Propose Adding a GetOrAdd(TKey key, Func<TKey, TValue> valueFactory) to Dictionary<TKey, TValue>

[shmuelie]

On ConcurrentDictionary<TKey, TValue> there is the very useful function TValue GetOrAdd(TKey key, Func<TKey, TValie> valueFactory). The pattern that the method represents is one that issued commonly on the "normal" Dictionary<TKey, TValue> as well. I propose adding such a method for this common use case. Besides simplifying it will also make switching between the two dictionaries simpler.

[JonHanna]

Suggested at #14676 within the comment thread too. Possible implementation at https://github.com/hackcraft/coreclr/commit/1a982053ae39ee0e418d370fdee3497ecefd0e5a

[shmuelie]

That's what I get for thinking of an idea, never getting around to suggesting it. And then remembering months later and forgetting to check again if it had been suggested. Thanks!

[JonHanna]

Never mind, I do check for duplicates and I still end up asking something that's been asked before, or submitting a PR that does the same as another.

[jnm2]

I assumed this meant Dictionary.GetOrAdd was added, but I'm not finding it or an IDictionary extension method. If they have been skipped, can this issue be reopened?

[svick]

@jnm2 As far as I can tell, @SamuelEnglard closed this issue in favor of https://github.com/dotnet/corefx/issues/1942, which was about TryAdd, but also discussed GetOrAdd. But then https://github.com/dotnet/corefx/issues/1942 was closed after TryAdd was added.

So yes, I think it makes sense to repoen this issue.

[jnm2]

Ugh, I can't believe this missed .NET Standard 2.0. @karelz no exceptions?

[jnm2]

Specifically:

public static TValue GetOrAdd<TKey, TValue>(this IDictionary<TKey, TValue> dictionary, TKey key, TValue value)
{
    if (dictionary == null) throw new ArgumentNullException(nameof(dictionary));

    if (!dictionary.TryGetValue(key, out var r))
        dictionary.Add(key, r = value);
    return r;
}

public static TValue GetOrAdd<TKey, TValue>(this IDictionary<TKey, TValue> dictionary, TKey key, Func<TKey, TValue> valueFactory)
{
    if (dictionary == null) throw new ArgumentNullException(nameof(dictionary));
    if (valueFactory == null) throw new ArgumentNullException(nameof(valueFactory));

    if (!dictionary.TryGetValue(key, out var r))
        dictionary.Add(key, r = valueFactory.Invoke(key));
    return r;
}

[karelz]

This needs API review. I don't want to rush API reviews. That is just begging for troubles later - you miss things and then you live with your mistakes forever.

It is convenience API, we lived without it for quite some time, we can IMO survive one more release. It is not the end of the world after all and not the last .NET Core release. It does not block scenarios.

Probably not the answer you expected, but this is how I feel about it. We should IMO focus primarily on the really impactful things. Makes sense?

[jnm2]

It is indeed the answer I expected. 😄

[jnm2]

(Don't worry about me, I'm not the type that goes around second-guessing the team's decisions. Fyi.)

[karelz]

Thanks for understanding!
IMO it is not that much about decisions, but us being open about prioritization. To ship high-quality SW, we gotta prioritize. And we gotta stabilize.

[jamesqo]

@jnm2 I don't think it would be a good idea to add GetOrAdd as an extension method on IDictionary-- it would give the impression that only 1 lookup is being made. I think it's best just to make this proposal about Dictionary<K, V>.

@karelz This is not necessarily a convenience API. For Dictionary, this helps avoid duplicate hashtable lookups, and the last time I checked I think I saw around 50 non-test instances of

if (!dict.ContainsKey(key))
{
    dict.Add(key, ...);
}

in corefx.

[jnm2]

@jamesqo Yeah, okay. After the BCL has it for a while and we get default interface methods it might make more sense.