[adrian_b]

The word "discriminated" by itself does not specify this property.

"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.