Avoid static variables in ASP.NET

Occasionally, I like to write static methods on classes. They are useful whenever the method loosely relates to to the class – but it doesn’t involve a specific instance of the class.

A good example of a static method in the .NET library is the int.Parse(string val) method. This method basically reads a string and tries to return an integer representation. The method logically fits with the int Class (I suppose a string could have a ToInt() instance method…but that would be overwhelming as you added ToFloat(), ToDecimal(), To…()) – but when you run it, there’s no reason to have an instance of int already. You run it to create a new instance of int.

Commonly, I follow similar logic in the business layer of my webapps. I write a static Add(param1,….,paramx) method that returns an instance of the class after it is persisted to the database. I could accomplish this other ways, but I think the resulting application code reads better like:

      User.Add(username,password);

Than:

      new User(username,password).Add();

or, worse:

      DataContext.Users.InsertOnSubmit(new User(username,password));
      DataContext.SubmitChanges();

The issue with static methods in your business logic is that you often need a common object to talk to the database through: a SqlConnection, a TableAdapter, A LINQ DataContext, etc. I could certainly create those objects locally inside each static method, but that’s time consuming and hard to maintain. I want instead to define a common property (in the business class or a parent class of it) that lazy-initializes and returns an instance of the object when I need it. The problem is that the property must also be static for a static method to have access to it.

The easiest way to accomplish this is something like:

      private static ModelDataContext dataContext=null;
      protected static ModelDataContext DataContext
      {
            get
            {
                 if(dataContext==null)
                     dataContext = new ModelDataContext();
                 return dataContext;
             }
       }

The tricky thing is that this will probably work in development and testing, until you get some load on your website. Then, you’ll start seeing all kinds of weird issues that may not even point to this code as the problem.

Why is this an issue? It’s all about how static variables are scoped inside a ASP.NET application. Most web programmers think of each page in their application as its own program. You have to manually share data between pages when you need to. So, they incorrectly assume that static variables are somehow tied to a web request or page in their application. This is totally wrong.

Really, your whole website is a single application, which spawns threads to deal with requests, and requests are dealt with by the code on the appropriate page. Think of a page in your webapp as a method inside of one big application, and not an application of its own – a method which is called by the url your visitor requests.

Why does this matter? Because static variables are not tied to any specific instance of any specific class, they must be created in the entire application’s scope. Effectively, ASP.NET static variables are the same as the global variables that all your programming teachers warned you about.

That means that, for the property above, every single request/page/user of your website will reuse the first created instance of DataContext created. That’s bad for several reasons. LINQ DataContexts cache some of the data and changes you make – you can quickly eat up memory if each instance isn’t disposed fairly quickly. TableAdapters hold open SQLConnections for reuse – so if you use enough classes of TableAdapters, you can have enough different static vars to tie up all of your db connections. Because requests can happen simultaneously, you can also end up with lots of locking/competing accesses to the variable. Etc.

What should you do about it? In my case, I take advantage of static properties that reference collections that are scoped to some appropriately short-lived object for my storage. For instance, System.Web.HttpContext.Current.Items:

      protected static ModelDataContext DataContext
      {
            get
            {
                 if(System.Web.HttpContext.Current.Items["ModelDataContext"]==null)
                     System.Web.HttpContext.Current.Items["ModelDataContext"] = new ModelDataContext();
                 return (ModelDataContext)System.Web.HttpContext.Current.Items["ModelDataContext"];
             }
       }

In this case, each instance of DataContext will automatically be disposed for each hit on the site – and DataContexts will never be shared between two users accessing the site simultaneously. You could also use collections like ViewState, Session, Cache, etc (and I’ve tried several of these). For my purposes, the HttpContext.Items collection scopes my objects for exactly where I want them to be accessible and exactly how long I want them to be alive.