Using C#'s ref features on (dictionary) indexers

Question

I was wondering whether it is possible to use C#'s ref return on (dictionary) indexers or properties which define a set and get accessor, e.g.:

readonly Dictionary<string, int> dictionary = ...;


ref int v = ref dictionary["foo"];
//          ^^^^^^^^^^^^^^^^^^^^^
// CS0206: A property or indexer may not be passed as an out or ref parameter
v = 42;

Is it possible to somehow provide ref functionality to properties or indexers (without using reflection)? If so, how?

---

I know, that the error message is clear in that sense - however, I was wondering which would be the optimal way to implement its semantics.

Answer 1

This requires the type implementing the indexer to provider ref-return in the indexer, so no: you can't use it with Dictionary<string, int>. But with something like this:

class MyRefDictionary<TKey, TValue>
{
    public ref TValue this[TKey key]
    {   // not shown; an implementation that allows ref access
        get => throw new NotImplementedException();
    }
}

You could indeed do:

ref var val = ref dictionary[key];

Note that arrays are a special case, as arrays have always allowed ref indexer access, i.e.

SomeMethod(ref arr[42]);

(the indexer access in arrays is implemented by the compiler, not the type)