Google Protocol Buffers: find out what message is encoded in byte[]

Is there a way to find out what type of message is encoded in byte[]? I mean, is it possible to develop things like:

byte [] buf;

if (typeOfMessage(buf) == "AProto") {
  AProto.parseFrom(buf);
} else if (typeOfMessage(buf) == "BProto") {
  BProto.parseFrom(buf);
}

...
Jon Skeet
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No, it isn't. Several different messages could all have the same byte[] representation. A message doesn't carry around with it any type information. The wire format indicates for each tag value what the encoding of the value is out of the various encodings, but that's as far as it goes.

If you want this functionality, however, it's relatively easy to wrap it up into your own message:

message MessageWithType {
  required string type_name = 1;
  required bytes data = 2;
}

Basically that just wraps each real message with the name of its type. This only works if you control the whole process, of course... if you don't get to change the data, you can't add that information.

As Kenton noted, with a fixed (and ideally small) set of types that you're interested in, you can declare a message with a field of each type (as per ursa's answer), using a oneof if you're using Protobuf 2.6.0+.

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