The word "interface" here does not refer to the C# keyword. If you know a better name for the method I'll describe in this question, please let me know.
I use a singleton in one of my program.
And while programming it, I just thought it would be great to have all the singleton's methods implemented in static as well, acting as an "interface" of the singleton.
The point would be to write
int ret = MyClass.GetValue();
instead of
int ret = MyClass.singleton.GetValue();
That's what I describe as being an "interface" of a singleton and here is how I would implement it.
public class MyClass : MonoBehaviour
{
private static MyClass singleton = null;
private int theValue = 123;
private int InstanceGetValue();
{
return theValue;
}
public static int GetValue()
{
if (singleton == null)
return default(int);
return singleton.InstanceGetValue();
}
private void OnEnable()
{
if (singleton == null)
singleton = this;
}
private void OnDisable()
{
if (singleton == this)
singleton = null;
}
}
So my question is:
What am I missing that could explain why this is not common practice when using singletons?
It makes every call look much better and it is simple to implement! Why is this not the default way of using singletons?
This is Unity code but you might be able to answer my question even if you don't use Unity. Just so you know, OnEnable is automatically called when this object become active in this scene and OnDisable is called when your object become inactive.
Let's not talk about whether using singletons is a good practice in C# in this topic. A lot of great topic are already answering this question.
What it seems you're doing here is trying to write an adapter to allow for accessing the singleton. If you look at the Wikipedia page on the Singleton pattern you'll note that the implementation details around it specifies two requisites.
ensure that only one instance of the singleton class ever exists
provide global access to that instance
What you're trying to do here is hiding that global access which brings me to believe that you're trying to hide the global access point or just plain want to write less code by excluding the singleton call from the following: MyClass.singleton.GetValue();
I think the main question you'd have to ask yourself here is which of the two your trying to achieve. If you're trying to hide the global access point I'd suggest looking at why you're using a singleton in the first place, but if you're trying to write less code I'd rather question why you're trying to write less code and if you're not using some hard to read coding practices.
EDIT: I understand why you'd only want one instance in the scene, but I'd rather just take full responsibility of managing that lifecycle instead of just trying to leverage the singleton pattern for this.
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 currently have two solutions inside my project.
Solution 1 is the GameEngine
Solution 2 is essentially the front end Adventure
In order to access classes from the GameEngine library a Singleton design approach is used, so for me to access my inventory class for example GameEngine.Instance.Inventory is used.
However, this is adding quite a lot of code now, doing GameEngine.Instance each time, is there a way I can invoke a method or return so I can simply refer to Inventory and it will return the GameEngine.Instance.Inventory class instead? Basically a shortcut.
Thanks In advance :)
One way is to set up a property:
public GameEngine.Inventory Inventory
{
get { return GameEngine.Instance.Inventory; }
}
Now you can use Inventory all over the class to refer to your singleton object. However this will work only within a particular class.
Also, as per #Leri's comment, this migth (and might not) affect performance.
Use a static property:
private static GameEngine.Inventory _inventory;
public static GameEngine.Inventory Inventory
{
get {
if (_inventory == null) {
_inventory = GameEngine.Instance.Inventory;
}
return _inventory;
}
}
You can also add a static method so it will be accessible through the entire project
public static ClsInventory GetInventory()
{
return GameEngine.Instance.Inventory;
}
If your singleton instance is never changed, you can safely use a variable for the instance:
var i = GameEngine.Instance;
then use it with:
i.Inventory
I would like to augment the answers.They all respond properly your question and I upvoted all. However, I would recomend instead of using a singleton, try to change your aproach to injection of dependencies to acces "global" instances.
What would happend when you want to acces other (global) object rather than the inventory? You will have to add another property to your GameEngine. Your engine will become a Frankestain with so many properties belongin to game engine internal instances. This violates the single responsablity pattern.
