Singletons are death to testability.  They cause tight coupling between different parts of your application.  If you must have them, then a common workaround is to add a Reset() or set() method to the Singleton so that test classes are able to change the instance that is being used, or to reset it to null before each test.  If you're not using an IOC Container to manage the liftetime of your objects, then the only way to test them effectively is to hack in back doors to your beautifully crafted Singleton pattern and then add a bunch of comments telling developers "don't call this outside of test classes".  It's crap.  It stinks.  Don't do it.