Singleton vs Cache ASP.NET

Question

I have created a Registry class in .NET which is a singleton. Apparently this singleton behaves as if it were kept in the Cache (the singleton object is available to each session). Is this a good practice of should I add this Singleton to the Cache? + do I need to wacth out for concurrency problems with the GetInstance() function?

namespace Edu3.Business.Registry
{
    public class ExamDTORegistry
    {
        private static ExamDTORegistry instance;
        private Dictionary<int, ExamDTO> examDTODictionary;

        private ExamDTORegistry()
        {
            examDTODictionary = new Dictionary<int, ExamDTO>();
        }

        public static ExamDTORegistry GetInstance()
        {
            if (instance == null)
            {
                instance = new ExamDTORegistry();
            }
            return instance;
        }
    }
}

Answer 1

Well, your GetInstance method certainly isn't thread-safe - if two threads call it at the same time, they may well end up with two different instances. I have a page on implementing the singleton pattern, if that helps.

Does your code rely on it being a singleton? Bear in mind that if the AppDomain is reloaded, you'll get a new instance anyway.

I don't really see there being much benefit in putting the object in the cache though. Is there anything you're thinking of in particular?

Answer 2

Despite their presence in GoF singletons are generally considered bad practice. Is there any reason why you wish to have only one instance?

Answer 3

HttpContext.Cache is available to all sessions, but items in the cache can be removed from memory when they expire or if there is memory pressure.

HttpContext.Application is also available to all sessions and is a nice place to store persistent, application-wide objects.

Since you've already created a singleton and it works, I don't see why should use one of the ones built-in singleton collections instead, unless you need the extra functionality that Cache gives you.