var someUser = new { Name = "SideburnsOfDoom", CommentValue = 3 };
What type is `someUser` ? Not one that you can reference by name in code, it is "anonymous" in that regard. But the compiler knows the type.
A type can be given at compile-time in a declaration, or generated at compile-time by the compiler like this. But it is still "Compiler-verified" and not ad-hoc or at runtime.
the type (Dog, Cat) pet seems similar, it's known at compile-time and won't change. A type without a usable name is still a type.
Is this "ad-hoc"? It depends entirely on what you mean by that.
> Cat, Dog and Bird don't have to inherit from the union, you can declare a union of completely random types, as opposed to saying "Animal has three subtypes, no more, no less"
"Animal has three subtypes" is more like the c# "sealed" modifier on a class, meaning that subtyping is not allowed. Except in this case I guess for three existing subtypes.
(Dog, Cat) pet = new Cat();
So without defining the union with an explicit name beforehand.