Answer by Tarmil for What is a "sealed" type in F# and why doesn't the type test pattern operator (:?) work on it out of the `box`?

Sealed types are a .NET concept. They are types that cannot be inherited from. This includes:

• F# records
• F# unions
• F# classes declared with the [<Sealed>] attribute
• C# classes declared with the sealed keyword

Now, the :? operator tests whether a value is an instance of a given type. For example, if a variable a has statically known type T, and U is a subtype of T, then a :? U checks whether a is type U. It's false if a is a concrete T or another subtype of T.

But if T is sealed, then it has no subtype. So the expression a :? U is trivially false. Unless U is T or a supertype of T, in which case it's trivially true. Either way, it's a useless test. So the F# compiler assumes you made a mistake.

The expression box a, on the other hand, upcasts a to type obj. This type is not sealed (obviously, since all other types inherit from it). So there's nothing wrong with using :? on it.