How to support C# dynamic types in an gRPC proto file

Question

We have a POST action in our asp.net core application that accepts a dynamic object.

[HttpPost]
public Task<ActionResult> SubmitAsync(dynamic unitOfWork)

We'd like to transform this POST action to a gRPC server and we'd like to continue receiving dynamic objects in the gRPC service. What is the equivalent of C# dynamic definition in gRPC protobuf file definition? Or if that cannot be achieved what's the best way to receive a dynamic object?

Answer 1

You can pass messages with fields whose type was not known in advance. You can also pass messages with fields that are not typed, such as dynamic objects that can take any scalar values, and collections null values are allowed. To do so, import the proto file "google/protobuf/struct.proto" and declare the dynamic type as google.protobuf.Value.

So, first add bellow line at the top of your proto file:

import "google/protobuf/struct.proto";

Here my sample message with two dynamic fields:

message BranchResponse {
 google.protobuf.Value BranchId = 1;
 google.protobuf.Value BranchLevel = 2;
}

Note that: the generated type in C# is Value and belongs to the Google.Protobuf.WellKnownTypes namespace, which belongs itself to the Google.Protobuf assembly. This type inherits from the IMessage, IMessage, IEquatable, IDeepCloneable, and IBufferMessage interfaces that all belong to the Google.Protobuf assembly, except for IEquatable, which comes from the .NET System.Runtime assembly. To write and read dynamic values, we have a set of methods available that shown bellow: (these are write static functions)

We can fill BranchResponse model like this:

var branch = new BranchResponse();
branch.BranchId = Value.ForNumber(1);
branch.BranchLevel = Value.ForStruct(new Struct
{
 Fields = {
 ["LevelId"] = Value.ForNumber(1),
 ["LevelName"] = Value.ForString("Gold"),
 ["IsProfessional"] = Value.ForBool(true)}
});

The read Value type is straightforward. The Value type has a set of properties that exposes its value in the wanted type. (these are read static functions)

At the end, you need to read data from your response model like this:

Here my c# classes that my response model is supposed to bind to them.

public class BranchModel
{
 public int BranchId { get; set; }
 public LevelModel Level { get; set; }
}
public class LevelModel
{
 public int LevelId{ get; set; }
 public string LevelName{ get; set; }
 public bool IsProfessional { get; set; }
}

Finally:

var branch = new BranchResponse(); // Received filled from a gRPC call
// Read
var branchModel = new BranchModel
{
 BranchId = Convert.ToInt32(branch.BranchId.NumberValue),
 Level= new LevelModel
   {
      LevelId = Convert.ToInt32(branchModel.Level.StructValue.
      Fields["LevelId"].NumberValue),
      LevelName = branchModel.Level.StructValue.
      Fields["LevelName"].StringValue,
      IsProfessional = branchModel.Level.StructValue.
      Fields["IsProfessional"].BoolValue,
   }
};

Answer 2

That isn't really a thing right now. In protobuf terms, Any is the closest thing, but I have not yet implemented that in protobuf-net (it is on my short term additions list). The legacy "dynamic types" feature in protobuf-net (that sends type metadata) is actively being phased out, with Any being the preferred route since it allows cross-platform usage and doesn't have the same metadata dependencies.

Frankly, though, I'd probably say "just don't do this"; instead, prefer oneof; it isn't likely that you actually mean "anything" - you probably just mean "one of these things that I expect, but I don't know which", and oneof expresses that intent. More: protobuf-net implements inheritance via oneof, so a good option is something like:

[ProtoContract]
[ProtoInclude(1, typeof(FooRequest))]
[ProtoInclude(2, typeof(BarRequest))]
public abstract class RequestBase {}

[ProtoContract]
public class FooRequest {} 
[ProtoContract]
public class BarRequest {}