C# generated code for Protobuf map<string, string> is failing

[maxipaolucci]

Hello team,

I have the following Protobuf schema

syntax = "proto3";
package confluent;

import "google/protobuf/descriptor.proto";

option java_package = "io.confluent.protobuf";
option java_outer_classname = "MetaProto";
option csharp_namespace = "Confluent.Protobuf";

message Meta {
  string doc = 1;
  map<string, string> params = 2;
  repeated string tags = 3;
}

extend google.protobuf.FileOptions {
  Meta file_meta = 1088;
}
extend google.protobuf.MessageOptions {
  Meta message_meta = 1088;
}
extend google.protobuf.FieldOptions {
  Meta field_meta = 1088;
}
extend google.protobuf.EnumOptions {
  Meta enum_meta = 1088;
}
extend google.protobuf.EnumValueOptions {
  Meta enum_value_meta = 1088;
}

Generated the C# classes using buf generate

version: v1
plugins:
  - name: csharp
    out: ../bindings
    opt: base_namespace

When I try to use the generated class Meta.cs in my .NET 6.0 project I am getting the following error
/Users/maxi.paolucci/Workspace/sdk.horizon-dotnet-examples/examples/Xero.Eventing.Horizon.SDK.Net6.ProducerSample/Confluent/Protobuf/Meta.cs(246,15): error CS1061: 'MapField<string, string>' does not contain a definition for 'MergeFrom' and no accessible extension method 'MergeFrom' accepting a first argument of type 'MapField<string, string>' could be found (are you missing a using directive or an assembly reference?) [/Users/maxi.paolucci/Workspace/sdk.horizon-dotnet-examples/examples/Xero.Eventing.Horizon.SDK.Net6.ProducerSample/Xero.Eventing.Horizon.SDK.Net6.ProducerSample.csproj]

I had been able to manually edit that file and replace that line 246 and do something like this as a work around and it worked

      // original line 246 commented out
      //params_.MergeFrom(other.params_);
     
      // workaround 
      foreach (var entry in other.params_)
      {
        if (!params_.ContainsKey(entry.Key))
        {
          params_.Add(entry.Key, entry.Value);
        }
        // You can also update existing values based on your logic here.
      }

But ideally we don't want to have to manually update these classes as they are generated by some automated process.

Any idea if this could be a bug in the buf libs?

Thank you!

[saquibmian]

Hi @maxipaolucci, thanks for raising an issue. This code is generated by protoc-gen-csharp, so most likely this is a mismatch between your Protobuf runtime version (Google.Protobuf) and the version of protoc-gen-csharp. Could you tell me which versions of both you are using?

[maxipaolucci]

Hey @saquibmian,

I don't have/find protoc-gen-csharp installed but have protoc version 26.1.0
Now sure what you mean with the version of Google.Protobuf or how to look at it?

I also have and I used it to generate these C# code buf version 1.47.2 which I installed with $ brew install bufbuild/buf/buf as detailed in here

[saquibmian]

Thanks, the protoc version is the one you're looking for.

In your hosting application (the application where you're using the generated code) you should have a dependency on the Google.Protobuf NuGet package, either directly or transitively. This would be somewhere in your csproj file. You should be using this package (it's basically 3.<protoc version>).

[maxipaolucci]

Ok got it, I was getting Google.Protobuf 3.15.0 transitively, when I explicitly added 3.26.1 it worked like a charm.

That means that the Google.Protobuf version has to match the protoc version always?
or the problem is just that the one I was using 3.15.0 was to old for the generated code?

[saquibmian]

Awesome, glad that was sorted for you!

Code generated by an older plugin is supported by newer runtimes (i.e., protoc version <= Google.Protobuf version), but the opposite is not true. As a best practice, it's best to keep them in sync to avoid any untoward issues.