Well, you can do this in c#:

  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.