Should I use a singleton?

Question

This is a semi-related question to question to the following question I just raised:

http://stackoverflow.com/questions/1671259/utility-classes-good-or-bad

After determining that a class is to act as a cache for Url parsing rules stored in an XML file, I have considered that a singleton would solve my problem, but introduce global state (although it is only traveling in one direction XML -> parser), and static dependencies.

The design consideration that led me to consider a singleton was: (note that this is a web application that uses a module to catch and parse ALL requests using the same parser)

• I need to cache the url parsing rules stored in the XML, so the class needs to hang around between requests. I also have a method that parses the Url, given the rules, which determines the routing of the request at the HttpModule level.

Is a singleton valid in this case? How would you solve this?

Answer 1

Your solution sounds good to me. Particularly if the consuming code isn't modifying data then using a singleton or static class sounds like it shouldn't be a problem. I'm not sure if labeling it as a "singleton" is necessary although I suppose that's how it's behaving.

You could use either a static class that is inherently "cached" because it's in the ASPNET worker process memory, or you could explicitly cache it into the web cache. If you do the latter, you could benefit by adding a file dependency to the cache entry so any file changes would force a reload from cache.

Answer 2

I would consider storing the rules in the HttpContext.Cache which is available to all sessions. You could rebuild the cache anytime it was unloaded (due to lack of usage).

Answer 3

It's so easy to get an IoC container to provide a singleton for you that I think it would be very hard to justify using the traditional Singleton pattern any more.

Answer 4

Singletons can be justified whenever you might need more than one instance of an object. The term "singleton" is slightly misleading because often the same software design pattern is used to control multiple instances of a class.

You can use a static class, or hang a singleton instance on a static reference, or store it in one of the asp.net web collections. Singletons can participate in inheritance in the future etc. etc...As for static class vs Singleton pattern, I'm staying out of that argument because it has been thoroughly addressed online, even on StackOverflow

Answer 5

Just quickly I would say No, just try to bind it to it's 'scope'. If it is application-wide then try to tie it to the application. If it is session-bound put it in the session etc. This helps you if you for instance want to run 2 apps in the same container. I have no experience with ASP.NET, but there seems to be a 'HttpApplicationState'. Could you use that?

Answer 6

I would not use a singleton since that makes an application harder to test. Most IoC containers can help you replace this pattern with a "single(ton)" instance that is shared between all classes that would use the singleton.

Answer 7

There's no need for a singleton with all of the associated drawbacks (customary anti-singleton link ).

I like the suggestion to store it in HttpContext.Cache. Some similar alternatives:

• Store it in HttpApplication.Application

• Add a property to your application class to store it, then in the relevant HttpModule, store a class-level reference to your application.