Reading a protobuf3 custom option from C#

Question

TL;DR

According to the doc, if I were doing C++, I could read the value of a custom option using string value = MyMessage::descriptor()->options().GetExtension(my_option);. There are similar examples for Java and Python. But I'm doing C# and I could find an equivalent. Can I do it, and if yes, how?

More details

I'm manipulating classes generated with protobuf3. The schemas are declaring a custom option. It looks like this:

import "google/protobuf/descriptor.proto";

extend google.protobuf.MessageOptions {
  string my_option = 51234;
}

message MyMessage {
  option (my_option) = "Hello world!";
}

My code is being provided an object generated from MyMessage, and I'd like to read the value of this option (here Hello world!)

---

Update: I'm not using protobuf-net. Now that C# is natively supported by protobuf, I'm using Google's protobuf3 C# library.

Answer 1

You can now access custom options in C#. First, define the custom option in your .proto:

import "google/protobuf/descriptor.proto";
extend google.protobuf.FieldOptions {
  string objectReferenceType = 1000; //Custom options are 1000 and up.
}

Next, apply the custom option to something. Here I attached it to a field:

message Item
{
  string name = 1;
  int32 id = 2;
  string email = 3;
  ObjectReference prefab = 4 [(objectReferenceType) = "UnityEngine.GameObject"];
}

Then you need to lookup the custom option field number. There's no nice way to do this, so just look up the extension from FileDescriptor of the file where you defined the custom option extension. You will have a C# generated class called protoFileNameReflection. From that, you can find the extension then the field number. Here's an example assuming the proto is called "Item.proto" so the generated class is called ItemReflection:

foreach (FieldDescriptor extensionFieldDescriptor in ItemReflection.Descriptor.Extensions.UnorderedExtensions)
    {   
        if (extensionFieldDescriptor.ExtendeeType.FullName == "google.protobuf.FieldOptions")
        {
            objectReferenceTypeFieldNumber = extensionFieldDescriptor.FieldNumber;
            break;
        }
    }

Then access the custom option in code using protobuf reflection:

FieldDescriptor fieldDescriptor = prefabFieldDescriptor;
CustomOptions customOptions = fieldDescriptor.CustomOptions;
if (customOptions.TryGetString(objectReferenceTypeFieldNumber, out string objectReferenceTypeText))
{
   Console.Log(objectReferenceTypeText); //logs: "UnityEngine.GameObject"
}