That will allocate for any constructed Either though. F#'s Result and ValueOption are value-types (structs), and value-type variants recently added support for sharing fields between variants when the name and type match.

Yes, that's the limitation until the value-type DUs arrive in C# 15.

In previous versions of language-ext, I defined Either as a struct with bespoke Match methods to pattern-match. But once pattern-matching appeared in C# proper, it didn't make sense to keep the struct type.