How to use protobuf-net with immutable value types?

Question

Suppose I have an immutable value type like this:

[Serializable]
[DataContract]
public struct MyValueType : ISerializable
{
private readonly int _x;
private readonly int _z;

public MyValueType(int x, int z)
    : this()
{
    _x = x;
    _z = z;
}

// this constructor is used for deserialization
public MyValueType(SerializationInfo info, StreamingContext text)
    : this()
{
    _x = info.GetInt32("X");
    _z = info.GetInt32("Z");
}

[DataMember(Order = 1)]
public int X
{
    get { return _x; }
}

[DataMember(Order = 2)]
public int Z
{
    get { return _z; }
}

public static bool operator ==(MyValueType a, MyValueType b)
{
    return a.Equals(b);
}

public static bool operator !=(MyValueType a, MyValueType b)
{
    return !(a == b);
}

public override bool Equals(object other)
{
    if (!(other is MyValueType))
    {
        return false;
    }

    return Equals((MyValueType)other);
}

public bool Equals(MyValueType other)
{
    return X == other.X && Z == other.Z;
}

public override int GetHashCode()
{
    unchecked
    {
        return (X * 397) ^ Z;
    }
}

// this method is called during serialization
public void GetObjectData(SerializationInfo info, StreamingContext context)
{
    info.AddValue("X", X);
    info.AddValue("Z", Z);
}

public override string ToString()
{
    return string.Format("[{0}, {1}]", X, Z);
}
}

It works with BinaryFormatter or DataContractSerializer but when I try to use it with protobuf-net (http://code.google.com/p/protobuf-net/) serializer I get this error:

Cannot apply changes to property ConsoleApplication.Program+MyValueType.X

If I apply setters to the properties marked with DataMember attribute it will work but then it breaks immutability of this value type and that's not desirable to us.

Does anyone know what I need to do to get it work? I've noticed that there's an overload of the ProtoBu.Serializer.Serialize method which takes in a SerializationInfo and a StreamingContext but I've not use them outside of the context of implementing ISerializable interface, so any code examples on how to use them in this context will be much appreciated!

Thanks,

EDIT: so I dug up some old MSDN article and got a better understanding of where and how SerializationInfo and StreamingContext is used, but when I tried to do this:

var serializationInfo = new SerializationInfo(
    typeof(MyValueType), new FormatterConverter());
ProtoBuf.Serializer.Serialize(serializationInfo, valueType);

it turns out that the Serialize<T> method only allows reference types, is there a particular reason for that? It seems a little strange given that I'm able to serialize value types exposed through a reference type.

Answer 1

Which version of protobuf-net are you using? If you are the latest v2 build, it should cope with this automatically. In case I haven't deployed this code yet, I'll update the download areas in a moment, but essentially if your type is unadorned (no attributes), it will detect the common "tuple" patten you are using, and decide (from the constructor) that x (constructor parameter)/X (property) is field 1, and z/Z is field 2.

Another approach is to mark the fields:

[ProtoMember(1)]
private readonly int _x;

[ProtoMember(2)]
private readonly int _z;

(or alternatively [DataMember(Order=n)] on the fields)

which should work, depending on the trust level. What I haven't done yet is generalise the constructor code to attributed scenarios. That isn't hard, but I wanted to push the basic case first, then evolve it.

I've added the following two samples/tests with full code here:

    [Test]
    public void RoundTripImmutableTypeAsTuple()
    {
        using(var ms = new MemoryStream())
        {
            var val = new MyValueTypeAsTuple(123, 456);
            Serializer.Serialize(ms, val);
            ms.Position = 0;
            var clone = Serializer.Deserialize<MyValueTypeAsTuple>(ms);
            Assert.AreEqual(123, clone.X);
            Assert.AreEqual(456, clone.Z);
        }
    }
    [Test]
    public void RoundTripImmutableTypeViaFields()
    {
        using (var ms = new MemoryStream())
        {
            var val = new MyValueTypeViaFields(123, 456);
            Serializer.Serialize(ms, val);
            ms.Position = 0;
            var clone = Serializer.Deserialize<MyValueTypeViaFields>(ms);
            Assert.AreEqual(123, clone.X);
            Assert.AreEqual(456, clone.Z);
        }
    }

Also:

it turns out that the Serialize method only allows reference types

yes, that was a design limitation of v1 that related to the boxing model etc; this no longer applies with v2.

Also, note that protobuf-net doesn't itself consume ISerializable (although it can be used to implement ISerializable).