ArcticFlare fix

Player.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Xml;

namespace Engine
{
    public class Player : LivingCreature
    {
        public int Gold { get; set; }
        public int ExperiencePoints { get; set; }
        public int Level
        {
            get { return ((ExperiencePoints / 100) + 1); }
        }
        public List<InventoryItem> Inventory { get; set; }
        public List<PlayerQuest> Quests { get; set; }
        public Location CurrentLocation { get; set; }       

        private Player(int currentHitPoints, int maximumHitPoints, int gold, int experiencePoitns) : 
            base (currentHitPoints, maximumHitPoints)
        {
            Gold = gold;
            ExperiencePoints = experiencePoitns;

            Inventory = new List<InventoryItem>();
            Quests = new List<PlayerQuest>();
        }

        public static Player CreateDefaultPlayer()
        {
            Player player = new Player(10, 10, 20, 0);
            player.Inventory.Add(new InventoryItem(World.ItemByID(World.ITEM_ID_RUSTY_SWORD), 1));
            player.CurrentLocation = World.LocationByID(World.LOCATION_ID_HOME);

            return player;
        }

        public static Player CreatePlayerFromXmlString(string xmlPlayerData)
        {
            try
            {
                XmlDocument playerData = new XmlDocument();

                playerData.LoadXml(xmlPlayerData);

                int currentHitPoints = Convert.ToInt32(playerData.SelectSingleNode("/player/Stats/CurrentHitPoints").InnerText);
                int maximumHitPoints = Convert.ToInt32(playerData.SelectSingleNode("/player/Stats/MaximumHitPoints").InnerText);
                int experiencePoints = Convert.ToInt32(playerData.SelectSingleNode("/player/Stats/ExperiencePoints").InnerText);
                int gold = Convert.ToInt32(playerData.SelectSingleNode("/player/Stats/Gold").InnerText);

                Player player = new Player(currentHitPoints, maximumHitPoints, gold, experiencePoints);

                int currentLocationID = Convert.ToInt32(playerData.SelectSingleNode("Player/Stats/CurrentLocation").InnerText);
                player.CurrentLocation = World.LocationByID(currentLocationID);

                foreach (XmlNode node in playerData.SelectNodes("/Player/InventoryItems/InventoryItem"))
                {
                    int id = Convert.ToInt32(node.Attributes["ID"].Value);
                    int quantity = Convert.ToInt32(node.Attributes["Quantity"].Value);

                    for (int i = 0; i < quantity; i++)
                    {
                        player.AddItemToInventory(World.ItemByID(id));
                    }
                }

                foreach (XmlNode node in playerData.SelectNodes("/Player/PlayerQuests/PlayerQuest"))
                {
                    int id = Convert.ToInt32(node.Attributes["ID"].Value);
                    bool isCompleted = Convert.ToBoolean(node.Attributes["IsCompleted"].Value);

                    PlayerQuest playerQuest = new PlayerQuest(World.QuestByID(id));
                    playerQuest.IsCompleted = isCompleted;

                    player.Quests.Add(playerQuest);
                }

                return player;
            }
            catch
            {
                return Player.CreateDefaultPlayer();
            }
        }

        public bool HasRequiredItemToEnterLocation(Location location)
        {
            if (location.ItemRequiredToEnter == null)
            {
                return true;
            }

            return Inventory.Exists(ii => ii.Details.ID == location.ItemRequiredToEnter.ID);
        }

        public bool HasThisQuest(Quest quest)
        {
            return Quests.Exists(pq => pq.Details.ID == quest.ID);
        }

        public bool CompletedQuest(Quest quest)
        {
            foreach (PlayerQuest pq in Quests)
            {
                if (pq.Details.ID == quest.ID)
                {
                    return pq.IsCompleted;
                }
            }
            return false;
        }

        public bool HasCompletionItems(Quest quest)
        {
            foreach (QuestCompletionItem qci in quest.QuestCompletionItems)
            {
                if (!Inventory.Exists(ii => ii.Details.ID == qci.Details.ID && ii.Quantity >= qci.Quantity))
                {
                    return false;
                }
            }
            return true;
        }

        public void RemoveQuestItems(Quest quest)
        {
            foreach (QuestCompletionItem qci in quest.QuestCompletionItems)
            {
                InventoryItem item = Inventory.SingleOrDefault(ii => ii.Details.ID == qci.Details.ID);

                if(item != null)
                {
                    item.Quantity -= qci.Quantity;
                }
            }
        }

        public void AddItemToInventory(Item itemToAdd)
        {
            InventoryItem item = Inventory.SingleOrDefault(ii => ii.Details.ID == itemToAdd.ID);

            if (item == null)
            {
                Inventory.Add(new InventoryItem(itemToAdd, 1));
            }
            else
            {
                item.Quantity++;
            }
        }

        public void MarkQuestCompleted(Quest quest)
        {
            PlayerQuest playerQuest = Quests.SingleOrDefault(pq => pq.Details.ID == quest.ID);

            if (playerQuest != null)
            {
                playerQuest.IsCompleted = true;
            }
        }

        public string ToXmlString()
        {
            XmlDocument playerData = new XmlDocument();

