protobuf的-CSHARP端口

Question

I'm using Jon Skeet's (excellent) port of Google's Protocol Buffers to C#/.Net.

For practice, I have written a dummy Instant Messenger app that sends some messages down a socket. I have a message definition as follows:-

message InstantMessage {<br/>
  required string Message = 1;<br/>
  required int64 TimeStampTicks = 2; <br/>
}

When the sender serialises the message, it sends it really elegantly:-

        ...
        InstantMessage.Builder imBuild = new InstantMessage.Builder();

        imBuild.Message = txtEnterText.Text;
        imBuild.TimeStampTicks = DateTime.Now.Ticks;

        InstantMessage im = imBuild.BuildPartial();

        im.WriteTo(networkStream);
        ...

This works great. But at the other end, I'm having trouble getting the ParseFrom to work.

I want to use:-

InstantMessage im = InstantMessage.ParseFrom(networkStream);

But instead I have had to read it to bytes and then parse it from here. This is obviously not ideal for a number of reasons. Current code is:-

while (true)
        {
            Byte[] byteArray = new Byte[10000000];

            int intMsgLength;
            int runningMsgLength = 0;

            DateTime start = DateTime.Now;

            while (true)
            {
                runningMsgLength += networkStream.Read(byteArray, runningMsgLength, 10000000 - runningMsgLength);

                if (!networkStream.DataAvailable)
                    break;

            }

            InstantMessage im = InstantMessage.ParseFrom(byteArray.Take(runningMsgLength).ToArray());

When I try to use ParseFrom , control does not return to the calling method even when I know a valid GB message is on the wire.

Any advice would be gratefully received,

PW

Answer 1

Sorry for taking a while to answer this. As Marc says, protocol buffers don't have a terminator, and they aren't length prefixed unless they're nested. However, you can put on the length prefix yourself. If you look at MessageStreamIterator and MessageStreamWriter, you'll see how I do this - basically I pretend that I'm in the middle of a message, writing a nested message as field 1. Unfortunately when reading the message, I have to use internal details (BuildImpl).

There's now another API to do this: IMessage.WriteDelimitedTo and IBuilder.MergeDelimitedFrom . This is probably what you want at the moment, but I seem to remember there's a slight issue with it in terms of detecting the end of the stream (ie when there isn't another message to read). I can't remember whether there's a fix for it at the moment - I have a feeling it's changed in the Java version and I may not have ported the change yet. Anyway, that's definitely the area to look at.

Answer 2

Protobuf has no terminator - so either close the stream, or use your own length prefix etc. Protobuf-net exposes this easily via SerializeWithLenghtPrefix / DeserializeWithLengthPrefix.

Simply: without this, it can't know where each message ends, so keeps trying to read to the end of the stream.