Why does CollectionMarshal.GetValueRefOrAddDefault returns a ref nullable TValue?

Question

The other available helper that is:

ref TValue GetValueRefOrNullRef<TKey, TValue>(Dictionary<TKey, TValue> dictionary, TKey key) where TKey : notnull

Can return a null ref (Unsafe.NullRef<TValue>()) but the value itself is what it is (since TValue is not constrained).

But:

ref TValue? GetValueRefOrAddDefault<TKey, TValue>(Dictionary<TKey, TValue> dictionary, TKey key, out bool exists) where TKey : notnull

Returns a TValue?. (And the reference itself cannot be null.)

There SHOULD be something I'm missing here and if anyone can explain, it'll be great!

Answer 1

I realized that this is required to accomodate reference types. Since the helper adds the default(TValue), the result may obviously be a null, regardless of the TValue parameter nullability. And this applies also to nullable value types!

(My mind was focused on value types).

This leads to a funny couple of signature where the OrDefault equivalent is not nullable (because it's a OrNullRef) and the OrAdd is nullable...