Question

I am trying to work out a relatively clean way of comparing two types of DateTime? objects without having to bother with lots of null checking.

Is something like the following a good idea or is there a much simpler or sensible way to go about it;

DateTime? test = DateTime.MinValue;
DateTime? date1 = new DateTime(2008, 6, 1, 7, 47, 0);

if (string.Format("{0:MM/dd/yyyy}", date1) == string.Format("{0:MM/dd/yyyy}", test))
{
    Console.WriteLine("They are the same");
}
else
{
     Console.WriteLine("They are different");
}

Answer 1

No, using text formatting to compare simple values is not a good idea IMO. One option to make things neater would be to write an extension method to effectively propagate nulls (like a maybe monad):

public static Nullable<TResult> Select<TSource, TResult>
    (this Nullable<TSource> input, Func<TSource, TResult> projection)
    where TSource : struct
    where TResult : struct
{
    return input == null ? (Nullable<TResult>) null : projection(input.Value);
}

You can then compare projections of nullable types easily:

if (date1.Select(x => x.Date).Equals(date2.Select(x => x.Date))

Or even add your own equality projection method:

public static bool EqualsBy<TSource, TResult>
    (this Nullable<TSource> x, Nullable<TSource> y,
     Func<TSource, TResult> projection)
    where TSource : struct
    where TResult : struct
{
    return x.Select(projection).Equals(y.Select(projection));
}

And:

if (date1.EqualsBy(date2, x => x.Date))

This is much more flexible and elegant than performing a text conversion when you really don’t care about the text.