How to use polymorphism on return type of grpc code first service?

[soroshsabz]

ITNOA

I want to use polymorphic return type in grpc service with code first approach, but I do not know how to do it?

I have service contract like below

    [ServiceContract]
    public interface IGreeterService
    {
        [OperationContract]
        BaseResponse SayHello(HelloRequest request,
            CallContext context = default);
    }

And I have two derived response like below

public class ResponseA : BaseResponse
{
string foo;
}

public class ResponseB: BaseResponse
{
string bar;
}

my BaseResponse exist in another library, and I do not want (and I can not) to change this library.

public abstract class BaseResponse
{
public string hoo;
}

So finally I want to implement SayHello like below

public class GreeterService : IGreeterService
{

  public BaseResponse SayHello(HelloRequest request, CallContext context = default)
  {
    if (request.Name == "Heelo")
    {
      return new ResponseA() {foo = "Hi", hoo = "Hey"};
    }
    else
      return new ResponseB() {bar = "bye", hoo = "You"};
  }
}

But I do not know how can I say to OperationContract that I have two type for return type?

[soroshsabz]

I think maybe related to #300

[soroshsabz]

I ask same question on Stack Overflow https://stackoverflow.com/q/77704495/1539100

[soroshsabz]

related to BSVN/Commons#68

[soroshsabz]

@mgravell response in stack overflow like below

If you can't alter BaseResponse, you will have to configure it manually - for example:

RuntimeTypeModel.Default.Add(typeof(BaseResponse), false)
    // fields of BaseResponse
    .Add(1, nameof(BaseResponse.hoo))
    // sub-types of BaseResponse
    .AddSubType(10, typeof(ResponseA))
    .AddSubType(11, typeof(ResponseB));

Here I'm assuming that ResponseA and ResponseB can at least be altered, i.e.

[ProtoContract]
public class ResponseA : BaseResponse
{
    [ProtoMember(1)]
    public string foo;
}

If that isn't possible, you'll also need to configure them manually, for example:

RuntimeTypeModel.Default.Add(typeof(ResponseA), false)
    .Add(1, nameof(ResponseA.foo));

[soroshsabz]

For automated this procedure, I write some attribute like below

namespace BSN.Commons.Utilities
{
    /// <summary>
    /// TODO
    /// </summary>
    [AttributeUsage(AttributeTargets.Class, AllowMultiple = true, Inherited = false)]
    public class ProtoImplementAttribute : Attribute
    {
        /// <summary>
        /// This constructor defines one required parameter
        /// </summary>
        /// <param name="BaseType">Type of base class or interface that you want to implement it</param>
        public ProtoImplementAttribute(Type BaseType)
        {
            this.BaseType = BaseType;
        }

        internal Type BaseType { get; }
    }

    /// <summary>
    /// Active polymorphism for protobuf-net code first approach.
    /// This class is used to enable polymorphism for protobuf-net code first approach on a specific assembly based on ProtoImplementAttribute.
    /// </summary>
    /// <remarks>
    /// You must call this class in the startup of your application.
    /// </remarks>
    public static class GrpcPolymorphismActivator
    {
        /// <summary>
        /// Enable polymorphism for protobuf-net code first approach on a specific assembly based on ProtoImplementAttribute.
        /// </summary>
        /// <param name="assembly"></param>
        /// <param name="extraTypes"></param>
        /// <exception cref="Exception"></exception>
        public static void Enable(Assembly assembly, (Type, Type)[] extraListOfDeriveds)
        {
            // TODO: Use C# source generator https://stackoverflow.com/q/64926889/1539100

            // ProtoBuf.Meta.RuntimeTypeModel.Default.Add(typeof(Response), false)
            //        .Add(1, nameof(Response.StatusCode))
            //        .Add(2, nameof(Response.Message))
            //        .Add(3, nameof(Response.InvalidItems))
            //        .AddSubType(100, typeof(ErrorResponse))
            //        .AddSubType(101, typeof(Response<SayHelloViewModel>));

            Dictionary<Type, List<Type>> listOfDerivedTypes = new Dictionary<Type, List<Type>>();
            foreach (var type in from type in assembly.GetTypes()
                                 where type.GetCustomAttribute<ProtoImplementAttribute>() != null
                                 select type)
            {
                ProtoImplementAttribute protoImplementAttribute = type.GetCustomAttribute<ProtoImplementAttribute>();

                if (listOfDerivedTypes.TryGetValue(protoImplementAttribute.BaseType, out List<Type> derivedTypes))
                    derivedTypes.Add(type);
                else
                    listOfDerivedTypes.Add(protoImplementAttribute.BaseType, new List<Type>() { type });
            }

            foreach ((Type @base, Type derived) in extraListOfDeriveds)
            {
                if (listOfDerivedTypes.TryGetValue(@base, out List<Type> derivedTypes))
                    derivedTypes.Add(derived);
                else
                    listOfDerivedTypes.Add(@base, new List<Type>() { derived });
            }

            foreach (var derivedType in listOfDerivedTypes)
            {
                MetaType @base = ProtoBuf.Meta.RuntimeTypeModel.Default.Add(derivedType.Key, false);
                int maxOrder = 0;
                foreach (var property in derivedType.Key.GetProperties())
                {
                    DataMemberAttribute dataMemberAttribute = property.GetCustomAttribute<DataMemberAttribute>();
                    if (property.GetCustomAttribute<DataMemberAttribute>() == null)
                        continue;
                    @base = @base.Add(dataMemberAttribute.Order, dataMemberAttribute.Name ?? property.Name);
                    maxOrder = Math.Max(maxOrder, dataMemberAttribute.Order);
                }

                // Reserve some order for well-known derived types
                maxOrder += 100;

                foreach (var type in derivedType.Value)
                {
                    @base.AddSubType(maxOrder + 1, type);
                }
            }
        }
    }
}

@mgravell Did you want to I create a PR for this helper and attribute?