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

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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>>)

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