Issue with IL2CPP where Default Constructor is possibly stripped from assembly

Questions & Answers
1

Apologies if this is not the right place to ask about this error. I’ve been at this for 4-5 days.

Specs: protobuf-net 3.0.73 integrated into a Unity 2019.4.6f1 project that targets .NET Standard 2.0 when building for Xbox console with IL2CPP set to Low Stripping.
Exact log: “MissingMthodException: Default constructor not found for type ProtoBuf.Internal.PrimaryTypeProvider”

The project built in Editor works exactly as intended and that’s why I think it’s the IL2CPP stripping.
Error is reported when attempting to assign a new data field of ProtoTest.
Might be important to note that the ProtoTest class is a C# Library DLL coded and programmed outside of the Unity environment, as required per ProtoBuf Custom serialization. (To my knowledge). I then built it and ported it into Unity Plugins folder along with the matching Serializer.

ProtoTest saveFile = new ProtoTest(); // This is the call made where the code will fail

----- // Decorated class
[ProtoContract()]
public class ProtoTest
{
    [ProtoMember(1)]
    public string testString;
    [ProtoMember(2)]
    public int testint;
    [ProtoMember(3)]
    public int testBool;
    [ProtoMember(4)]
    public int[] testFixedIntArray;
    [ProtoMember(5)]
    public List<int> testList = new List<int>();
    [ProtoMember(6)]
    public Dictionary<int, string> testDictionary = new Dictionary<int, string>();

    // This is the constructor that is being stripped (to my knowledge)
    public ProtoTest()  { }
}

I suspect that some off the protobuf files are being stripped during the build, I don’t have a lot of experience with handling this. However I attempted to omit the protobuf namespace from stripping by writing a Linker referencing the namespaces used to create all the protobuf data I know of.

Might be important to note that the ProtoTest class is a C# Library DLL coded and programmed outside of the Unity environment, as required per ProtoBuf Custom serialization. (To my knowledge).

Attempt 1

<linker>
  <assembly fullname="ProtoBuf">
	<namespace fullname="ProtoBuf.Internal.PrimaryTypeProvider" preserve="all"/>
  </assembly>
</linker>

Attempt 2

<linker>
  <assembly fullname="ProtoSave" preserve="all"/>
  <assembly fullname="ProtoBuf" preserve="all"/>
  <assembly fullname="ProtoBuf.Internal" preserve="all"/>
  <assembly fullname="ProtoBuf.Meta" preserve="all"/>
</linker>

Attempt 3

<linker>
  <namespace fullname="ProtoSave" preserve="all"/>
  <namespace fullname="ProtoBuf" preserve="all"/>
  <namespace fullname="ProtoBuf.Internal" preserve="all"/>
  <namespace fullname="ProtoBuf.Meta" preserve="all"/>
  <namespace fullname="ProtoBuf.Serializers" preserve="all"/>
  <namespace fullname="DataSerializer" preserve="all"/>
</linker>
2

I would recommend reviewing the documentation about the linker if you have not done so: Unity - Manual: Managed code stripping.

I think the question is, which assembly does the ProtoTest type live in? Once you know that, you can start by preserving that entire assemly:

<linker>
    <assembly fullname="ProtoTestAssembly" preserve="all"/>
</linker>

This is probably not a good long term solution, but it will help isolate the issue. Once you know this, then you can more specific preserve only the types in that assembly that you need.