Question

I’m calling my custom factory that I created (PhotoServiceFactory), which is a singleton that allows me to get at a specific custom service type back (in this case FacebookService). FacebookService is also a singleton. In FacebookService I’ve exposed an instance of FacebookAlbumPhoto through a property. I did this because then I don’t have to have a ton of the same code over and over again creating a new instance of FacebookAlbumPhoto…I can get an instance using the FacebookService’s property.

PhotoServiceFactory service = PhotoServiceFactory.CurrentPhotoServiceFactory;
FacebookService facebookService = (FacebookService)service.GetAPIService(APIType.Facebook);

FacebookAlbumPhoto facebookPhoto = facebookService.FacebookAlbumPhoto.GetFacebookAlbumPhoto(selectedPhotoID);

So this is all set up now, I created all this and just testing it now.

What’s happening is my code is bombing out at this line:

FacebookAlbumPhoto facebookPhoto = facebookService.FacebookAlbumPhoto.GetFacebookAlbumPhoto(selectedPhotoID);

The error I get is when I try to reference the facebookService.FacebookAlbumPhoto instance:

CurrentSession = ‘_singletonInstance.CurrentSession’ threw an exception of type ‘System.Threading.ThreadAbortException’

So I don’t know if it’s because the service singleton is on one thread and then it tries to reference another singleton that’s on a completely different thread and that’s just not possible? That it’s not possible to nest singletons like this? Or could this be another issue altogether? Cause I can’t see it.

Here’s my ServiceFactory:

public class PhotoServiceFactory
{
    private static PhotoServiceFactory _singletonInstance;
    private PhotoServiceFactory(){}

    public static PhotoServiceFactory CurrentPhotoServiceFactory
    {
        get
        {
            _singletonInstance = _singletonInstance ?? (_singletonInstance = new PhotoServiceFactory());
            return _singletonInstance;
        }
    }

    public object GetAPIService(APIType apiType)
    {
        object apiService = null;
        switch (apiType)
        {
            case APIType.Facebook:
                apiService = FacebookService.CurrentService;
                break;
            // rest of code
        }

        return apiService;
    }

So the main singleton here Service has a property to get its related Session:

Here’s the FacebookServiceClass:

public class FacebookService
{
    private static FacebookService _singletonInstance;

    private FacebookService(){}

    public FacebookSession CurrentSession
    {
        get
        {
            return FacebookSession.GetCurrentSession();
        }
    }

    /// <summary>
    /// Gets the current facebook service singleton instance.
    /// </summary>
    /// <value>The current facebook service.</value>
    public static FacebookService CurrentService
    {
        get
        {
            _singletonInstance = _singletonInstance ?? (_singletonInstance = new FacebookService());
            return _singletonInstance;
        }
    }

    public FacebookAlbumPhoto FacebookAlbumPhoto
    {
        get
        {
            return new FacebookAlbumPhoto(); // create an instance automatically so we can start working with this object
        }
    }
}

Here’s the session class:

public class FacebookSession
{
    const string loginCallbackUrl = "http://localhost/PhotoUpload/FacebookOauth.aspx";

    private FacebookSession()
    {
    }

    public string UserID { get; private set; }

    public static FacebookSession GetCurrentSession()
    {
        //....bunch of other logic is here
        FacebookSession facebookSession = CreateNewSession();
        return facebookSession;
    }

    public FacebookSession CreateNewSession()
    {
        //...some code here
        FacebookSession newFacebookSession = new FacebookSession
        //... rest of code...
        return newFacebookSession;
    }
    // ... rest of code
}

UPDATED:

As requested here’s my FacebookAlbumPhoto class that I created:

    public class FacebookAlbumPhoto : FacebookPhotoBase
    {
        private FacebookSession currentSession;

        public FacebookAlbumPhoto()
        {
            currentSession = FacebookService.CurrentService.CurrentSession;             
        }

        #region Methods

        public FacebookAlbumPhoto GetFacebookAlbumPhoto(string photoID)
        {
            ...more code
             FacebookPhotoRequest request = new FacebookPhotoRequest(currentSession.UserID, photoID);
            ...more code

            FacebookAlbumPhoto facebookPhoto = ParseFacebookPhoto(json);

            return facebookPhoto;
        }
...rest of code
}

Answer 1

Two things. First, remember to read over Skeet’s catalogue of singleton implementations.
Second, try breaking your code just before the spot where the exception occurs, and then bring up your “Exception” dialogue (ctrl-alt-e). Click the “throw” checkbox next to the CLR (second row of dialogue) and hit ok. Continue debugging your code. The results may tell you where the real problem is.

Don’t forgot to go back to the Exception dialogue and remove that check from the check box, after you are done. 🙂