Problems serializing a list of nullable doubles using Protobuf-net

Question

I've been able to serialize nullable doubles without an issue, and can serialize Lists of nullable other types, but can't serialize Lists of nullable doubles.

If I do this:

        List<double?> aList = new List<double?>();
        aList.Add(0.1);
        aList.Add(null);
        Serializer.Serialize(ms, aList);

I get this error:

System.NullReferenceException: Object reference not set to an instance of an object. at ProtoBuf.Meta.TypeModel.TrySerializeAuxiliaryType(ProtoWriter writer, Type type, DataFormat format, Int32 tag, Object value, Boolean isInsideList) in c:\Dev\protobuf-net\protobuf-net\Meta\TypeModel.cs:line 169 at ProtoBuf.Meta.TypeModel.SerializeCore(ProtoWriter writer, Object value) in c:\Dev\protobuf-net\protobuf-net\Meta\TypeModel.cs:line 188 at ProtoBuf.Meta.TypeModel.Serialize(Stream dest, Object value, SerializationContext context) in c:\Dev\protobuf-net\protobuf-net\Meta\TypeModel.cs:line 217 at ProtoBuf.Meta.TypeModel.Serialize(Stream dest, Object value) in c:\Dev\protobuf-net\protobuf-net\Meta\TypeModel.cs:line 201 at ProtoBuf.Serializer.Serialize[T](Stream destination, T instance) in c:\Dev\protobuf-net\protobuf-net\Serializer.cs:line 87

should this work? Am I doing something wrong?

I put a test together with a List<int?>, a List<double?> and a List<string> - they all behaved identically here — Marc Gravell♦
I get the same behaviour will all nullable types. I think previously whilst I had nullable types, it was only the nullable double that had actual null values. Apologies for the misinformation. Ben — mcmillab

Marc Gravell Marc Gravell · Accepted Answer · 2013-05-26T07:45:53

The main problem here is that the protobuf specification simply has no concept of null - explicitly null / missing values cannot be expressed in the protobuf format.

On a per-library basis, the library itself could choose to spoof an extra layer to allow for this kind of thing, but:

it would take extra bytes on the wire
it would complicate the code and require extra configuration
it would (by necessity) disable optimisations like "packed" encoding

It should probably detect the null and behave better, though!

I would encourage you to serialize a list of things that have a nullable-value, rather than a list of null able values themselves. For example:

[ProtoContract]
public class Foo {
    [ProtoMember(1)] public double? Value {get;set;}
}

A list of the above can express null values. And is basically exactly the same as what I would write if I added inbuilt support for spoofing nulls.

Problems serializing a list of nullable doubles using Protobuf-net

1 Answers