Is there a way to define alternative conversion functions (from/to interfaces) in a protobuf-net surrogate class

Question

Using protobuf-net v2 build 668, I’m trying to serialize/deserialize a class which contains a member defined as an interface while doing on-the-fly conversions at the same time. Normally, the surrogate approach would work just fine but since C# doesn’t allow user-defined conversions for interfaces, I can’t define the conversions.

Thanks,

namespace ProtoBufNetTest
{
    using System.Diagnostics;
    using System.IO;

    using ProtoBuf;
    using ProtoBuf.Meta;

    class Program
    {
        static void Main()
        {
            RuntimeTypeModel.Default.Add(typeof(IDummy), false)
                .SetSurrogate(typeof(DummySurrogate));

            var container = new Container { Data = new Dummy { Positive = 3 } };

            using (var file = File.Create("test.bin"))
            {
                Serializer.Serialize(file, container);
            }

            using (var file = File.OpenRead("test.bin"))
            {
                container = Serializer.Deserialize<Container>(file);
                Debug.Assert(container.Data.Positive == 3);
            }
       }
    }

    // Outside of the project, cannot be changed
    public interface IDummy
    {
        int Positive { get; set; }
    }

    [ProtoContract]
    public class Container
    {
        [ProtoMember(1)]
        public IDummy Data { get; set; }
    }

    public class Dummy : IDummy
    {
        public int Positive { get; set; }
    }

    [ProtoContract]
    class DummySurrogate
    {
        [ProtoMember(1)]
        public int Negative { get; set; }

        // Does not compile : user-defined conversions to or from an interface are not allowed
        public static explicit operator IDummy(DummySurrogate value)
        {
            return value == null ? null : new Dummy { Positive = -value.Negative };
        }

        // Does not compile : user-defined conversions to or from an interface are not allowed
        public static explicit operator DummySurrogate(IDummy value)
        {
            return value == null ? null : new DummySurrogate { Negative = -value.Positive };
        }

        // Fake attribute, does not exist but could work if it did
        [ProtoConvertFrom]
        public static IDummy From(DummySurrogate value)
        {
            return value == null ? null : new Dummy { Positive = -value.Negative };
        }

        // Fake attribute, does not exist but could work if it did
        [ProtoConvertTo]
        public static DummySurrogate To(IDummy value)
        {
            return value == null ? null : new DummySurrogate { Negative = -value.Positive };
        }
    }
}

Answer 1

In the current build: no, there is not.

However, in the next build, this works fine:

[ProtoContract]
class DummySurrogate
{
    [ProtoMember(1)]
    public int Negative { get; set; }

    [ProtoConverter]
    public static IDummy From(DummySurrogate value)
    {
        return value == null ? null : new Dummy { Positive = -value.Negative };
    }

    [ProtoConverter]
    public static DummySurrogate To(IDummy value)
    {
        return value == null ? null : new DummySurrogate
           { Negative = -value.Positive };
    }
}

Basically, a static method marked [ProtoConverter] takes precedence over an implicit or explicit conversion operator that is defined, with the further advantage that [ProtoConverter] methods are not subject to the same syntactic rules as operators. It is not necessary to define separate *To / *From attributes, as the intent is clear from the signature.

As a side note: the attributes on Dummy are unnecessary and are never used.