public interface IReagent
{
    int ItemId { get; set; }
    int Quantity { get; set; }
    ConcurrentBag<IReagent> Reagents { get; set; }
}

public class Craft : IReagent
{
    public int ItemId { get; set; }
    public int Quantity { get; set; }
    public int SpellId { get; set; }
    public int Skill { get; set; }
    public Profession Profession { get; set; }
    public ConcurrentBag<IReagent> Reagents { get; set; }
}

public class Reagent : IReagent
{
    public int ItemId { get; set; }
    public int Quantity { get; set; }
    public ConcurrentBag<IReagent> Reagents { get; set; }
}

/* 28 */
{
  "_id" : ObjectId("4e497efa97e8b617f0d229d4"),
  "ItemId" : 52186,
  "Quantity" : 0,
  "SpellId" : 0,
  "Skill" : 475,
  "Profession" : 8,
  "Reagents" : { }
}

{
    "_id" : ObjectId("4e497efa97e8b617f0d229d4"),
    "ItemId" : 52186,
    "Quantity" : 0,
    "SpellId" : 0,
    "Skill" : 475,
    "Profession" : 8,
    "Reagents" : [
        {
            "ItemId" : 521833,
            "Quantity" : 3,
            "SpellId" : 0,
            "Skill" : 400,
            "Profession" : 7,
            "Reagents" : [
                {
                    "ItemId" : 52186,
                    "Quantity" : 3,
                    "SpellId" : 0,
                    "Skill" : 475,
                    "Profession" : 8,
                    "Reagents" : [
                        {
                            "ItemId" : 52183,
                            "Quantity" : 2,
                            "Reagents" : []
                        },
                        {
                            "ItemId" : 521832,
                            "Quantity" : 1,
                            "Reagents" : []
                        }
                    ]
                },
                {
                    "ItemId" : 52386,
                    "Quantity" : 2
                    "SpellId" : 0,
                    "Skill" : 400,
                    "Profession" : 8,
                    "Reagents" : [
                        {
                            "ItemId" : 52383,
                            "Quantity" : 2,
                            "Reagents" : []
                        },
                        {
                            "ItemId" : 523832,
                            "Quantity" : 1,
                            "Reagents" : []
                        }
                    ]
                }
            ]
        }
    ]
}

public class FieldsWrapper : IBsonSerializable
    {
        public List<DataFieldValue> DataFieldValues { get; set; }


        public object Deserialize(MongoDB.Bson.IO.BsonReader bsonReader, Type nominalType, IBsonSerializationOptions options)
        {
        if (nominalType != typeof(FieldsWrapper)) throw new ArgumentException("Cannot deserialize anything but self");
        var doc = BsonDocument.ReadFrom(bsonReader);
        var list = new List<DataFieldValue>();
        foreach (var name in doc.Names)
        {
            var val = doc[name];
            if (val.IsString)
                list.Add(new DataFieldValue {LocalIdentifier = name, Values = new List<string> {val.AsString}});
            else if (val.IsBsonArray)
            {
                DataFieldValue df = new DataFieldValue {LocalIdentifier = name};
                foreach (var elem in val.AsBsonArray)
                {
                    df.Values.Add(elem.AsString);
                }
                list.Add(df);
            }
        }
        return new FieldsWrapper {DataFieldValues = list};
        }


        public void Serialize(MongoDB.Bson.IO.BsonWriter bsonWriter, Type nominalType, IBsonSerializationOptions options)
        {
            if (nominalType != typeof (FieldsWrapper))
                throw new ArgumentException("Cannot serialize anything but self");
            bsonWriter.WriteStartDocument();
            foreach (var dataFieldValue in DataFieldValues)
            {

                bsonWriter.WriteName(dataFieldValue.LocalIdentifier);
                if (dataFieldValue.Values.Count != 1)
                {
                    var list = new string[dataFieldValue.Values.Count];
                    for (int i = 0; i < dataFieldValue.Values.Count; i++)
                        list[i] = dataFieldValue.Values[i];
                    BsonSerializer.Serialize(bsonWriter, list);
                }
                else
                {
                    BsonSerializer.Serialize(bsonWriter, dataFieldValue.Values[0]);
                }
            }
            bsonWriter.WriteEndDocument();
        }

