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Bad sorting and other pitfalls
While tracing a bug, I ended up with the following minimum reproduction:
The error you’ll get is:
Unable to sort because the IComparer.Compare() method returns inconsistent results. Either a value does not compare equal to itself, or one value repeatedly compared to another value yields different results.
The nasty thing about this is that if we had just 16 items in the array, this code would work. So this would appear to successfully work most times, and then break.
The underlying issue is that Array.Sort will use different sorting algorithms based on the size of the array to be sorted. Under 16 items, it’ll use an insertion sort, but over that, an introspection sort will be used (up to a limit, and then heap sort. Go read the code.).
What is key here is that our comparison function is broken. It doesn’t understand that two values can be equal. Because of that, comparing two equal values result in both of them being smaller than one another. That cause an error, and .NET issues this error. When you know what went wrong, the fix is pretty easy:
Now we properly handle this scenario, and everything will work.
Comments
I don't think your fix works...
retis a boolean, it can't be equal to zero.The proper fix would be
Array.Sort(s, (a, b) => DateTime.Parse(a).CompareTo(DateTime.Parse(b))). Or even simpler,Array.Sort(s), since ISO8601 is sortable alphabetically (well, except for the timezone part... it would only works if they're all in the same timezone)Thomas, Correct, I fixed the code.
While technically the code works, I would probably avoid parsing dates twice...
Sometimes you can’t. I faced a very similar issue recently and the issue popped up with a javascript map reduce building a multidimensional pre aggregated result set. In the reduce it faces duplicate date dimensions,
I would have written the code like this
Pop Catalin,
If I did it that way, it would be much harder to show the issue :-)
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