Why does F# compiler check `this != null`?

Asked Aug 03 '18 at 08:51

Active Aug 03 '18 at 08:57

Viewed 61 times

F# compiler does a lot of work for us. By default it implements IComparable<T>.

However, the code is embarrassing

type Either = | This | That

Yields:

[CompilerGenerated]
public sealed override int CompareTo(Either obj)
{
    if (this != null)
    {
        if (obj != null)
        {
            int tag = _tag;
            int tag2 = obj._tag;
            if (tag == tag2)
            {
                return 0;
            }
            return tag - tag2;
        }
        return 1;
    }
    if (obj != null)
    {
        return -1;
    }
    return 0;
}

Why is the check this != null here?!

asked Aug 03 '18 at 08:51

Pavel Voronin

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This is answered (for the case of IEquatable) here https://stackoverflow.com/a/8097472/59371 – marklam Aug 03 '18 at 09:44
@marklam Funny, but it looks like reflection does not allow invoking instance methods against `null` target. – Pavel Voronin Aug 03 '18 at 10:04
You can see exactly why that check is there if you try something like this: `(None :> IComparable).CompareTo(Some ())` – Aaron M. Eshbach Aug 03 '18 at 12:37
@AaronM.Eshbach It's not quite valid example. After cast to interface F# emits `callvirt`. – Pavel Voronin Aug 03 '18 at 13:00

Why does F# compiler check `this != null`?

0 Answers0