It seems to me that singletons enforce a single instance but why would an application that understands that there should be only on such instance even attempt to instantiate a second instance?
I think its also about managing the single shared instance. Synchronization to be precise.You don't want someone to use the resource when you are using it.
Private constructor and static method simply enforces the application to do so.
A lot of developers refrain from using it since its also difficult to test and causes multi-threading issues.
I have a direct question: Is a good pratice use Singleton pattern to control a class responsable for Login tasks, or another pattern is more appropriated? Or do not exist a patter to do this kind os issue?
Thanks.
Few people use singletons these days as they are almost becoming anti-patterns. I would recommend to learn Dependency Injection (DI).
With DI you can register the object you wish to use as singleton with a container and that container will serve or give that object to all other objects that need it. Of course you can register the object as a singleton - but not the usual singleton - and the container will guarantee that all objects which need it will receive the same instance.
Nevertheless, if you are building a small application then It would be better to use the Singleton pattern and avoid DI.
Jon Skeet has a very nice article about Singleton pattern or if you are using Java then you could use Enumerations to implement it, look implementations techniques on Google.
The singleton pattern is used when you have to prevent the creation of more than one instance of the same class. I do not really see the situation in which a login class should only have one instance so i would say using this pattern for a login class is overkill.
Then again, introducing a DI framework when you only need a simple singleton... now thats overkill :)
Yes and no. There's no 'best' or 'worse' practise.
Just do it, if using a singleton makes it easier to test and if the approach will get you to the pub earlier than learning and implementing DI just for the purposes of login.
I saw a resource online
http://www.labelmedia.co.uk/blog/posts/design-patterns-factory-method.html
From what I know this is not really the pattern. It's more of a mix between the the Simple Factory Idiom and Simple Factory Method.
Did they get it wrong ?
Hmm, yes. It looks more like a Simple Factory indeed. However, it might be they are refering to what is sometimes called "Static Factory Method".
Once the singleton pattern is understood, writing subsequent singleton classes in C# is a brainless exercise. I would hope that the framework would help you by providing an interface or a base class to do that. Here is how I envision it:
public sealed class Schablone : ISingleton<Schablone>
{
// Stuff forced by the interface goes here
// Extra logic goes here
}
Does what I am looking for exist?
Is there some syntactic sugar for constructing a singleton class - whether with an interface, a class attribute, etc.?
Can one write a useful and bullet-proof ISingleton themselves? Care to try?
Thanks!
How often do you find yourself writing singleton classes?
I'd suggest that if it's a sufficiently frequent task for you that you feel you're losing productivity, you may well be overusing the pattern. I find that it's very rarely appropriate - but can easily lead to hard-to-test code.
Writing a singleton class in C# isn't just brainless - it's almost entirely painless. The boiler plate required is tiny, assuming you don't require absolute laziness to the point where the singleton shouldn't be instantiated just by calling other static methods.
public sealed class Singleton
{
private static readonly Singleton instance = new Singleton();
public static Singleton Instance { get { return instance; } }
private Singleton() {}
// Omit if you don't care about type initializer laziness
static Singleton() {}
}
Obviously you'd need a class declaration anyway, so you've saved either three or four lines, depending on whether you need the static constructor. Personally I don't think that's so time consuming for the rare occasions on which a singleton is valid that it's worth being part of the framework.
Indeed, it's hard to see how it could be a framework feature rather than a language feature... at the very least the compiler would have to be aware of it, to understand that there will be some magic "Instance" property (or whatever) that it should make available to callers.
As Eric Lippert is fond of pointing out (entirely reasonably, I might add) language features have great costs associated with them in terms of design, implementation and testing. Furthermore, there's the burden of each developer learning about the new language feature. In short, they need to really earn their value - and for me, the singleton pattern isn't sufficiently useful and important that it makes the bar.
What is there to force? A singleton can be implemented with as little as a single property and a field. It doesn't seem like there is a very good value/cost ratio here for any framework developers to bother with.
Not to mention, the singleton pattern is often a poor one to use, and IMO it'd be bad to have the framework promoting them. To me, a singleton is almost always a code smell.
As others and me (in comments) have pointed out, the singleton pattern is rarely appropriate.
Still, since some believe that this could not work, here's the helper class to implement such a singleton (inspired by the linked article about how to implement singletons):
public static class Singleton<T> where T: class, new() {
public static T Instance {
get {
return Nested.instance;
}
}
private static class Nested {
public static readonly T instance = new T();
static Nested() {}
}
}
The class to be used as singleton needs to have a public default constructor.
You could then use it as follows:
Singleton<MyClass>.Instance.DoSomething();
No, there is nothing in the .Net framework like this. You have to write singletons yourself. It would be a bit hard to have a definative API for singletons as there are so many ways of implementing them, and depending on your goals depends on what approach you will take.
I would suggest you read this article on singletons. (Written by Jon - He's a wise guy, listen to him) for guidance and tips on the various techniques. It covers some stuff that most people might not have thought about.
[A lot of people consider the singleton pattern to be a dubious design pattern. I won't go into the reasons here, but if your interested google has loads of info. Just be careful you don't overuse it unnecessarily.]
I don't think that is possible.
How would an interface prohibit you from adding a public constructor to your class?
After all, writing a singleton is not that much work once you understood the basic pattern (and the caveats of making it thread-safe).
Cut. Paste. Next.
Use an Inversion of Control container to manage it for you.
With StructureMap, you can declare any type as a singleton:
ObjectFactory.Initilialize(x => x.ForSingletonOf<Schablone>());
To retrieve it:
ObjectFactory.GetInstance<Schablone>();
.Net 4.0 comes with MEF. And you can use MEF to instantiate a singleton. Thru the shared creation policy. I'll dig out a code sample....
EDIT ----
better still, here's some doco....
http://mef.codeplex.com/wikipage?title=Parts%20Lifetime&referringTitle=Guide
I have a singleton class which needs to be intialized by castle? I'm a little newbie in castle. I have looked for singleton in castle and I saw setting the lifestyle attribute of component to "Singleton" seems enough. But in that case it seems I don't need to implement the class in "Singleton Pattern". Just plain class definition for my class seems ok.
Is castle provide my class as singleton to me even I define it as a normal class? Or I am missing something?
No, you are absolutely right - the container will instantiate your class the first time it is needed, and from then on the instance will live inside your container.
Stay away from the singleton pattern - it is evil! :)
(no, seriously - there are very few cases where implementing a singleton is acually necessary... and if you are using an IoC container, you will never have to implement singletons by using static instances, which can seriously hurt your design by coupling your classes to those static instances)