You won't be able to leverage C#'s pattern-matching effectively with a library.

Really though, you don't need a library to do sum-types in C#:

It's best to just use records for now:

    public abstract record Maybe<A>
    {
        private Maybe() { }

        public sealed record Just(A Value) : Maybe<A>;
        public sealed record Nothing : Maybe<A>;
    }

The private constructor and sealed case-types stops anybody else deriving from `Maybe<A>`, so the type is effectively closed and finite.

You can then construct like so:

    var mx = new Maybe<int>.Just(123);
    var my = new Maybe<int>.Nothing();

Then you can use the pattern-matching:

   var r = mx switch
   {
      Maybe<int>.Just (var x) => x,
      Maybe<int>.Nothing      => 0
   };

Of course, we don't get exhaustiveness checking, that'll have to wait for the proper sum-types (although the compiler does some pretty good work on this right now); but until then this is the most expressive and powerful way of doing sum-types in C#.

And, if you want a load of pre-built ones, then my library language-ext will help [1]

[1] https://github.com/louthy/language-ext/