Singleton Data Repository - Gang of Four Singleton Pattern Simple Sample

In this article we create a singleton Data Repository for MVC. A Singleton class is a class of which only a single instance can exist: is one of the 23 software patterns by the Gang of Four. The Singleton Pattern become really useful in an MVC application, because instead of re-creating the Repository 1) each time a request comes to the MVC app, and 2) also for every different user which browser to the site, it is created ONLY ONE time and kept for the whole lifetime of the MVC application, that means, until the IIS application pool is reset.

First i give you the whole c# code to create a Singleton Data Repository for Asp.Net MVC. It will be  connected to a single XML file , and will be exposing all CRUD operations, using the XDocument class. The basic class will be "Message", and the Repository , "IMessagesRepository". The initial XML file is placed at the end of this post, if you want to use it as bootstrap.
After giving you the Repository code for COPY-PASTE, we'll explain how it works, and we'll compare its behavior with a non-singleton MVC application.

  The whole application can show as follows (i'm using the Twitter Bootstrap: HERE you have a tutorial for installing it in 5 minutes):

This is the code for the Singleton Data Repository. Just COPY-PASTE it in your Models folder and use it inside the Controller: 

namespace RichFormApplication.Models
{

    public class Message
    {
        public int ID { get; set; }
        public string To { get; set; }
        public string Sender { get; set; }
        public string Title { get; set; }
        public string Contents { get; set; }
    }


    public interface IMessagesRepository
    {
        IEnumerable GetAll();
        Message Get(int id);
        Message Add(Message item);
        bool Update(Message item);
        bool Delete(int id);
    }


    public class MessagesRepository : IMessagesRepository
    {
        static private int test;
        private List<Message> Messages = new List<Message>();
        private int iNumberOfMsgs = 1;
        private XDocument doc;

        #region Singleton Init
        private static MessagesRepository _RepositoryInstance = new MessagesRepository();
        public static MessagesRepository RepositoryInstance
        {
            get
            {
                test = 1;
                return _RepositoryInstance;
            }
        }
        #endregion

        private MessagesRepository()
        {
            doc = XDocument.Load(HttpContext.Current.Server.MapPath("~/App_Data/data.xml"));
            foreach (var node in doc.Descendants("note"))
            {
                Messages.Add(new Message
                {
                    ID = Int32.Parse(node.Descendants("id").FirstOrDefault().Value),
                    To = node.Descendants("to").FirstOrDefault().Value,
                    Sender = node.Descendants("from").FirstOrDefault().Value,
                    Title = node.Descendants("heading").FirstOrDefault().Value,
                    Contents = node.Descendants("body").FirstOrDefault().Value
                });
            }

            iNumberOfMsgs = Messages.Count;
        }

        public IEnumerable GetAll()
        {
            return Messages;
        }
        public Message Get(int id)
        {
            return Messages.Find(p => p.ID == id);
        }
        public Message Add(Message item)
        {
            if (item == null)
            {
                throw new ArgumentNullException("item");
            }

            item.ID = iNumberOfMsgs++;

            XElement newNode = new XElement("note");
            XElement id = new XElement("id"); id.Value = item.ID.ToString();
            XElement to = new XElement("to"); to.Value = item.To;
            XElement from = new XElement("from"); from.Value = item.Sender;
            XElement Title = new XElement("heading"); Title.Value = item.Title;
            XElement Contents = new XElement("body"); Contents.Value = item.Contents;
            newNode.Add(id, to, from, Title, Contents);
            doc.Root.Add(newNode);
            SaveToXML();
            return item;
        }
        public bool Update(Message item)
        {            
            XElement Message = doc.Descendants("Message").Where(n => Int32.Parse(n.Descendants("id").FirstOrDefault().Value) == item.ID).FirstOrDefault();
            Message.Descendants("to").FirstOrDefault().Value = item.To;
            Message.Descendants("from").FirstOrDefault().Value = item.Sender;
            Message.Descendants("heading").FirstOrDefault().Value = item.Title;
            Message.Descendants("body").FirstOrDefault().Value = item.Contents;
            SaveToXML();
            return true;
        }
        public bool Delete(int id)
        {
            doc.Root.Descendants("note").Where(n => Int32.Parse(n.Descendants("id").First().Value) == id).Remove();
            SaveToXML();
            return true;
        }

        private void SaveToXML()
        {
            doc.Save(HttpContext.Current.Server.MapPath("~/App_Data/data.xml"));
        }
    }
}

Take a look at the red code : the steps for creating the Singleton are:
1) change the "public" for a "private" constructor;
2) declare a "public static" property which returns an INSTANCE of the class
3) that static instance will be created by the .Net CLR runtime , at the private static field containing the
         instance, at the precise instant in which someone calls at least one of the properties or methods
         of the class. The static instance of the class will survive for all lifetime of the MVC application.

To use the Repository just type the following code in the Controller:

        public ActionResult Index()
        {
            return View(MessagesRepository.RepositoryInstance.GetAll());
        }

Now let's test the singleton: browse to the website and put in debugger two breakpoints inside the Repository:

There is no initialization of the Repository , as it is static and its constructor is private.
You'll see that the first time some user browse to the site, the repository is initialized:

Try a second time, refreshing the web page: the repository will not be created again, because it already exists:

Try now as a new user, opening a second session in another browser tab, to see that you keep using the same repository as the first user does.

This is the XML initial file:

<?xml version="1.0" encoding="utf-8"?>
<notes>
  <note>
    <id>0</id>
    <to>Fry</to>
    <from>Leela</from>
    <heading>Reminder</heading>
    <body>Don't forget me this weekend!!!</body>
  </note>
  <note>
    <id>1</id>
    <to>Leela</to>
    <from>Fry</from>
    <heading>Finally!!!</heading>
    <body>Leela, have you asked permission from the Professor?</body>
  </note>
  <note>
    <id>2</id>
    <to>Fry</to>
    <from>Leela</from>
    <heading>Rejection</heading>
    <body>You know what? In second thoughts, i'm going out with Lars this weekend. Sorry...</body>
  </note>

</notes>

That's all concerning to the Singleton Repository.

If you are asking yourself about the XML related code, some relevant points about the XML include:

 We are using the Descendants("") method and the Load() method, for reading the XML file into memory and then inside the List<>.
The Get methods use Find(p => p):

To add a new node, we CREATE an XElement with all its XElement CHILDREN , and then add it to the ROOT (be careful adding them to the root) of the XDocument:

That's all!!  

Happy programming.....

        By Carmel Schvartzman

כתב: כרמל שוורצמן