I use the singleton pattern in a lot of places, sometimes the constructor does nothing, other times it's initialising things.
I wondered if there was a way to set up an abstract class to minimise my code repetition a bit, i.e. I don't need public static readonly Singleton _Instance = new Singleton(); in every single class, just one base class.
I understand interfaces are not an option.
I've tried using the following (taken from here);
public abstract class Singleton<T> where T : new()
{
static Singleton()
{
}
private static readonly T _Instance = new T();
public static T Instance
{
get { return _Instance; }
}
}
The problem with this is that I can't override the constructor for the cases where I need to initialise things.
Is what I'm trying to do even possible? Or should I just keep doing what I'm doing and not worry about a base singleton class?
I wondered if there was a way to set up an abstract class to minimise my code repetition a bit
No, there isn't. As soon as you've got an abstract class, you've got a class which can be instantiated multiple times. I've seen various people try to do something like this, but the end result is either not a singleton or is more complicated than just doing what you're already doing.
Is what I'm trying to do even possible? Or should I just keep doing what I'm doing and not worry about a base singleton class?
You shouldn't try to create a base singleton class - but if I were you I'd try to stop using quite so many singletons in the first place. The singleton pattern is very easily overused, and it's tantamount to an anti-pattern. See if you can refactor towards dependency injection, using a configuration which happens to create only one instance of each of these classes, but where that's a matter of configuration rather than enforced by the class itself.
Aside from anything else, unit testing involving singletons tends to be a pain, simply because it's global state which needs cleaning up between tests etc.
Related
I am writing a C# program that records and processes video for the Microsoft Kinect. I have created a KinectManager class which checks the state of the sensor, and does things like activating, deactivating, and recording the color stream.
Since there can only be one Kinect sensor plugged in, I will always only need one instance of the KinectManager class every time the program is run. In this case, is it good practice to make the class or its methods static?
While static methods could be handy, keep in mind statics aren't object oriented design. E.g. inheritance won't work and static classes can't implement interfaces.
Also you get more coupled code as it's more work to to replace static calls than passing another instance.
Static methods are better when not dealing with state (e.g. ToUpperCase(string)) - as #Wazner also mentioned.
In this case you could use the Singleton pattern, this will ensure there is only one instance.
For example:
public class KinectManager {
// instance, another option is to make it lazy, but be aware if it's needs to be threadsafe.
private static KinectManager _instance = new KinectManager();
//private ctor to ensure it won't be created outside this class .
private KinectManager () {}
// The instance
public static KinectManager Instance {
get { return _instance ;}
}
}
But be aware that the singleton pattern is overused in the real world. If something goes wrong when you have two instances, then singleton is a great way to ensure that this won't happen. But if two instances are OK, then maybe the singleton pattern is over-design.
I'm writing a PCL in .NET and I have a wrapper class around HttpClient that loads an HtmlAgilityPack.HtmlDocument from a URI in multiple different methods. It is stateless, so I would really like to make it static, since in my opinion instantiating something with new gives the impression that it contains state. However, I have a couple of interfaces that I want it to inherit from, so it can't be static. This is where I thought of making it a singleton. Here are a few snippets from the code:
public class ConcurrentClient : IAsyncClient<HtmlDocument>
{
private static readonly ConcurrentClient _Instance = new ConcurrentClient();
private ConcurrentClient() { }
public static ConcurrentClient Instance
{
get { return _Instance; }
}
public HtmlDocument LoadUri(string uri)
{
return LoadUriAsync(uri).Result;
}
// ...
public async Task<HtmlDocument> LoadUriAsync(string uri,
Encoding e, NetworkCredential creds, Action<HtmlDocument> prehandler)
{
// ...
}
}
I'm wondering, though, if I should change the beginning parts to this:
private static readonly ConcurrentClient _SharedInstance = new ConcurrentClient();
public static ConcurrentClient SharedInstance
{
get { return _SharedInstance; }
}
The reason for this is I'm not that sure about using the Singleton pattern, mainly because I've rarely seen it in use in other libraries (maybe WinRT's Application.Current?), and I think it would encourage users of my PCL to write coupled code, since it's much easier to just call ConcurrentClient.Instance everywhere than it is to pass it in as a parameter.
However, I do want to encourage the use of a shared instance because excluding the reasons above, there's very little point in calling new ConcurrentClient() because all it does is create more memory overhead. Also, I can't think of a better way to implement inheritance with methods that don't really rely on state.
Your Singleton already implements 2 interfaces. The question really is, where are the dependencies to this Singleton and why are they there ?
If the answer is that these dependencies are there because they need to get to the implementation of those interfaces then I would say that this is wrong.
