protocol buffers - Error when migrating WCF service to gRPC service

Question

I have a WCF service that I want to rewrite into a gRPC service. There is a specific endpoint that gives me some trouble. Right now the method looks like this:

public List<Dictionary<string, string> GetData(GetDataRequest request)
{
    List<Dictionary<string, string>> results = new List<Dictionary<string, string>>();
    /// ...
    /// Code that populates the results list
    /// ...
    return results;
}

I have composed the proto file like this:

message GetDataRequest {
    string code = 1;
}

message GetDataResponse {
    message KeyValuePair {
        map<string, string> pairs = 1;
    }
    repeated KeyValuePair results= 1;
}

service Demo {
    rpc GetData(GetDataRequest) returns (GetDataResponse);
}

And the service implementation:

public class DemoService : Demo.DemoBase
{
    public override async Task<GetDataResponse> GetData(GetDataRequest request, ServerCallContext context)
    {
        List<Dictionary<string, string>> results = new List<Dictionary<string, string>>();

        /// ...
        /// Code that populates the results list
        /// ...

        return await Task.FromResult(new GetDataResponse
        {
            Results = results
        });
    }
}

My problem is when I try to return the list of dictionaries I get the this error:

What changes I need to make in order to return the response properly? I use the Visual Studio 2019 gRPC Service template.

This is the GetDataResponse generated code from protobuf compiler:

public sealed partial class GetDataResponse : pb::IMessage<GetDataResponse>
  #if !GOOGLE_PROTOBUF_REFSTRUCT_COMPATIBILITY_MODE
      , pb::IBufferMessage
  #endif
  {
    private static readonly pb::MessageParser<GetDataResponse> _parser = new pb::MessageParser<GetDataResponse>(() => new GetDataResponse());
    private pb::UnknownFieldSet _unknownFields;
    [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
    public static pb::MessageParser<GetDataResponse> Parser { get { return _parser; } }

    [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
    public static pbr::MessageDescriptor Descriptor {
      get { return global::GrpcService.Protos.DemoReflection.Descriptor.MessageTypes[1]; }
    }

    [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
    pbr::MessageDescriptor pb::IMessage.Descriptor {
      get { return Descriptor; }
    }

    [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
    public GetDataResponse() {
      OnConstruction();
    }

    partial void OnConstruction();

    [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
    public GetDataResponse(GetDataResponse other) : this() {
      results_ = other.results_.Clone();
      _unknownFields = pb::UnknownFieldSet.Clone(other._unknownFields);
    }

    [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
    public GetDataResponse Clone() {
      return new GetDataResponse(this);
    }

    /// <summary>Field number for the "results" field.</summary>
    public const int ResultsFieldNumber = 1;
    private static readonly pb::FieldCodec<global::GrpcService.Protos.GetDataResponse.Types.KeyValuePair> _repeated_results_codec
        = pb::FieldCodec.ForMessage(10, global::GrpcService.Protos.GetDataResponse.Types.KeyValuePair.Parser);
    private readonly pbc::RepeatedField<global::GrpcService.Protos.GetDataResponse.Types.KeyValuePair> results_ = new pbc::RepeatedField<global::GrpcService.Protos.GetDataResponse.Types.KeyValuePair>();
    [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
    public pbc::RepeatedField<global::GrpcService.Protos.GetDataResponse.Types.KeyValuePair> Results {
      get { return results_; }
    }

    [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
    public override bool Equals(object other) {
      return Equals(other as GetDataResponse);
    }

    [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
    public bool Equals(GetDataResponse other) {
      if (ReferenceEquals(other, null)) {
        return false;
      }
      if (ReferenceEquals(other, this)) {
        return true;
      }
      if(!results_.Equals(other.results_)) return false;
      return Equals(_unknownFields, other._unknownFields);
    }

    [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
    public override int GetHashCode() {
      int hash = 1;
      hash ^= results_.GetHashCode();
      if (_unknownFields != null) {
        hash ^= _unknownFields.GetHashCode();
      }
      return hash;
    }

    [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
    public override string ToString() {
      return pb::JsonFormatter.ToDiagnosticString(this);
    }

    [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
    public void WriteTo(pb::CodedOutputStream output) {
    #if !GOOGLE_PROTOBUF_REFSTRUCT_COMPATIBILITY_MODE
      output.WriteRawMessage(this);
    #else
      results_.WriteTo(output, _repeated_results_codec);
      if (_unknownFields != null) {
        _unknownFields.WriteTo(output);
      }
    #endif
    }

    #if !GOOGLE_PROTOBUF_REFSTRUCT_COMPATIBILITY_MODE
    [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
    void pb::IBufferMessage.InternalWriteTo(ref pb::WriteContext output) {
      results_.WriteTo(ref output, _repeated_results_codec);
      if (_unknownFields != null) {
        _unknownFields.WriteTo(ref output);
      }
    }
    #endif

    [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
    public int CalculateSize() {
      int size = 0;
      size += results_.CalculateSize(_repeated_results_codec);
      if (_unknownFields != null) {
        size += _unknownFields.CalculateSize();
      }
      return size;
    }

