Cap'n Proto 1.0(capnproto.org)

    // the framework's general channel type from which messages are read
    public interface IChannel
    {
        T Read<T>() where T : interface;
    }

    // clients declare the interface they operate on:
    public interface IClientFields
    {
        public int Foo { get; set; }
        public string? Name { get; set; }
    }
    ...
    // client middleware function
    Task MiddlewareFn(IChannel chan)
    {
        var client = chan.Read<IClientFields>();
        ... // do something with client before resuming at next stage
    }

    message FooRequest {
        required string query = 1;
        optional list<string> options = 2;   // added later
    }