how are nullable types implemented under the hood in .net?

Question

In our own Jon Skeet's C# in depth, he discusses the 3 ways to simulate a 'null' for value types:

• Magic value (e.g. earliest possible DateTime is taken to be 'null')
• Reference type wrapper
• boolean flag

It is mentioned that nullable types use the third method. How exactly do nullable types work under the hood?

Answer 1

Ultimately, they are just a generic struct with a bool flag - except with special boxing rules. Because structs are (by default) initialized to zero, the bool defaults to false (no value):

public struct Nullable<T> where T : struct {     private readonly T value;     private readonly bool hasValue;     public Nullable(T value) {         this.value = value;         hasValue = true;     }     public T Value {         get {            if(!hasValue) throw some exception ;-p            return value;         }     }     public T GetValueOrDefault() { return value; }     public bool HasValue {get {return hasValue;}}     public static explicit operator T(Nullable<T> value) {         return value.Value; }     public static implicit operator Nullable<T>(T value) {         return new Nullable<T>(value); } } 

Extra differences, though:

• special boxing rules (you can't normally do this)
• special C# rules for comparing to null etc
• "lifted" operators in C# (and in .NET via EqualityComparer<T>, Comparer<T> etc)
• special rules on generic type constraints (to prevent Nullable<Nullable<T>>)