Please introduce a reusable implementation of ILookup

[weitzhandler]

Hi,

The Lookup class' constructor is private, and its Create methods are internal.
I'd like to request to make the Create methods in lookup public, and/or introduce a public/protected constructor to it, so we can benefit of having custom Lookup implementations.

Alternatively (preferable option), please provide a comprehensive solution to deal with mutable multi-value dictionaries (i.e. single key with multiple values).
The MultiValueDictionary in the corefxlab repo, implements IReadOnlyDictionary, which is immutable. I'd prefer a permanent editable multi-value dictionary.
Here's a shallow way of how I achieve this, but it'll only work if the keys are immutable (just like KeyedCollection itself will).

Gist

private abstract class Lookup<TKey, TElement> : KeyedCollection<TKey, ICollection<TElement>>
{
  protected override TKey GetKeyForItem(ICollection<TElement> item) =>
    item
    .Select(b => GetKeyForItem(b))
    .Distinct()
    .SingleOrDefault();

  protected abstract TKey GetKeyForItem(TElement item);

  public void Add(TElement item)
  {
    var key = GetKeyForItem(item);
    if (Dictionary != null && Dictionary.TryGetValue(key, out var collection))
      collection.Add(item);
    else
      Add(new List<TElement> { item });
  }

  public void Remove(TElement item)
  {
    var key = GetKeyForItem(item);
    if (Dictionary != null && Dictionary.TryGetValue(key, out var collection))
    {
      collection.Remove(item);
      if (collection.Count == 0)
        Remove(key);
    }
  }
}

[danmoseley]

@weitzhandler hello, please format your proposal as per example linked here https://github.com/dotnet/corefx/blob/master/Documentation/project-docs/api-review-process.md

[weitzhandler]

@danmosemsft updated

[danmoseley]

@weitzhandler please present the API in reference format similar to Proposed API in https://github.com/dotnet/corefx/issues/271. Also it's good to break out Motivation, and Potential Alternatives. In some cases Performance is also worth breaking out. You already have an Example of use which is good.
I know it's paperwork but it makes it much easier to formally review to have sections organized.

[danmoseley]

You can copy/paste relevant part of https://github.com/dotnet/corefx/blob/master/src/System.Linq/ref/System.Linq.cs and add in your API.

[weitzhandler]

I'm sorry but I currently don't have the time to put it all up like that.
My motive was indeed to simply allow reusing the Lookup code rather than having to copy paste it.
I'm gonna close this issue.

Thanks for your effort @danmosemsft.