The whole point of doing a SOLID design is to have dependencies towards interfaces and not towards any concrete implementation. So anyone who needs any of these 2 interfaces should be given those interfaces via dependency injection. So that means that the interfaces would be passed by their constructor or via an extra parameter in a method call, a strategy pattern, ...
Also see this : http://blogs.msdn.com/b/scottdensmore/archive/2004/05/25/140827.aspx
There can be a reason to make a singleton, but based on your explanation this is not that clear.
Investigate more of your time in using dependency injection. If you have that under control move one step further and investigate on how you can use an inversion of control container.
Also, it's easy to forget DI and passing around the object as a parameter when you can just access it by Singleton.Instance.
You are forgetting about unit testing. If you pass interfaces to your class constructors you can easily mock those interfaces and test your class functionality. With your singleton in place, your classes really need that singleton. Unit testing will be harder.
Of course Instance is easy to access, it's a global and since people revert back to old habits of programming towards objects all the time, that is why it is so popular.
I have a singleton object in C#.
This singleton object works based on some state assigned to it.
I dont have any method to switch state of singleton object at run time. Also I dont need it as application always start in one state and remains in same state.
Problem is while writing the test cases. I have written the test cases for each state. But I cant run it because the for all test cases I have single object with one state.
How to run the tests for other state. How to re-create the object for each test?
I dont want to change the singleton object code for test cases.
Any thought or idea will be much appreciated.
This is one of the reasons why it is handy not to manage the lifetime of a class yourself, but to have an Inversion of Control (IoC) container such as Autofac or Unity do it for you. You then simply create a class, that looks like any other class, and tell your IoC container to instantiate it as a singleton.
See also An Autofac Lifetime Primer.
In the case when you cannot use an IoC container (can't think of any, but let's be flexible), you can create an internal class that contains your "singleton"'s logic -- and this internal class is just that, an internal class, not a singleton...
internal class MyLogic
{
...
}
And then you wrap it in a public class, and make that a singleton. If you put both these classes together in a single Project, then the internal class (the implementation of your business logic) is not accessible to your application, only the public singleton version is accessible.
public sealed class MySingleton
{
private MySingleton() { Implementation = new MyLogic(); }
public static MySingleton Instance { ... }
private MyLogic Implementation { get; set; }
...
}
But then you can point out in your AssemblyInfo that your unit-test project does have access to the internal class by using
[assembly: InternalsVisibleTo("MySolution.UnitTests")]
This way, you can unit test your logic class while your application(s) can only use it as a Singleton.
Frankly I prefer the IoC way but if that's new to you, it's probably faster to implement the above solution instead.
Good luck!
I dont want to change the singleton object code for test cases.
Maybe it's the time to think about changing it for your whole program. Is there a reason you need this to be singleton? Singleton is a great pattern if you need it, but it's often misused by people who want to use global variables but have heard that they are evil. Now they program singletons, because they work the same while being one of those "patterns" that are cool OOP.
But a singleton is nothing but a global. And it has the same problems as a global. If you use that pattern, make sure you actually need it, because it comes with problems attached and you need to weight the benefits against the drawbacks. If you don't use the benefits, you only have drawbacks.
I would suggest you to avoid singletons when possible, see this talk by Misko Hevery: The Clean Code Talks
Using a Singleton class guarantees one instance of a class to give control to the programmer. Really useful.
I was wondering if for example a Singleton Interface existed in a given framework to bypass the need to create/add one explicitly?
Would be handy to decalre:
public sealed class MySingleton : ISingleton //or a different class
{ ... }
And then expect the class to only ever be instantiated once.
Is this a good idea, or am I thinking a bit off the mark? :)
I was wondering if for example a Singleton Interface existed in a given framework to bypass the need to create/add one explicitly?
It doesn't and can't exist. A singleton basically requires a static Singleton getInstance() method, but because it's static, it cannot be definied as an abstract (interface) method. It also makes sense, there can be only one singleton implementation, not multiple. Abstracting it is pointless.
You'll need to boilerplate complete singletons yourself. I however highly question how that's useful. It's certainly not its sole purpose to prevent stackoverflow or memory errors. Writing good code prevents that. Singletons are only useful if you want to have the enduser to deal with the same instance all the time. Which can be done as good without the singleton pattern by the way. Either just declare it static or make use of the "application scope" concept the average framework can provide you.
Instead of singletons, rather look for inversion of control (dependeny injection). That's by the way also exactly what Spring is doing. They do not use "pure" singletons. It was a poor word choice they made.
See also:
Singletons are evil
Patterns I hate #1: Singleton
Inversion of Control and Dependency Injection pattern
A Spring Singleton is not a Singleton
Singleton does not prevent stack overflow, not sure what you are getting at with that.
