I understand well the benefit of option, but in this case, I want to avoid using option for performance reasons. option wraps a type in a class, which just means more work for the garbage collector -- and I want to avoid that.
In this case especially, I have multiple fields that are all Some under the same circumstances, but I don't want to put them in a tuple because, again, tuples are classes -- and puts additional stress on the GC. So I end up accessing field.Value -- which defeats the purpose of option.
So unless there's an optimization I don't know about that causes option types to be treated as references that are potentially null, I want to just use null. Is there a way that I can do that?
Edit: To expand on what I'm doing, I'm making a bounding volume hierarchy, which is really a binary tree with data only at the leaf nodes. I'm implementing it as a class rather than as a discriminated union because keeping the items immutable isn't an option for performance reasons, and discriminated unions can't have mutable members, only refs -- again, adding to GC pressure.
As silly as it is in a functional language, I may just end up doing each node type as an inheritance of a Node parent type. Downcasting isn't exactly the fastest operation, but as far as XNA and WP7 are concerned, almost anything is better than angering the GC.
According to this MSDN documentation, if you decorate your type with the [<AllowNullLiteral>] attribute, you can then call Unchecked.defaultof<T>() to build a null for you.
That seems to be the only way within F# to do what you want. Otherwise, you could marshall out to another .net language and get nulls from there... but I'm guessing that is not what you want at all