In Nullable micro-optimizations, part one, Eric mentions that Nullable<T> has a strange boxing behaviour that could not be achieved by a similar user-defined type.
What are the special features that the C# language grants to the predefined Nullable<T> type? Especially the ones that could not be made to work on a MyNullable type.
Of course, Nullable<T> has special syntactic sugar T?, but my question is more about semantics.
What I was getting at is: there is no such thing as a boxed nullable. When you box an int, you get a reference to a boxed int. When you box an int?, you get either a null reference or a reference to a boxed int. You never get a boxed int?.
You can easily make your own Optional<T> struct, but you can't implement a struct that has that boxing behaviour. Nullable<T>'s special behaviour is baked into the runtime.
This fact leads to a number of oddities. For example:
And FYI there are other ways in which the Nullable<T> type is "magical". For instance, though it is a struct type, it does not satisfy the struct constraint. There's no way for you to make your own struct that has that property.