Besides, when you want to unit test your game, you wil have to setup the ENTIRE GameEngine for each test / fixture. IOD also helps you to decouple your code.
every one know how to write code for Singleton Design Pattern.say for example
public class Singleton
{
// Private static object can access only inside the Emp class.
private static Singleton instance;
// Private empty constructor to restrict end use to deny creating the object.
private Singleton()
{
}
// A public property to access outside of the class to create an object.
public static Singleton Instance
{
get
{
if (instance == null)
{
instance = new Singleton();
}
return instance;
}
}
}
it is very clear that when we create a instance of any class many time the memory is allocated for each instance but in case of Singleton design pattern a single instance give the service for all calls.
1) i am bit confuse and really do nor realize that what are the reasons...that when one should go for Singleton Design Pattern. only for saving some memory or any other benefit out there.
2) suppose any single program can have many classes then which classes should follow the Singleton Design Pattern? what is the advantage of Singleton Design Pattern?
3 in real life apps when should one make any classes following Singleton Design Pattern?
thanks
Here is thread safe singleton
public sealed class MultiThreadSingleton
{
private static volatile MultiThreadSingleton instance;
private static object syncRoot = new Object();
private MultiThreadSingleton()
{
}
public static MultiThreadSingleton Instance
{
get
{
if (instance == null)
{
lock (syncRoot)
{
if (instance == null)
{
instance = new MultiThreadSingleton();
}
}
}
return instance;
}
}
}
To assure only one and same instance of object every time.
Take a scenario, say for a Company application, there is only one CEO. If you want to create or access CEO object, you should return the same CEO object every time.
One more, after logging into an application, current user must return same object every time.
Other answers are good, as well. But they are providing examples of behavioural characteristics of the pattern. But, Singleton is more about creation. Thus one of the most important benefit of the pattern is that it is resource friendly. You are not wasting memory for a new object when you actually do not need a new one.
This causes another benefit, which is the instantiation overhead is avoided.
Benefits of Singleton Pattern:
• Instance control: Singleton prevents other objects from instantiating their own copies of the Singleton object, ensuring that all objects access the single instance.
• Flexibility: Since the class controls the instantiation process, the class has the flexibility to change the instantiation process.
The advantage of Singleton over global variables is that you are absolutely sure of the number of instances when you use Singleton, and, you can change your mind and manage any number of instances.
Real time usages/benefits of Singleton Design Pattern.
While using multi-threading, to manage the multi-thread Pool.
to manage the "service host repositories" in SOA (service oriented architecture).
for Logging Framework implementation
in automation Testing/Unit Testing project i.e. Coded UI projects.
While implementing the Caching in a big application.
for configuration settings to make proper control over the application.
One useful place to use a singleton is if it is accessing some resource that you only want to have a single access point for. For example, I've used it when writing some code to talk to a device. I only want one piece of code talking to the device so I use a singleton. Any attempt to create another instance of the object that talks to the device will just give you the same object back so I never have to worry about two instances maintaining out-of-sync data about the device or getting messages to and from the device mixed up or out-of-order.
But, of course, you are under no obligation to use them. They are just a tool that is sometimes useful.
Generally singleton is considered an anti-pattern in OOP because it means a class is asserting that with respect to the entire program - which in OOP it should have no knowledge of - it knows it's going to be the only one. That being said, singleton is the proper way to implement a constant in my experience. In general if something I was about to hard-code into a program (say a database username) then it can be moved to a Config file or a singleton.
One of few areas Java beats C# (in my opinion...) is its support for enums. Java offers truly OO constants via enums, and so this is how I will always implement a singleton in Java. C# has no ready-equivalent.
It can improve the way that memory is handled in the JVM and with memory being used properly, better performance will be reached. You are not creating multiple objects but trying to create only one, that way, there is less work for the Garbage collector and less memory occupation in the JVM heap.
for me i use singleton when i dont want to always getting data from my database if its not necessary. for example i created a singleton class for getting the data from my database once and only once, and use that data across my system, then i expose a method that will get the data(refresh) again when necessary or get the data when theres a new/modified data.
Let us assume there is one printer and all have to access that printer then while creating an object u should give access to only one person to print as it doesnt allow another person at the same time thats why in this situations in real life we need single ton classes where we can manage the tasks one by one with better clarity...
