protobuf-net v3 with surrogate and inheritance hierarchies

Question

I'm struggling migrating from protobuf-net v2.4.6 to v3.0.100 (or any 3.0.x) in regards to an existing type hierarchy used as ProtoContracts with one of the subtypes requiring a surrogate due to one of its property being of type object.

With previous configuration in place, I get the following exception thrown on creating the runtime model: System.InvalidOperationException: 'Types with surrogates cannot be used in inheritance hierarchies'

Hence, my question is how to properly deal with this scenario using protobuf-net 3.0.x?

Here's my (over-)simplified repro of the issue:

class Program
{
    static void Main(string[] args)
    {
        var model = RuntimeTypeModel.Create();
        _ = model[typeof(Base)]; // <-- InvalidOperationException thrown here

        Base value = new Complex();
        var copy = model.DeepClone(value);
    }
}

[ProtoContract]
[ProtoInclude(1, typeof(Simple))]
[ProtoInclude(2, typeof(Complex))]
public abstract class Base
{
}

[ProtoContract]
public class Simple : Base
{
}

[ProtoContract(Surrogate = typeof(ComplexSurrogate))]
public class Complex : Base
{
}

[ProtoContract(Name = nameof(Complex))]
public class ComplexSurrogate
{
    [ProtoConverter]
    public static ComplexSurrogate Convert(Complex source) => new ComplexSurrogate();

    [ProtoConverter]
    public static Complex Convert(ComplexSurrogate source) => new Complex();
}

As a side note: When compiling protobuf-net from source with the above mentioned exception suppressed, I'm able to defined a surrogate for the Base class which seems to serve as a workaround.

Answer 1

Right now, that scenario isn't supported. While reworking the code for v3, some ambiguous outcomes/intents were found, and it needs work to go in and figure out what the correct outcomes are in each case, design how to achieve that, implement it, and test it. That time has not yet been found, so right now it is safer to prevent a configuration that could lead to big problems downstream, than to just shrug and assume that whatever happens is correct. It is on my list of things to do, but: ultimately this is a project that comes entirely out of my own spare time - it isn't sponsored or part of my paid work, so: it'll get there when it gets there.