MongoDB - override default Serializer for a C# primitive type

I'd like to change the representation of C# Doubles to rounded Int64 with a four decimal place shift in the serialization C# Driver's stack for MongoDB. In other words, store (Double)29.99 as (Int64)299900

I'd like this to be transparent to my app. I've had a look at custom serializers but I don't want to override everything and then switch on the Type with fallback to the default, as that's a bit messy.

I can see that RegisterSerializer() won't let me add one for an existing type, and that BsonDefaultSerializationProvider has a static list of primitive serializers and it's marked as internal with private members so I can't easily subclass.

I can also see that it's possible to RepresentAs Int64 for Doubles, but this is a cast not a conversion. I need essentially a cast AND a conversion in both serialization directions.

I wish I could just give the default serializer a custom serializer to override one of it's own, but that would mean a dirty hack.

Am I missing a really easy way?

Solution

You can definitely do this, you just have to get the timing right. When the driver starts up there are no serializers registered. When it needs a serializer, it looks it up in the dictionary where it keeps track of the serializers it knows about (i.e. the ones that have been registered). Only it it can't find one in the dictionary does it start figuring out where to get one (including calling the serialization providers) and if it finds one it registers it.

The limitation in RegisterSerializer is there so that you can't replace an existing serializer that has already been used. But that doesn't mean you can't register your own if you do it early enough.

However, keep in mind that registering a serializer is a global operation, so if you register a custom serializer for double it will be used for all doubles, which could lead to unexpected results!

Anyway, you could write the custom serializer something like this:

public class CustomDoubleSerializer : BsonBaseSerializer
{
    public override object Deserialize(BsonReader bsonReader, Type nominalType, Type actualType, IBsonSerializationOptions options)
    {
        var rep = bsonReader.ReadInt64();
        return rep / 100.0;
    }

    public override void Serialize(BsonWriter bsonWriter, Type nominalType, object value, IBsonSerializationOptions options)
    {
        var rep = (long)((double)value * 100);
        bsonWriter.WriteInt64(rep);
    }
}

And register it like this:

BsonSerializer.RegisterSerializer(typeof(double), new CustomDoubleSerializer());

You could test it using the following class:

public class C
{
    public int Id;
    public double X;
}

and this code:

BsonSerializer.RegisterSerializer(typeof(double), new CustomDoubleSerializer());

var c = new C { Id = 1, X = 29.99 };
var json = c.ToJson();
Console.WriteLine(json);

var r = BsonSerializer.Deserialize<C>(json);
Console.WriteLine(r.X);