MongoDB C#: ID Serialization best pattern

I have a class User and I need to work with them in web services.

Then problem is that if I try to serialize Id that is type of BsonObjectId, I see that have an empty property, that have an empty property, and so on ...

I have write this workaround in order, it's is a good solution?

public partial class i_User 
{
    [BsonId(IdGenerator = typeof(BsonObjectIdGenerator))]
    [NonSerialized]
    public BsonObjectId _id;

    public String Id
    {
        get
        {
            return this._id.ToString();
        }
    }   
}   

In this way, I can keep _Id as BsonObjectId but I send an string representation over the web in the property Id.

Another solution is to work with StringObjectIdGenerator

public partial class i_User 
{
    [BsonId(IdGenerator = typeof(StringObjectIdGenerator))]
    public String id;
}

But is see that MongoDB will store a string into database instead of ObjectId.

What is the best approach in order to work in a serialization environmental like web services and/or an client-server (Flash+C#)?