TryGetValue - Null or Value

[TonyValenti]

How do I nullable-annotate this:

        public static TValue? TryGetValue2<TKey, TValue>(this IReadOnlyDictionary<TKey, TValue>? This, TKey? Key, TValue? Default = default) {
            var ret = Default;

            if (This is { } && Key is { } && This.TryGetValue(Key, out var NewRet)) {
                ret = NewRet;
            }


            return ret;
        }

So that I can call it using any combination of struct/class parameters.

Ie:

            var V1 = new Dictionary<long, object>();
            var V2 = V1.ToImmutableDictionary();

            var V3 = new Dictionary<object, long>();
            var V4 = V3.ToImmutableDictionary();

            var T1 = V1.TryGetValue2(0, new object());
            var T2 = V1.TryGetValue2(0, null);
            var T3 = V1.TryGetValue2(null, new object());
            var T4 = V1.TryGetValue2(null, null);

            var T5 = V2.TryGetValue2(new object(), 1);
            var T6 = V2.TryGetValue2(new object(), null);
            var T7 = V2.TryGetValue2(null, 1);
            var T8 = V2.TryGetValue2(null, null);

[huoyaoyuan]

[return: NotNullIfNotNull("Default")] should be appropriate?

[huoyaoyuan]

Well, it is not. When TValue is nullable and a null value is found, in this case it will be wrong.

[TonyValenti]

@huoyaoyuan Any other thoughts? I feel like this should be a really easy method to build but I just can't figure it out.

[huoyaoyuan]

Well, the logic is still too complex for attribute.

[333fred]

You simply cannot write such a method. There is no constraint today that allows you to make a T be substituted with the nullable version of whatever type you substitute, regardless of whether it's a reference type or a value type.