The ASP.NET Singleton, Singleton classes for ASP.NET

This is a little trick I learned recently. I had a situation where I needed to use a static class in one of my ASP.NET web apps. I wanted it to provide functionality on a per-user basis, but when I put this in to practice I got some funny results and then I realised the obvious. Static classes are shared throughout any single instance of a .NET application, and each website runs within it's own w3wp.exe process, as a self contained application. This explained the funny behaviour.

So I did a little digging around and found the following code, which adapts the singleton pattern to store the instance in a per-user manner.

Each user is assigned an HttpContext for thier time visiting an ASP.NET web application. This Context object allows you to store objects.

So a classic Singleton might look something like this

class Singleton {
  
    // private instance field
    private static Singleton _instance;
  
    // public read-only instance property
    public static Singleton Instance {
        get{
            if(_instance == null){
                _instance = new Singleton();
            }
          
            return _instance;
        }
    }

    // private constructor
    private Singleton() {
        // constructor logic here
    }
}

All we have to do is to replace the private field for an Item in the HttpContect object, like so...

class Singleton {
  
    // public read-only instance property
    public static Singleton Instance {
        get{
            if(HttpContext.Current.Items["SingltonInstance"] == null){
                HttpContext.Current.Items.Add("SingltonInstance", new

Singleton());
            }
          
            // cast the item to a singleton because it is stored as a generic object
            return (Singleton)HttpContext.Current.Items["SingltonInstance"];
        }
    }

    // private constructor
    private Singleton() {
        // constructor logic here
    }
}