[Proposal] Allow custom per-property equality for records

[sab39]

Proposal: For a record type R which has a positional property Prop of type T, if R also has a static property or field PropEquality whose type implements IEqualityComparer<T>, the generated Equals and GetHashCode methods for R will use that equality comparer for Prop.

public record Name(string FirstName, string LastName)
{
    private static readonly StringComparer FirstNameEquality = StringComparer.OrdinalIgnoreCase;
    private static readonly StringComparer LastNameEquality = StringComparer.OrdinalIgnoreCase;
}

In much the same way that Collection Expression Arguments were motivated by not wanting collection expressions to fall off an ergonomic cliff if a developer needs to pass a comparer to a dictionary, this would remove the current ergonomic cliff that exists if a developer wants to customize the equality behavior of a single property within a record.

Intentionally, this approach would be easy to build on using source generators to improve the ergonomics further, eg:

public record Name(
    [ValueEquality.OrdinalIgnoreCase] string FirstName,
    [ValueEquality.OrdinalIgnoreCase] string LastName);

This proposal is inspired by Collections in records should be compared with value equality instead of reference equality, but isn't limited to collections and doesn't propose a breaking change to default behavior. With this proposal, you could get value equality of collections in records by using an EqualityComparer type based on Enumerable.SequenceEqual (which I'm surprised doesn't seem to exist in the BCL today, but could be added, or is easy enough to write by hand).

[333fred]

Our position in the past is that this is too much customization for a language feature, and that this should be done as a source generator. Personally, my opinion hasn't changed.

[HaloFour]

I'd think this entire proposal as written could be accomplished by a source generator. I wonder if there are modest modifications the language could offer to lower the barrier to this customization, especially for simpler cases where a source generator would be overkill, like emitting private partial declarations. Then again, a general purpose source generator could do that too.