    }

/* 3131 */
{
  "_id" : ObjectId("4e4d58df77d2cf00691aaef2"),
  "ItemId" : 28432,
  "Quantity" : 0,
  "Reagents" : [{
      "ItemId" : 23448,
      "Quantity" : 0,
      "Reagents" : [{
          "ItemId" : 23447,
          "Quantity" : 0,
          "Reagents" : [{
              "ItemId" : 23427,
              "Quantity" : 2,
              "Reagents" : []
            }]
        }, {
          "ItemId" : 23445,
          "Quantity" : 0,
          "Reagents" : [{
              "ItemId" : 23424,
              "Quantity" : 2,
              "Reagents" : []
            }]
        }]
    }, {
      "ItemId" : 23572,
      "Quantity" : 8,
      "Reagents" : []
    }, {
      "ItemId" : 28431,
      "Quantity" : 0,
      "Reagents" : [{
          "ItemId" : 23571,
          "Quantity" : 0,
          "Reagents" : [{
              "ItemId" : 21885,
              "Quantity" : 1,
              "Reagents" : []
            }, {
              "ItemId" : 21884,
              "Quantity" : 1,
              "Reagents" : []
            }, {
              "ItemId" : 22451,
              "Quantity" : 0,
              "Reagents" : [{
                  "ItemId" : 21885,
                  "Quantity" : 1,
                  "Reagents" : []
                }]
            }, {
              "ItemId" : 22452,
              "Quantity" : 1,
              "Reagents" : []
            }, {
              "ItemId" : 22457,
              "Quantity" : 0,
              "Reagents" : [{
                  "ItemId" : 21884,
                  "Quantity" : 1,
                  "Reagents" : []
                }]
            }]
        }, {
          "ItemId" : 22456,
          "Quantity" : 0,
          "Reagents" : [{
              "ItemId" : 21885,
              "Quantity" : 1,
              "Reagents" : []
            }]
        }, {
          "ItemId" : 23573,
          "Quantity" : 0,
          "Reagents" : [{
              "ItemId" : 23446,
              "Quantity" : 0,
              "Reagents" : [{
                  "ItemId" : 23425,
                  "Quantity" : 2,
                  "Reagents" : []
                }]
            }]
        }, {
          "ItemId" : 23448,
          "Quantity" : 0,
          "Reagents" : [{
              "ItemId" : 23447,
              "Quantity" : 0,
              "Reagents" : [{
                  "ItemId" : 23427,
                  "Quantity" : 2,
                  "Reagents" : []
                }]
            }, {
              "ItemId" : 23445,
              "Quantity" : 0,
              "Reagents" : [{
                  "ItemId" : 23424,
                  "Quantity" : 2,
                  "Reagents" : []
                }]
            }]
        }]
    }]
}

public void Serialize(MongoDB.Bson.IO.BsonWriter bsonWriter, Type nominalType, MongoDB.Bson.Serialization.IBsonSerializationOptions options)
{
    if (nominalType != typeof(IReagent) && nominalType != typeof(Reagent) && nominalType != typeof(Craft))
    {
        throw new ArgumentException("Cannot serialize anything but self");
    }

    bsonWriter.WriteStartDocument();

    bsonWriter.WriteInt32("ItemId", this.ItemId);
    bsonWriter.WriteInt32("Quantity", this.Quantity);

    if (this.Reagents != null)
    {
        bsonWriter.WriteName("Reagents");
        bsonWriter.WriteStartArray();

        foreach (var r in this.Reagents)
        {
            BsonSerializer.Serialize(bsonWriter, r.GetType(), r, options);
        }

        bsonWriter.WriteEndArray();
    }

    bsonWriter.WriteEndDocument();

}