Question

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.

Answer 1

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:

• C# 4: Dynamic and Nullable<>

• C# Reflection: How to get the type of a Nullable<int>?

• Cannot change type to nullable in generic method

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.