[API Proposal]: AlternateLookup.TryAdd overload with actualKey output + similar CollectionsMarshal overloads

Open
#112,554 9 comments 5 reactions 0 assignees View on GitHub

Nobody has claimed this yet.

Assessment

Difficulty
5/5
Estimated time
Over a week
Newbie friendliness
42/100
Issue type
Feature
Clarity
Mostly clear
Activity status
Quiet
Tech stack
csharp
Domain
api, backend-api-design

Research direction

Start with the proposed Dictionary<TKey,TValue>.AlternateLookup.TryAdd overload and the two CollectionsMarshal signatures. Compare their requested behavior—returning the existing or newly added actual key—and resolve the nullable-annotation question for GetValueRefOrNullRef. Done means an agreed API shape for all three proposals.

Written by the indexing model from the issue text.

Description

api-suggestion area-System.Collections help wanted
Background and motivation

I am finding myself in situations where I need the actual key after a call to TryAdd, which currently requires me to either do an additional subsequent lookup or additional allocation to create it again using the alternate comparer. It would be nice if TryAdd had an overload that output the actual key (existing key if it exists, otherwise the new key it creates).

API Proposal
partial class Dictionary<TKey, TValue>
{
    partial struct AlternateLookup<TAlternateKey>
    {
        public bool TryAdd(TAlternateKey key, TValue value, out TKey actualKey);
    }
}

partial class CollectionsMarshal
{
    public static ref TValue? GetValueRefOrAddDefault<TKey,TValue,TAlternateKey>(
        System.Collections.Generic.Dictionary<TKey,TValue>.AlternateLookup<TAlternateKey> dictionary,
        TAlternateKey key,
        out TKey actualKey,
        out bool exists);

    // Not 100% sure about this one, the nullable annotation on the actual key is a bit wonky
    // but I don't think there's anything that can be done about that
    public static ref TValue GetValueRefOrNullRef<TKey,TValue,TAlternateKey>(
        System.Collections.Generic.Dictionary<TKey,TValue>.AlternateLookup<TAlternateKey> dictionary,
        TAlternateKey key,
        out TKey? actualKey);
}
API Usage
ReadOnlySpan<char> key = ...;
int value = ...;

if (altLookup.TryAdd(key, value, out string actualKey)
{
    Console.WriteLine($"Added '{actualKey}'.");
}
else
{
    Console.WriteLine($"Key '{actualKey}' already exists.");
}
Alternative Designs

No response

Risks

None

Dominant language
C#
Stars
18.3k
Forks
5.6k
PR metrics pending

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

More from dotnet/runtime

All issues in dotnet/runtime

Similar issues

More C# issues

Get new issues in your inbox

A short digest of beginner-friendly GitHub issues.