In C#, all instances have a class, so there is already a discriminant, the class itself.
In the article, the example with the switch works because it switches on the class of the instance.
In the article, the example with the switch works because it switches on the class of the instance.
The fact that they flatten a union of optional types into an optional union of the corresponding non-optional types is indeed a weird feature, which I do not like, because I think that a union must preserve the structural hierarchy of the united types, e.g. a union of unions must be different from a union of all types included in the component unions, and the same for a union of optional types, where an optional type is equivalent with a union between the void/null type and the non-optional type, but this C# behavior still does not make the C# unions anything else but discriminated unions, even if with a peculiar feature.
This is exactly the difference between simple union types and discriminated unions. This c# feature is what typescript has, not what Haskell/java/f#, etc.
"Discriminated" just means that at run time you can discriminate the values of a union type by their current type, so you can use them correctly in expressions that expect one of the component types.
I agree that the right implementation of discriminated types is that mentioned by you and which is that of many important programming languages, but even if I disapprove of this property that the C# unions have, which in my opinion may lead to unexpected behavior that can cause subtle bugs, the C# unions are still discriminated unions, where you can discriminate the current type at run-time, with a "switch".
In my opinion, one should avoid this weird behavior of C#, by always defining only unions of non-optional types. Where needed, one should then define an optional type having as base a union type. Then these unions will behave like the discriminated unions of other languages.
Whether you use or not this policy, there are types that the C# unions cannot express, but if you use this policy, at least the limitations become explicit.