As an exercise, I'm translating parts of our large and battle-hardened Delphi-framework to C#.
Included in this framework is a generic singleton parent class. Of course, implementing a singleton in C# is fairly easy (there is even a Jon Skeet article, so what more could I wish for), but our Delphi singleton has a slightly different take on the pattern: as opposed to publishing an 'instance' property/method, it has a "fake" constructor that always returns the same instance. The essential characteristic of this approach is that the user of the singleton class doesn't know that he is dealing with a singleton:
as far as they know, they just construct any old class and request some information from it.
I want to accomplish the same thing in C# (as an exercise, so it doesn't have to be production-quality code, evil hackery is fine), but so far I've failed.
Any suggestion to make a simple myInstance = new MyClass(); always return the same instance is most welcome!
Additional info
We are talking a convenience-implementation of the singleton pattern, as offered by the framework. It doesn't necessarely have to be a parent-class, but it does have to assist the developers in creating their own singletons as well. Requiring them to manually redirect all their method calls to the single-instance will not make them overflow with joy. :-)
I'm not really interested in debating whether or not this is the right way to deal with singletons, for now I'm just interested in the finer art of c#-tweaking.
You would do a Proxy (Edit: As Tom points out below, the proper design pattern is Monostate):
public class MyClass {
MyActualClass _actual;
public MyClass() {
_actual = MyActualClass. Instance;
}
public DoStuff() {
_actual.DoStuff();
}
}
internal class MyActualClass {
private MyActualClass {
}
public DoStuff() {
...
}
MyActualClass _instance;
public static Instance {
get {
if(_instance == null)
_instance = new MyActualClass()
return _instance;
}
}
}
....
public static void Main() {
var my1 = new MyClass();
var my2 = new MyClass();
}
my1 != my2 but my1.DoStuff() calls the same instance of the method as my2.DoStuff()
This would be simplified even further if you programmed of an interface only.
Edit: The equality problem could partially be solved by making _actual protected internal and overwriting MyClass.Equals(object obj) to check whether this._actual == obj._actual
I believe the Monostate pattern will give you what you need:
"The Monostate gives us the singularity of state that we so treasure in the Singleton, but without all of the static headaches that come along with it."
More here:
http://jeremyjarrell.com/archive/2008/04/21/88.aspx
As far as I know, this cannot be done for real because of how C# handles object instances. In order for a constructor to be called, the instance has to actually be created, and you can't just "return" another object from a constructor.
The best thing I can come up with (other than using a factory method) is to treat the class internally as a Singleton, and create "dummy" instances that all just point back to that original instance, when created. So for example, in your constructor you would check to see if the singleton has been initialized, and if not, would initialize it, then you would basically just proxy each instance's methods and properties back to the singleton.
In this implementation, the singleton needn't even be necessarily the same class, but you could if you wanted to keep things contained.
Update: One drawback of this approach is that although each instance would behave as a singleton, it would still have its own object reference and therefore you might also want to override Equals() for equality comparisons.
I think you could possibly roll something with Remoting.
Update:
A better way would be to wrap a proper singleton class in a struct or lightweight class.
Create the singleton as a static member and make all methods access the single static instance.
class SingletonWrapper {
private static RealSingleton instance = new RealSingleton();
public void methodA() {
instance.methodA();
}
public String getA() {
return instance.getA();
}
}
(This is actually Java code but C# is similar enough, I think. :)
I don't see how you can, as constructors don't use a return statement. You could clone all of the relevant links to the singleton, but it would have local copies of local variables, and be very messy.
Instead have the constructor check if the singleton is instantiated, and if it has been already, throw an exception, or log a warning to the developer who is using the old style code.
How about using a static function to return an object insted of using the new keyword.
static private m_obj my_obj;
static private bool my_new_obj;
static public m_obj create_m_obj()
{
if (my_new_obj == false)
{
my_new_obj = true;
my_obj = new my_obj();
}
return my_obj;
}
Then you have easy full controle over the creation of the object, if I an not mistaken.