MongoDb PreserveValueIfNull

gistfile1.cs

    static class MongoSerializationExtensions
    {
        public static BsonMemberMap PreserveValueIfNull(this BsonMemberMap memberMap)
        {
            var originalSetter = memberMap.Setter;
            Action<object, object> preserveSetter = (obj, value) => {
                
                if (value != null)
                    originalSetter(obj, value);
            };

            var field = typeof(BsonMemberMap).GetField("_setter", BindingFlags.NonPublic | BindingFlags.Instance);
            if (field == null)
                throw new InvalidOperationException("BsonMemberMap format has changed, please correct this dirty hack.");

            field.SetValue(memberMap, preserveSetter);

            return memberMap;
        }
    }

// Usage
   ...
        public static void Install()
        {
            var database = MongoDatabase.Create("server=localhost; database=test");
            if (!database.CollectionExists("test"))
                database.CreateCollection("test");

            BsonClassMap.RegisterClassMap<Test>( cm => {
                cm.AutoMap();
                cm.MapField(t => t.Collection).PreserveValueIfNull(); 
            });
        }
   ...