Comment by jaen

These are not what are commonly called tagged unions. It's actually closer to an untagged union.

The C# union does not store any discriminator. Just look at the implementation - it's a single `object?` field.

The discriminative part is handled by run-time type information, which is stored in the object itself, not the union - which is why the C# built-in implementation requires boxing.

Also, you can use the same class/record type ("discriminator") in several different unions - again, a feature which ADTs/sum-types/tagged unions in most functional languages do not have.

You can even store one single object (ie. identical by reference equality) in several different union values at the same time, theoretically, which in combination with mutability is... uhh, certainly not common functional/mathematical semantics.