Custom BSON Key Value Serializer

Question

I ran into some trouble trying to convert a class to a BSON document.

I have Custom1 and Custom2 which should behave a little different. How do I create a custom serializer which "unfold" the KeyValuePair so it generates the expected result (See below)? You can see the code example below together with the expected result.

Moreover, I'm using Mongo BSON library for serializing the object.

public class UserData
{
    public UserData()
    {
        Id = 100;
        Name = "Superuser";
        Custom1 = new KeyValuePair<string, double>("HelloWorld1", 1);
        Custom2 = new KeyValuePair<string, double>("HelloWorld2", 2);
    }

    public int Id { get; set; }
    public string Name { get; set; }
    public KeyValuePair<string, double> Custom1 { get; set; }
    public KeyValuePair<string, double> Custom2 { get; set; }
}

Execute test code:

var userdata = new UserData();
var doc = userdata.ToBsonDocument();

Current result:

{
    "Id": 100,
    "Name": "Superuser",
    "Custom1": {
                    "Key": "HelloWorld1",
                    "Value": 1
               },
    "Custom2": {
                    "Key": "HelloWorld2",
                    "Value": 2
               }
}

Expected result:

{
    "Id": 100,
    "Name": "Superuser",
    "HelloWorld1": 1,
    "HelloWorld2": 2
}

Answer 1

For such a complex serialization case you have to implement custom IBsonSerializer converter for your class.

Here is the working example:

public class UserDataSerializer : SerializerBase<UserData>
{
    public override void Serialize(BsonSerializationContext context, BsonSerializationArgs args, UserData value)
    {
        context.Writer.WriteStartDocument();
        context.Writer.WriteName("Id");
        context.Writer.WriteInt32(value.Id);
        context.Writer.WriteName("Name");
        context.Writer.WriteString(value.Name);

        WriteKeyValue(context.Writer, value.Custom1);
        WriteKeyValue(context.Writer, value.Custom2);

        context.Writer.WriteEndDocument();
    }

    private void WriteKeyValue(IBsonWriter writer, KeyValuePair<string, double> kv)
    {
        writer.WriteName(kv.Key);
        writer.WriteDouble(kv.Value);
    }

    public override UserData Deserialize(BsonDeserializationContext context, BsonDeserializationArgs args)
    {
        //TODO: implement data deserialization using context.Reader
        throw new NotImplementedException();
    }
}

To make it work you also need to register our UserDataSerializer somehow. IBsonSerializationProvider is the easiest way to achieve that.

public class UserDataSerializationProvider : IBsonSerializationProvider
{
    public IBsonSerializer GetSerializer(Type type)
    {
        if (type == typeof(UserData)) return new UserDataSerializer();
        return null;
    }
}

And finally we can use it.

//register our serialization provider
BsonSerializer.RegisterSerializationProvider(new UserDataSerializationProvider());

var userdata = new UserData();
var doc = userdata.ToBsonDocument();

Here is the result:

{ "Id" : 100, "Name" : "Superuser", "HelloWorld1" : 1.0, "HelloWorld2" : 2.0 }

You may also want to implement UserDataSerializer.Deserialize() method in order to provide the backward conversion. It can be done in the same way using context.Reader.

More information about custom serialization process can be found here.