Header.cs

public interface IHeader
{
    string Value { get; }
    IEnumerable<string> Values { get; }
}

public sealed class Header : IHeader
{
    readonly IEnumerable<string> values;
    readonly string value;

    public Header(IEnumerable<string> values)
    {
        this.values = values.ToArray();
        value = this.values.FirstOrDefault() ?? string.Empty;
    }

    public string Value
    {
        get { return value; }
    }

    public IEnumerable<string> Values
    {
        get { return values; }
    }
}

It could be that trying to wedge my idea into a generic container is a bad idea. I'll write some code and try some ideas out. It could be I go back to IDictionary<string, IEnumerable<string>> or it could be I end up at a specialized IHeaderCollection.

I'm against being reliant on extension methods for completely standard interactions. They aren't discoverable if you don't have the right namespace imported and force you to use methods rather than properties or indexes.

Thanks for your feedback by the way, it's much appreciated.