`StringComparer` overloads for hash/dictionary types

[ZzZombo]

Is it intentional they do not support it?

[badeend]

Yes, this is intentional as it simplifies the implementation and public interface quite a bit.

It is mentioned in the docs: https://badeend.github.io/ValueCollections/#other-notable-features--omissions

Custom IComparer parameters are not supported. All operations use the type's Default(Equality)Comparer.

You can trivially work around this by creating a new wrapper type with the desired IEquatable behavior and using that as the key type. For example:

public readonly struct CaseInsensitiveString(string value) : IEquatable<CaseInsensitiveString>
{
  public string Value => value;

  public bool Equals(CaseInsensitiveString other) => Value.Equals(other.Value, StringComparison.OrdinalIgnoreCase);
  public override int GetHashCode() => Value.ToLowerInvariant().GetHashCode();

  // ...
}

Another benefit of this method is that: .NET has special logic to devirtualize calls to EqualityComparer<T>.Default.Equals(T, T). So it may even perform better than a constructor-provided IEqualityComparer<T> instance. YMMV.

In my personal experience, whenever I had the need to pass an IComparer instance to a collection, I've been better off using a domain specific type anyway.