    [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
    public void MergeFrom(GetDataResponse other) {
      if (other == null) {
        return;
      }
      results_.Add(other.results_);
      _unknownFields = pb::UnknownFieldSet.MergeFrom(_unknownFields, other._unknownFields);
    }

    [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
    public void MergeFrom(pb::CodedInputStream input) {
    #if !GOOGLE_PROTOBUF_REFSTRUCT_COMPATIBILITY_MODE
      input.ReadRawMessage(this);
    #else
      uint tag;
      while ((tag = input.ReadTag()) != 0) {
        switch(tag) {
          default:
            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
            break;
          case 10: {
            results_.AddEntriesFrom(input, _repeated_results_codec);
            break;
          }
        }
      }
    #endif
    }

    #if !GOOGLE_PROTOBUF_REFSTRUCT_COMPATIBILITY_MODE
    [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
    void pb::IBufferMessage.InternalMergeFrom(ref pb::ParseContext input) {
      uint tag;
      while ((tag = input.ReadTag()) != 0) {
        switch(tag) {
          default:
            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, ref input);
            break;
          case 10: {
            results_.AddEntriesFrom(ref input, _repeated_results_codec);
            break;
          }
        }
      }
    }
    #endif

    #region Nested types
    /// <summary>Container for nested types declared in the GetDataResponse message type.</summary>
    [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
    public static partial class Types {
      public sealed partial class KeyValuePair : pb::IMessage<KeyValuePair>
      #if !GOOGLE_PROTOBUF_REFSTRUCT_COMPATIBILITY_MODE
          , pb::IBufferMessage
      #endif
      {
        private static readonly pb::MessageParser<KeyValuePair> _parser = new pb::MessageParser<KeyValuePair>(() => new KeyValuePair());
        private pb::UnknownFieldSet _unknownFields;
        [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
        public static pb::MessageParser<KeyValuePair> Parser { get { return _parser; } }

        [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
        public static pbr::MessageDescriptor Descriptor {
          get { return global::GrpcService.Protos.GetDataResponse.Descriptor.NestedTypes[0]; }
        }

        [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
        pbr::MessageDescriptor pb::IMessage.Descriptor {
          get { return Descriptor; }
        }

        [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
        public KeyValuePair() {
          OnConstruction();
        }

        partial void OnConstruction();

        [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
        public KeyValuePair(KeyValuePair other) : this() {
          pairs_ = other.pairs_.Clone();
          _unknownFields = pb::UnknownFieldSet.Clone(other._unknownFields);
        }

        [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
        public KeyValuePair Clone() {
          return new KeyValuePair(this);
        }

        /// <summary>Field number for the "pairs" field.</summary>
        public const int PairsFieldNumber = 1;
        private static readonly pbc::MapField<string, string>.Codec _map_pairs_codec
            = new pbc::MapField<string, string>.Codec(pb::FieldCodec.ForString(10, ""), pb::FieldCodec.ForString(18, ""), 10);
        private readonly pbc::MapField<string, string> pairs_ = new pbc::MapField<string, string>();
        [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
        public pbc::MapField<string, string> Pairs {
          get { return pairs_; }
        }

        [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
        public override bool Equals(object other) {
          return Equals(other as KeyValuePair);
        }

        [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
        public bool Equals(KeyValuePair other) {
          if (ReferenceEquals(other, null)) {
            return false;
          }
          if (ReferenceEquals(other, this)) {
            return true;
          }
          if (!Pairs.Equals(other.Pairs)) return false;
          return Equals(_unknownFields, other._unknownFields);
        }

        [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
        public override int GetHashCode() {
          int hash = 1;
          hash ^= Pairs.GetHashCode();
          if (_unknownFields != null) {
            hash ^= _unknownFields.GetHashCode();
          }
          return hash;
        }

        [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
        public override string ToString() {
          return pb::JsonFormatter.ToDiagnosticString(this);
        }

        [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
        public void WriteTo(pb::CodedOutputStream output) {
        #if !GOOGLE_PROTOBUF_REFSTRUCT_COMPATIBILITY_MODE
          output.WriteRawMessage(this);
        #else
          pairs_.WriteTo(output, _map_pairs_codec);
          if (_unknownFields != null) {
            _unknownFields.WriteTo(output);
          }
        #endif
        }

        #if !GOOGLE_PROTOBUF_REFSTRUCT_COMPATIBILITY_MODE
        [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
        void pb::IBufferMessage.InternalWriteTo(ref pb::WriteCont

Answer 1

In ASP.NET Core gRPC for WCF developers the Repeated fields for lists and arrays section explains that

repeated fields are represented by read-only properties of the Google.Protobuf.Collections.RepeatedField type rather than any of the built-in .NET collection types. This type implements all the standard .NET collection interfaces, such as IList and IEnumerable. So you can use LINQ queries or convert it to an array or a list easily.

You'll have to use Add/AddRange to add items to that property , eg:

var response=new GetDataResponse();
response.AddRange(results);
return response;

You don't need to wrap response in a Task to return it because your method has the async signature, so it already wraps return values. In any case, await Task.FromResult(x) just returns x