namespace myNS
{
    public enum ReplaceOpt{notReplace, oldReplace, alwaysReplace};

    [DataContract]
    class BUProject
    {
        [DataMember]
        public List<Position> Positions;

        public BUProject()
        {
            this.Positions = new List<Position>();
        }

        public void addPosition(Position p)
        {
            this.Positions.Add(p);
        }

        public void toJSON()
        {
            DataContractJsonSerializer serializer = new DataContractJsonSerializer(typeof(BUProject));
            MemoryStream stream = new MemoryStream();
            XmlWriter writer = new XmlTextWriter("D:\\test.xml", null);

            serializer.WriteObject(stream, this);
            writer.Close();
        }
    }

    [DataContract]
    public class Position
    {
        [DataMember]
        public string SourceString;
        [DataMember]
        public FileReplaceOpt frOpt;
        [DataMember]
        public List<String> Targets;

        public Position()
        {
            this.Targets = new List<String>();
        }

        public override string ToString() { return SourceString; }

        public void addTarget(string s)
        {
            this.Targets.Add(s);
        }
    }
}

 public void toJSON()