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Nullable Reference Types

Starting with version 3.0 support for nullable reference types (NRTs) is enabled.

Support for NRTs is limited in this library because it uses generics extensively, and generics don't play nicely with NRTs.

Since match expressions can work will null, their return types are marked as such. Match results may also contain null values. This may be inconvenient if you know that null values are not used as match results.

In addition, extensions in Matchmaker.Linq also don't fully support NRTs. Look at the (simplified) implementation of the Select extension for match results:

public static MatchResult<TResult> Select<T, TResult>(this MatchResult<T> result, Func<T, TResult> mapper) =>
    result.IsSuccessful ? MatchResult.Success(mapper(result.Value!)) : MatchResult.Failure<TResult>();

Here's the interesting part: mapper(result.Value!). There is no way to tell C# that mapper can receive null inputs, because T is generic and we cannot use T?.

Since extensions for patterns use extensions for match results, this also applies to extensions for patterns. There is no way to tell C# that functions passed to those extensions can receive null, but they can, and this should be accounted for.

When using this library with NRTs enabled, remember two things:

  • MatchResult<T> may contain null and so patterns can return null as their results as well
  • Match expressions' results may be null

Next article: Migration guide