For Java, what came to mind is Spring. By default, every Spring bean you write is a singleton. You can use it in 100 places, and they will all be set automagically via injection, and all 100 references will go to the same object (i.e. a singleton). When you set up a project in Spring, you can make any class you want a singleton just by following the conventions.
Google Guice is a dependency-injection framework that supports a #Singleton annotation.
Note that classes annotated with #Singleton aren't "true" singletons - there's nothing stopping client code from creating many instances of such a class. However, Guice-managed dependencies will all share the same instance.
See http://code.google.com/p/google-guice/wiki/Scopes
Maybe not what you're looking for, but here's my favorite version of the singleton pattern in C#. It's thread-safe, uses lazy instantiation, and doesn't require any locks. It's also pretty painless to write... no frameworks needed. ;)
class MyClass
{
// ...
#region Singleton pattern
private MyClass() { }
public static MyClass Instance { get { return Singleton.instance; } }
class Singleton
{
static Singleton() { }
internal static readonly MyClass instance = new MyClass();
}
#endregion
// ...
}
To get the object instance:
MyClass m = MyClass.Instance;
In Java you can do this simply with an enumerated type. You specify the number of instances so that there can be none (also called a utility class), one (also called a singleton) or more as you choose.
public enum MySingleton {
INSTANCE;
}
.NET 4.0 has the Lazy(T) Class, which will lazily-initialize a value on first access, in a thread-safe manner. There are lots of examples at the Lazy Initialization topic.
Also, if you are using Unity, there is a lifetime manager which you can configure with the ContainerControlledLifetimeManager to ensure a single instance.
Ruby has a module called singleton that makes the class which includes it a singleton. This module is built into the standard library.
The intention behind the singleton pattern is "Configure once. Use multiple times". This is typically used to share any kind of data or resources as mentioned in one of the answers above. But it is also useful to enable any kind of "management" application. (think JMX if it is Java)
You have one instance of a certain class that you can use multiple times. Since there is only one instance, by configuring that instance appropriately, you can reflect the configuration changes across the app. Hence the singleton pattern gives the ability to enable a "management dashboard" to your app.
Spring or Spring.NET (the .NET implementation of Spring) are useful for configuring and injecting singletons. The same arguments apply for any kind of dependency injection framework. You should read about dependency injection in general to harness the full power. A true singleton, across multiple JVMs or clusters, is usually harder to create and manage. and might require tool support. In practice, it is not necessary to create and maintain that.
Don't confuse singletons with statics! The construct looks similar but it can be pretty different. Now to drum my own trumpet! Here is a link to an article that I had written about static methods.
public class Singleton<T> where T : class, new()
{
static class SingletonCreator
{
internal static readonly T instance = new T();
}
public static T Instance
{
get
{
return SingletonCreator.instance;
}
}
}
It is lazy and versatile. Define constructors on demand.
In a web app, I need to have only one instance of a class called ProcessManager. One way is to make it a singleton. The other way is to use the HttpApplicationState to make sure I always access the same instance, like this:
public static ProcessManager ProcessManager
{
get
{
HttpApplicationState applicationState = HttpContext.Current.Application;
if (applicationState["ProcessManager"] == null)
{
applicationState["ProcessManager"] = new ProcessManager();
}
return (ProcessManager)applicationState["ProcessManager"];
}
}
Which method is better and why?
Based on the limited description you've given, I would choose a Singleton, because then it doesn't have a dependency on HttpContext.Current, and can be used outside of the ASP.Net pipeline (for example, when you want to write unit tests.)
(As an aside, when you set something into ApplicationState, you also need to first call Lock() on it, and then Unlock() it after you're done writing to it, to make sure it's thread safe.)
Alternately, allow injection of an HttpContext when you create your ProcessManager, such that you can use it with a mocked HttpContext.
if you plan to implement it as singleton,as per Jon Skeet (a.k.a C# guru),he personally prefers the code below
public sealed class Singleton
{
static readonly Singleton instance=new Singleton();
// Explicit static constructor to tell C# compiler
// not to mark type as beforefieldinit
static Singleton()
{
}
Singleton()
{
}
public static Singleton Instance
{
get
{
return instance;
}
}
}
(I'm assuming your ProcessManager constructor is private.)
Making it an actual singleton would be best, because such an approach would make it structurally impossible for other programmers that are maintaining your code to accidentally create multiple instances. There's nothing stopping a consumer from accessing the HttpApplicationState directly and removing and replacing the ProcessManager instance. So you must rely on convention to protect the instance of ProcessManager in HttpApplicationState.
Only if there is an actual use case for multiple instances of the class to exist does it make sense to allow multiple instantiations while relying on convention to protect the instance in HttpApplicationState.