Question

In a web app, I need to have only one instance of a class called ProcessManager. One way is to make it a singleton. The other way is to use the HttpApplicationState to make sure I always access the same instance, like this:

public static ProcessManager ProcessManager
        {
            get 
            {
                HttpApplicationState applicationState = HttpContext.Current.Application;
                if (applicationState["ProcessManager"] == null)
                {
                    applicationState["ProcessManager"] = new ProcessManager();
                }

                return (ProcessManager)applicationState["ProcessManager"];
            }
        } 

Which method is better and why?

Answer 1

Based on the limited description you’ve given, I would choose a Singleton, because then it doesn’t have a dependency on HttpContext.Current, and can be used outside of the ASP.Net pipeline (for example, when you want to write unit tests.)

(As an aside, when you set something into ApplicationState, you also need to first call Lock() on it, and then Unlock() it after you’re done writing to it, to make sure it’s thread safe.)

Alternately, allow injection of an HttpContext when you create your ProcessManager, such that you can use it with a mocked HttpContext.