csharp marshaller : emulate legacy serializer via contextual API, or provide ability to specify legacy *and* contextual serializer

[mgravell]

Right now when creating a Marshaller<T>, you can only specify either the legacy or contextual serializer. If you use the contextual API, it creates intentionally broken delegates for the legacy API:

// gRPC only uses contextual serializer/deserializer internally, so emulating the legacy
// (de)serializer is not necessary.
this.serializer = (msg) => { throw new NotImplementedException(); };
this.deserializer = (payload) => { throw new NotImplementedException(); };

This is fine for Grpc.Core, but breaks if another host wants to use the legacy API (possibly to avoid the "experimental" API surface).

What would be ideal is if we had another constructor, to allow the configuring code to specify both a contextual and legacy API; if the legacy API is omitted, then sure: omit them. By which I mean:

public Marshaller(
    Action<T, SerializationContext> serializer,
    Func<DeserializationContext, T> deserializer)
        : this(serializer, deserializer, null, null) {}
public Marshaller(
    Action<T, SerializationContext> contextualSerializer,
    Func<DeserializationContext, T> contextualDeserializer,
    Action<T, SerializationContext> legacySerializer,
    Func<DeserializationContext, T> legacyDeserializer)
{
    this.contextualSerializer = GrpcPreconditions.CheckNotNull(contextualSerializer, nameof(contextualSerializer));
    this.contextualDeserializer = GrpcPreconditions.CheckNotNull(contextualDeserializer, nameof(contextualDeserializer));
    // gRPC only uses contextual serializer/deserializer internally, so emulating the legacy
    // (de)serializer (when they are not provided by the caller) is not necessary.
    this.serializer = legacySerializer ?? ((msg) => { throw new NotImplementedException(); });
    this.deserializer = legacyDeserializer ?? ((payload) => { throw new NotImplementedException(); });
}

This would allow callers to supply "best and fallback" implementations, so that it can at least work on all hosts, even if it doesn't get the full advantages if the host only consumes the legacy API.

Happy to do a PR if it is viable.

[mgravell]

Another option of course being to emulate it with a spoof ctx; that should be pretty easy too - happy to help

[mgravell]

on reflection, I think emulation is the better option - then there's only one API for the caller to implement either way; added a PR

[jtattermusch]

I think the right solution is to switch grpc-dotnet to contextual serialization API which should solve the problems you're seeing
#19471 (comment)