            //Create top level XML node
            XmlNode player = playerData.CreateElement("Player");
            playerData.AppendChild(player);

            //Create the 'Stats' child node to hold the other player stat nodes
            XmlNode Stats = playerData.CreateElement("Stats");
            player.AppendChild(Stats);

            //Create child nodes for 'stats' node
            XmlNode currentHitPoints = playerData.CreateElement("CurrentHitPoints");
            currentHitPoints.AppendChild(playerData.CreateTextNode(this.CurrentHitPoints.ToString()));
            Stats.AppendChild(currentHitPoints);

            XmlNode maximumHitPoints = playerData.CreateElement("MaximumHitPoints");
            maximumHitPoints.AppendChild(playerData.CreateTextNode(this.MaximumHitPoints.ToString()));
            Stats.AppendChild(maximumHitPoints);

            XmlNode gold = playerData.CreateElement("Gold");
            gold.AppendChild(playerData.CreateTextNode(this.Gold.ToString()));
            Stats.AppendChild(gold);

            XmlNode experiencePoints = playerData.CreateElement("ExperiencePoints");
            experiencePoints.AppendChild(playerData.CreateTextNode(this.ExperiencePoints.ToString()));
            Stats.AppendChild(experiencePoints);

            XmlNode currentLocation = playerData.CreateElement("CurrentLocation");
            currentLocation.AppendChild(playerData.CreateTextNode(this.CurrentLocation.ID.ToString()));
            Stats.AppendChild(currentLocation);

            //Create an InventoryItems node to hold inventory items
            XmlNode inventoryItems = playerData.CreateElement("InventoryItems");
            player.AppendChild(inventoryItems);

            foreach (InventoryItem item in this.Inventory)
            {
                XmlNode inventoryItem = playerData.CreateElement("InventoryItem");

                XmlAttribute idAttribute = playerData.CreateAttribute("ID");
                idAttribute.Value = item.Details.ID.ToString();
                inventoryItem.Attributes.Append(idAttribute);

                XmlAttribute quantityAttribute = playerData.CreateAttribute("Quantity");
                quantityAttribute.Value = item.Quantity.ToString();
                inventoryItem.Attributes.Append(quantityAttribute);

                inventoryItems.AppendChild(inventoryItem);
            }

            //Create the "PlayerQuests" node to hold my quests
            XmlNode playerQuests = playerData.CreateElement("PlayerQuests");
            player.AppendChild(playerQuests);

            foreach (PlayerQuest quest in this.Quests)
            {
                XmlNode playerQuest = playerData.CreateElement("PlayerQuest");

                XmlAttribute idAttribute = playerData.CreateAttribute("ID");
                idAttribute.Value = quest.Details.ID.ToString();
                playerQuest.Attributes.Append(idAttribute);

                XmlAttribute isCompletedAttribute = playerData.CreateAttribute("IsCompleted");
                isCompletedAttribute.Value = quest.IsCompleted.ToString();
                playerQuest.Attributes.Append(isCompletedAttribute);

                playerQuests.AppendChild(playerQuest);
            }

            return playerData.InnerXml; //  The XML document, as a string, so we can save the data.
        }
    }
}

PlayerData_FIXED.xml

<Player>
  <Stats>
    <CurrentHitPoints>10</CurrentHitPoints>
    <MaximumHitPoints>10</MaximumHitPoints>
    <Gold>20</Gold>
    <ExperiencePoints>0</ExperiencePoints>
    <CurrentLocation>4</CurrentLocation>
  </Stats>
  <InventoryItems>
    <InventoryItem ID="1" Quantity="1" />
  </InventoryItems>
  <PlayerQuests>
    <PlayerQuest ID="1" IsCompleted="False" />
  </PlayerQuests>
</Player>

PlayerData_ORIGINAL.xml

<Player>
  <Stats>
    <CurrentHitPoints>10</CurrentHitPoints>
    <MaximumHitPoints>10</MaximumHitPoints>
    <Gold>20</Gold>
    <ExperiencePoints>0</ExperiencePoints>
    <CurrentLocation>Engine.Location</CurrentLocation>
  </Stats>
  <InventoryItems>
    <InventoryItem Quantity="1" ID="1"/>
  </InventoryItems>
  <PlayerQuests>
    <PlayerQuest ID="1" IsCompleted="False"/>
  </PlayerQuests>
</Player>