLine("Press 'Enter' to exit");

The output of this program surprised us:

Do you think we will get here?
Apparently these DateTimes refer to the same instant
Press 'Enter' to exit

Wait… what?

Those two times should be different. Granted, they wont be different by much, but they should be demonstrably different on a Ticks level (since a Tick represents one ten millionth of a second). For applications that assume time resolution on a Tick level, this is a big deal.

Confronted with this problem, we visited the Intellisense for AddSeconds and found the following:

This seems fairly reasonable and our team was scratching our heads trying to determine if we had discovered some bug in .NET (spoiler… we hadn’t), or if there was just some key piece of information we were missing from the documentation. At this point, we turned to our second-tier source for documentation: the MSDN. The MSDN’s reference on AddSeconds, and found the following (emphasis mine):

This method does not change the value of this DateTime. Instead, it returns a new DateTime whose value is the result of this operation.
The fractional part of value is the fractional part of a second. For example, 4.5 is equivalent to 4 seconds, 500 milliseconds, and 0 ticks.
The value parameter is rounded to the nearest millisecond.

This is a key piece of information that would be, in my opinion, good information to have in the Intellisense. By having the type of the parameter be double it implies to the developer full double precision, when in fact, it is rounding to three decimal places.

The Takeaway
If you think your API violates the Principle of Least Astonishment, not only should you document it, but you should ensure it is documented in the place your Developers are most likely to look first in addition to your primary source of documentation.