Use IReadonlyDictionary<TKey, TValue> instead of IEnumerable<KeyValuePair<TKey, TValue>>

I’m proposing that we change methods that use IEnumerable<KeyValuePair<TKey,TValue>> to IReadonlyDictionary<TKey,TValue>. I believe that using IReadonlyDictionary maintains the semantics of an immutable map but is easier to work with e.g. we can use indexer map[key] and also access Count property.

If agreed we can make the following changes:

interface ISpanContext {
 IReadonlyDictionary<string, string> BaggageItems { get; }
}

interface ISpan
{
  Log(IReadonlyDictionary<string, string> fields);
  Log(DateTimeOffset timestamp, IReadonlyDictinary<string, string> fields)
}

Using IReadonlyDictionary<TKey,TValue> simplifies implementations e.g.

1. Duplicate key behavior is undefined for Log(IEnumerable) so using IReadOnlyDictionary averts that behavior
2. In the Log(IEnumerable) it’s simpler to check for semantic fields e.g. Brave library checks for event key

[gideonkorir]

Maybe we also change string ISpan.GetBaggageItem(string key) to bool ISpan.TryGetBaggageItem(string key, out string value). This makes explicit that the key could be missing and doesn’t require the client to check for nulls.

[gideonkorir]

@ndrwrbgs whilst the specification doesn’t explicitly tell us how to handle duplicate keys, I think that the ISpan.GetBaggageItem & ISpan.SetTag implicitly expect single keys in the baggage items otherwise the 1st API should return IEnumerable<string> and the 2nd API should be ISpan.AddTag.

The idea about the un-serialized baggage item is interesting but one could make an implementation of IReadOnlyDictionary that deserializes on access to any of it’s APIs. Also we expect the client to have as few baggage items as possible; I think the simplicity of using IReadOnlyDictionary far outweighs the deserialization costs.

Your comment about null is spot on 😃, if we agreed to change baggage items to IReadOnlyDictionary I’d probably change ISpanContext to:

public interface ISpanContext
{
  IReadOnlyDictionary<string,string> BaggageItems { get; }
}

The readonly property has the same semantics as the currently defined method.