Generic XMLgateway

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Linq.Expressions;
using System.Reflection;
using System.Text;
using System.Threading.Tasks;
using System.Xml;
using AR.Entities;
using AR.Gateways.XML;

namespace AR.Gateways
{
    public class UniversalGateway_XML<T> : IGateway<T> where T : BaseEntity
    {
        private XmlDocument document;
        private string mainName;
        private string mainListString;
        private XmlNode mainList;
        private string databaseFile;

        public UniversalGateway_XML(string p_databaseDirectory) {
            this.mainName = typeof(T).Name;
            this.mainListString = this.mainName + "List";
            this.databaseFile = p_databaseDirectory.TrimEnd('\\') + @"\" + mainName + "s.xml";

            if (!File.Exists(this.databaseFile))
            {
                using (FileStream fs = File.Create(this.databaseFile))
                {
                    using (StreamWriter sw = new StreamWriter(fs))
                    {
                        sw.WriteLine("<?xml version=\"1.0\"?><" + this.mainListString + "></" + this.mainListString + ">");
                    }
                }
            }

            this.document = new XmlDocument();
            this.document.Load(this.databaseFile);

            this.mainList = this.document.GetElementsByTagName(this.mainListString)[0];
        }

        private void saveXML()
        {
            this.document.Save(this.databaseFile);
        }

        private int getNextID()
        {
            int res = 1;

            try
            {
                XmlNodeList entitys = this.mainList.ChildNodes;
                foreach (XmlNode entit in entitys)
                {
                    int lastID = -1;
                    try
                    {
                        PropertyInfo[] ppi = typeof(BaseEntity).GetProperties();
                        lastID = int.Parse(entit[ppi[0].Name].InnerText);
                    }
                    catch (Exception e)
                    {
                        res = -1;
                        break;
                    }

                    if (lastID > -1)
                    {
                        if (lastID >= res)
                        {
                            res = lastID + 1;
                        }
                    }
                }
            }
            catch (Exception e)
            {
                res = -1;
            }

            return res;
        }

        public IEnumerable<T> List
        {
            get
            {
                throw new NotImplementedException();
            }
        }

        public bool Add(T entity)
        {
            bool res = false;

            try
            {
                XmlNode entityNode = this.document.CreateNode(XmlNodeType.Element, this.mainName, "");
                int newID = this.getNextID();
                if (newID > -1)
                {
                    entity.Id = newID;
                    foreach (PropertyInfo propertyInfo in typeof(User).GetProperties())
                    {
                        XmlNode node_property = this.document.CreateNode(XmlNodeType.Element, propertyInfo.Name, "");
                        node_property.InnerText = propertyInfo.GetValue(entity, null).ToString();
                        entityNode.AppendChild(node_property);
                    }
                    this.mainList.AppendChild(entityNode);

                    this.saveXML();
                    res = true;
                }
            }
            catch (Exception e)
            {
                res = false;
            }

            return res;
        }

        public bool Delete(T entity)
        {
            throw new NotImplementedException();
        }

        public T FindById(int Id)
        {
            throw new NotImplementedException();
        }

        public IQueryable<T> SearchFor(Expression<Func<T, bool>> predicate)
        {
            throw new NotImplementedException();
        }

        public bool Update(T entity)
        {
            throw new NotImplementedException();
        }
    }
}