Change IComparer interface argument so that it accepts nulls.

[davidroth]

Currently ISemVersionComparer and its concrete implementations implements IComparer<SemVersion> which forces one to use the bang operator when using it with some apis like the Linq Order APIs.

Current usage example:

SemVersion?[] versions = [null, SemVersion.Parse(1, 0), null, SemVersion.Parse(2, 0)];

var ordered = versions
.OrderByDescending(x => x, SemVersion.PrecedenceComparer); // Shows CS8620: because x is SemVersion? but the comparer contract documents as SemVersion only.

To get rid of the warning, one has to use the !bang operator to make the compiler happy like so:

.OrderByDescending(x => x, SemVersion.PrecedenceComparer!);

However, I think this is unncessary as the implementation rightly supports nullable comparison.

With this change, the nullability is also documented on the exposing interface to make usage in some APIS easier without any warnings.

[WalkerCodeRanger]

I certainly agree that the comparer accepts null and the bang operator should not be necessary. However, the handling of nullability of generics should mean you don't have to specify T? when implementing IComparer<T> (see this stack overflow answer). I'll have to look at what is going on a little more carefully. However, I would like to put out a release addressing this. Perhaps even a patch release since this is really the behavior I expected.

Notice that the IComparer<T>.Compare is declared with the signature public int Compare (T? x, T? y);.

[davidroth]

The problem is not with the IComparer<T>.Compare method, which is annotated to allow nulls as you mentioned.
The problem is that the nullability is not exposed on the implementation of the IComparer<T> and therefore it does not communicate this when flowing as arguments to other methods.

See also the dotnet runtime, where for example they also implement the StringComparer with IComparer<string?> to correctly advertise this on the implementation so that it can flow into other signatures without problems.