C＃类库 - 单例设计模式

Question

Background/Question:

I'm fairly new to the singleton design pattern. I've used it once in a web application (with the help of the SO community):

public static AppGlobal Instance
{
    get
    {
        if (HttpContext.Current.Session != null)
        {
            HttpSessionState session = HttpContext.Current.Session;

            if (session["AppGlobalInstance"] == null)
            {
                session["AppGlobalInstance"] = new AppGlobal();
            }

            return (AppGlobal)session["AppGlobalInstance"];
        }
        else
        {
            return null;
        }
    }
}

The above implementation makes sense to me because the instance of the AppGlobal is stored in the session. When the session dies, AppGlobal dies. What happens if I use the same design pattern in a class library that is called by a web application ? For example, the users requests a page that calls methods in a DLL that doesn't know about the session. Will the data stored in the singleton instance be persisted through multiple calls?

private static readonly Singleton instance = new Singleton();
private Singleton() { }

public static Singleton Instance
{
    get
    {
        return instance;
    }
}

Additional Information:

Here's what I'm trying to accomplish: I have a web application that is going to receive XML requests from a third party application. This XML will tell my web application to do one of three things (or all three of them). I would like to have a singleton instance of a class that stores data that can be accessed by multiple classes. I want the singleton instance to DIE after each request. If the above doesn't accomplish this, what's the best way to accomplish it?

Note: This web application runs on a single server and will never run on a farm.

EDIT 1:

Based on the suggestion below, I've used System.Web.HttpContext.Current.Session to store my class instance. Does this look like the correct approach for a singleton that will be unique to each session (remember I'm in a class library)?

    public static Ariba Instance
    {
        get
        {
            if (HttpContext.Current.Session != null)
            {
                HttpSessionState session = HttpContext.Current.Session;

                if (session["AribaInstance"] == null)
                {
                    session["AribaInstance"] = new Ariba();
                }

                return (Ariba)session["AribaInstance"];
            }
            else
            {
                return null;
            }
        }
    }

Answer 1

It will be persisted through multiple calls, but there is one caveat. The static variables are scoped to the AppDomain, so any time the IIS worker process is recycled, any data stored in a static variable will be lost. The same is true of session data, if you're storing it "in proc."

If you want an object that will only exist for the duration of the HTTP request, you can use the HttpContext.Items property.

Answer 2

Because the singleton is static, your data will be available for all the requests in your web application, so it will not be available only for the session.

But in ASP.NET applications, you should avoid using Singletons. Instead you should use the Application object. Main reason for that is that if you will use a web farm then your singleton is no longer singelton for the application scope but only on the machine.

Answer 3

Oh! If you want to use the instance PER REQUEST, why don't you pass it as a parameter to the methods you are calling or as a constructor parameter for the classes that requires the xml. This will be the best design approach, I think.