public class MainMongo 
    {
        public MongoDB.Bson.ObjectId Id { get; set; }
    }

    public class DbRef<T> : MongoDB.Driver.MongoDBRef
    {
        public DbRef(string ColName, MongoDB.Bson.BsonValue Id) : base(ColName, Id)
        {

        }

        [MongoDB.Bson.Serialization.Attributes.BsonIgnore()]
        public T Value
        {
            get
            { 
                return new MongoDB.Driver.MongoClient("mongodb://localhost").GetServer().GetDatabase("Wik").FetchDBRefAs<T>(this);
            }
        }

    }

 
    public class Car : MainMongo
    { 
        public DateTime BuildDate { get; set; }

    }

    public class Player : MainMongo
    { 

        public string Name { get; set; }
        public List<DbRef<Car>> Cars { get; set; }

    }


    //Here is code that uses database

            var CarCol = mdb.GetCollection<Car>("Car");
            var PlayerCol = mdb.GetCollection<Player>("Player");
            CarCol.Drop();
            PlayerCol.Drop();


            var Ca = new Car() { BuildDate = DateTime.Now };
            CarCol.Save<Car>(Ca);

            var Pl = new Player() { Name = "Mohsen", Cars = new List<DbRef<Car>>(new DbRef<Car>[] { new DbRef<Car>("Car", Ca.Id) }) };
            PlayerCol.Save<Player>(Pl);

            var GetCarDbRef = PlayerCol.AsQueryable().OfType<Player>().FirstOrDefault().Cars.FirstOrDefault();
            bool IsNull1 = GetCarDbRef.CollectionName == null; //true
            bool IsNull2 = GetCarDbRef.DatabaseName == null;   //true
            bool IsNull3 = GetCarDbRef.Id == null;             //true