ASP.NET Threading - Double checked locking

Question

I have an extension method on the HttpApplicationState object for getting my IoC container out of the application. This same code will also create the container if it doesn't exist.

I have 2 questions:

1. Is my code actually thread safe as I intend it to be
2. Is this considered the best practice for dealing with the application state

Code as follows:

private const string GlobalContainerKey = "UnityContainerKey";

public static IUnityContainer GetContainer(this HttpApplicationState application)
{
    var container = application[GlobalContainerKey] as IUnityContainer;

    if (container == null)
    {
        try
        {
            application.Lock();
            container = application[GlobalContainerKey] as IUnityContainer;

            if (container == null)
            {
                container = new UnityContainer();
                application[GlobalContainerKey] = container;
            }
        }
        finally
        {
            application.UnLock();
        }
    }

    return container;
}

Answer 1

Technically, that won't work given the EMCA specification. Jon Skeet goes into this in his C# FAQ:

http://www.yoda.arachsys.com/csharp/singleton.html

Specifically, see the section with the "Third version"

I would read further down and use his suggestion for how to implement the singleton to see how to implement what you are trying to do.

Answer 2

Why dou you check for "container == null" the first time? I think you should lock first and then check for container being null. All sort of dodgy stuff may happen between the first if and the return in other threads.

Answer 3

You need to put

var container = application[GlobalContainerKey] as IUnityContainer;

in the lock as well, otherwise many threads may create a new container in sequence.

private const string GlobalContainerKey = "UnityContainerKey";
private const object lockObject = new object();

public static IUnityContainer GetContainer(this HttpApplicationState application)
{
    var IUnityContainer container = null;

    lock (lockObject)
    {
        container = application[GlobalContainerKey] as IUnityContainer;
        if (container == null)
        {
            container = new UnityContainer();
            application[GlobalContainerKey] = container;
        }
    }

    return container;
}

Answer 4

Double check with lock is used inside the .NET Framework's code, for singletons (see System.Web.Profile.ProfileManager for example).

So I think your implementation is OK.