Dear Developers,

I can't quite fully understand the details of the Instance machanism in this framework, could someone please explain how it works? For example, lets take Articles:


The text says that
   "Once the provider is created for the first time, it is saved in a static private property and won't be recreated again until the web application is shut down and restarted (for example, when IIS is stopped and restarted, or when the web.config file is changed"



There are calls such as this:

---

List<ArticleDetails> recordset = SiteProvider.Articles.GetArticles(
               categoryID, GetPageIndex(startRowIndex, maximumRows), maximumRows);

---

In my understanding this call references the static private property and calls its GetArticles() method, right???


But where is the statement that calls the following code in order to create the instance the first time the application is started?

      static private ArticlesProvider _instance = null;
      /// <summary>
      /// Returns an instance of the provider type specified in the config file
      /// </summary>
      static public ArticlesProvider Instance
      {
         get
         {
            if (_instance == null)
               _instance = (ArticlesProvider)Activator.CreateInstance(
                  Type.GetType(Globals.Settings.Articles.ProviderTyp e));
            return _instance;
         }
      }
   }
}

Thanks!

__________________
Alex

- TheBeerHouse Mods Repository
http://www.sashka.com/TheBeerHouse/thebeerhouse.html

A static property doesn't need an instance for the caller to use it. A caller just specifies the classname followed by ".Instance" and that will perform the actions in the getter. That checks to see if there's already an instance constructed by examining the private static member named _instance to see if it's null. If it's not null, then he knows there's already an instance created so he passes that back. If it is null that means he needs to create an instance, and then he passes that back (and remembers it in _instance).

This is similar to the "lazy loading" pattern to cache data for this site. The idea is that you don't create an instance until someone needs one, and after that you keep handing out a pointer to the same instance. The caller won't know if he's the first to use an instance or not.

You have to be very careful with private instance fields in this case. You can't store user specific data in private fields unless you first get a lock. Locking would not be desireable in this kind of code, and you'd normally put user-specific data elsewhere, like in session state if you need to persist it.

You also have to be careful about garbage collection in this model. Once he creates the instance and sticks it in the private static field, that prevents it from being garbage collected for the life of the app domain. That is normally the behavior you want when you use this model, but you must be careful about that data that instance might be accumulating since it won't be cleaned up.

This is a good choice for a class containing only methods, which could also be implemented as a fixed static class.

You might be wondoring why he went to so much trouble, when a simple static class (or even an instance class) might have worked fine. The reason is that he wants to be able to instantiate a different flavor of class depending on the provider specified in configuration. Marco designed this site to support different back-ends. He only implemented a back-end for SQL Server due to time and space constraints in the book, but the infrastructure is there for people who wish to expand on it.

Eric

Thanks, I think I got it now - when the call is made such as:
   commentCount = SiteProvider.Articles.GetCommentCount();

a call is implicitly made to .Articles first before calling the GetCommentCount() method ?

No, you need to look at how SiteProvider was created before that line of code. You can just search in all the files for ".Instance" (case sensitive) to see where these calls are being made. I don't have the code with me now, but it should be easy to find with a search.

Eric

This is a snippet of code in SiteProvider:

namespace MB.TheBeerHouse.DAL
{
   public static class SiteProvider
   {
      public static ArticlesProvider Articles
      {
         get { return ArticlesProvider.Instance; }
      }


but I couldn't find any code that calls .Articles, or that creates the SiteProvider...

I still haven't had any luck finding the code that makes the call to "public static ArticlesProvider Articles" in SiteProvider.cs, which in turn calls "get { return ArticlesProvider.Instance; }"

Can somebody please post the module and line number which makes this call?