Are static class instances unique to a request or a server in ASP.NET?

Question

On an ASP.NET website, are static classes unique to each web request, or are they instantiated whenever needed and GCed whenever the GC decides to disposed of them?

The reason I ask is because I've written some static classes before in C# and the behavior is different than I would have expected. I would have expected static classes to be unique to each request, but it doesn't seem like that is the case.

If they are not unique to each request, is there a way to allow them to be?

UPDATE:
The answer driis gave me was exactly what I needed. I was already using a singleton class, however it was using a static instance and therefore was being shared between requests even if the users were different which in this case was a bad thing. Using HttpContext.Current.Items solves my problem perfectly. For anyone who stumbles upon this question in the future, here is my implementation, simplified and shortened so that it easy to understand the pattern:

using System.Collections;
using System.Web;

public class GloballyAccessibleClass
{
    private GloballyAccessibleClass() { }

    public static GloballyAccessibleClass Instance
    {
        get
        {
            IDictionary items = HttpContext.Current.Items;
            if(!items.Contains("TheInstance"))
            {
                items["TheInstance"] = new GloballyAccessibleClass();
            }
            return items["TheInstance"] as GloballyAccessibleClass;
        }
    }
}

Answer 1

Since the types are contained in an app domain, I would expect static classes to be present as long as the app domain is not recycled, or if the request gets served by a different app domain.

I can think of several ways to make objects specific to a particular request depends on what you want to do, for e.g. you could instantiate the object in Application.BeginRequest and then store it in HttpRequest object so that it can be accessed by all objects in the request processing pipeline.

Answer 2

If they are not unique to each request, is there a way to allow them to be?

Nope. Static members are owned by the ASP.NET process and shared by all users of the Web app. You'll need to turn to other session management techniques such as session variables.

Answer 3

Your static classes and static instance fields are shared between all requests to the application, and has the same lifetime as the application domain. Therefore, you should be careful when using static instances, since you might have synchronization issues and the like. Also bear in mind, that static instances will not be GC'ed before the application pool is recycled, and therefore everything that is referenced by the static instance, will not be GC'ed. This can lead to memory usage problems.

If you need an instance with the same lifetime as a request, I would suggest to use the HttpContext.Current.Items collection. This is by design meant to be a place to store stuff that you need througout the request. For nicer design and readability, you can use the Singleton pattern to help you manage these items. Simply create a Singleton class that stores its instance in HttpContext.Current.Items. (In my common library for ASP.NET, I have a generic SingletonRequest class for this purpose).

Answer 4

Normally static methods, properties and classes are common at the Application level. As long as the application lives, they are shared.

You can specify a different behaviour by using the ThreadStatic attribute. In that case they will be specific to the current thread, which, I think, is specific for each request.
I would not advise this though as it seems overcomplicated.

You can use HttpContext.Current.Items to set stuff up for one request, or HttpContext.Current.Session to set stuff up for one user (across requests).

In general though, unless you have to use things like Server.Transfer, the best way is basically creating things once and then passing them explicitly via method invocation.