I want to implement simple Singleton and after some investigation have following to working fine. I test it using a simple console app but will be helpful if someone else can comment on it. The reason I've a doubt because new instance of Singleton is created within a static constructor and not sure if that's has any side effects.
sealed class SingletonEx
{
public static readonly SingletonEx Instance;
static SingletonEx()
{
if (null == Instance)
{
Instance = new SingletonEx();
}
}
private SingletonEx() { }
}
In case you're curious I found http://csharpindepth.com/Articles/General/Singleton.aspx quite helpful on this topic.
This is not a typical usage for static constructor, although it is possible to do so. Side effect would be that constructor would run before first using of any SingletonEx instance will be used in your code. Having a static constructor wouldn't change much about the "Singleton" behaviour. But, pay attention that initilization would happend in another stage than it would usually be if it wern't static.
I'd be worried for the following line:
public static readonly SingletonEx Instance;
Here are your problems:
public - Means it could be changed outside of the class.Because the Singleton instance is referenced by a private static member variable, the instantiation does not occur until the class is first referenced by a call to the Instance property. This solution therefore implements a form of the lazy instantiation property, as in the Design Patterns form of Singleton. When implement design pattern follow as much as you can on it's design.
readonly - shouldn't be here either. It is usually editable, unless you have a good reason for that.
Related
I am trying to understand singleton pattern in C#. I got 1 structure for that but i have some doubt in the implementation. Since I'm new to C#, i want to know how this is working.
public class Class1
{
public static void Main(string[] args)
{
Singleton.Object.Func1();
}
}
public sealed class Singleton
{
private static readonly Singleton _obj = new Singleton();
static Singleton() { } // Make sure it's truly lazy
private Singleton() { } // Prevent instantiation outside
public static Singleton Object { get { return _obj; } }
public void Func1() { }
}
Is this pattern correct for a singleton class?
What is the purpose of this line "static Singleton() { } // Make sure it's truly lazy"
How the class identify only 1 instance is created in a particular time
Your pattern is valid. There are some other ways to do it but this is fine.
static Singleton() { } I don't believe is necessary but I could be wrong, it's more akin to something you need to do in C++ to make sure when you call for it, it grabs the singleton.
As _obj is static, the class can only have 1 version of it at any given time, meaning whenever you call it with .Object, it returns this one valid copy.
Yes this pattern is pretty much bare bones implementation of the singleton pattern.
While not strictly necessary, the purpose of that line looks like it might be trying to teach you a bit about C#, since static constructors are not the same as regular constructors.
https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/static-constructors
Note that:
A static constructor is called automatically to initialize the class before the first instance is created or any static members are referenced.
The class achieves this by having a private constructor, which means that only code inside the class implementation could instantiate an instance.
The singleton pattern is commonly used when you only want one instance of your class to be created. For example you might want to have an InputManager class who deals will processing all of the mouse and keyboard events for your engine. It doesn't make sense to have multiple instances of this class because then you would have to keep multiple copies of the manager up to date and then determine whose job it is to process the event. Instead you can use a singleton to make sure that input is all dealt with by the one manager.
I have to change around 10 classes into singletons and I thought that instead of copy-pasting the code it makes sense to use mixins, like it is described here: http://msdn.microsoft.com/en-us/vstudio/bb625996.aspx
However, I need not that much of additional methods, but more additional changes to the class itself, I have problems applying those instructions.
I tried to create an empty interface ISingleton and then to add the singleton part as extension to the new class public static class Singleton
This is singleton part which I would like to use:
public static SomeClass Instance
{
get { return _instance ?? (_instance = new SomeClass ()); }
}
private static SomeClass _instance;
But when adding it as extension I had a problem - how to define the Instance property, so it will be reusable by many classes?
And the second issue - I still need to change the constructors to private manually.
Does this approach makes any sense?
I haven't used mixins before, maybe this is just not the right scenario for it?
The mixins link you gave shows extension methods being used to add functionality to all objects supporting an interface. You still need to create the objects first. As singleton patterns handle the creation of objects it's basicly too soon to apply these techiniques.
Singleton needn't be so complicated, you're reading Jon Skeets article, a simple:
public sealed MyClass
{
private MyClass(){}
public static MyClass Instance = new MyClass();
}
Is often all you need. I'd happily repeat that code 10 times if needed. Or one can use a service locator or IoC container to manage the lifetime of objects.
Considering the 'standard' C# implementation of a singleton pattern with type initialisation:
public sealed class Singleton
{
private static readonly Singleton instance = new Singleton();
public static Singleton Instance
{
get
{
return instance;
}
}
private Singleton() { }
}
Is there any point in the static property? If the static field is marked as readonly, then surely it cannot be written to from anywhere, including from outside the class. For a more terse implementation, would this work?:
public sealed class Singleton
{
public static readonly Singleton Instance = new Singleton();
private Singleton() { }
}
It seems fine to me but I've only ever seen the top one used so I wonder if there is anything wrong that I have missed.
Is there any point in the static property?
Yes. It hides the implementation detail that the value is retrieved from a field in the exact same class. If you expose the field, you're stuck with that implementation forever (assuming you can't later change/rebuild all code referring to it). If you use a property, you could later:
Change to use Lazy<T>
Use a field within a nested class to get potentially lazier initialization if you want to have other static methods in the class
Use some new and funky scheme yet to be devised
Basically, with a property you've got options for future change. With a field, you haven't.
I used not to think this was a relevant difference, but in Noda Time I've had situations where complex type initialization ordering issues have made it really important that I can control this sort of thing. Where I've exposed a public static readonly field in an earlier version, I've later regretted it :(
A public static readonly field works in terms of still being a singleton - it just gives you less control.
As an aside, you may wish to add an empty static constructor to your class. See my articles on singleton implementations and beforefieldinit for more details.
In case of the Singleton pattern it's not very harmful to have a public readonly field rather than just a getter.
The reason the 'standard' implementation is using a getter is to be consistent with the rest of public getters and make you adopt good habits for all the other cases where a getter is more appropriate than a poublic field (for adding functionality without changing the interface for example)
Readonly will mean that it is an instance variable that cant be written to, static is a variable that can be read without instantiating the object. For example you would call them as follows:
//Readonly
var readonlySingleton = new singleton();
var result = readnlySingleton.Instance;
//static readonly
var result = singleton.Instance;
I need to know how to prevent a class from being Instantiated in .net?
I know few methods like making the class Abstract and Static.
Is there any more way to achieve this?
Making the class static is the best approach, if you absolutely don't want any instances. This stops anyone from creating instances. The class will be both sealed and abstract, and won't have any constructors.
Additionally, the language will notice that it's a static class and stop you from using it in various places which imply instances, e.g. type arguments and variables. This indicates the intention more clearly than just having a private constructor - which could mean that there are instances, created within that class (e.g. for a singleton implementation).
Oh, and making the class static will stop you from introducing any pointless instance members in the class, too :)
See MSDN for more information about static classes.
Mark the constructor(s) private, protected or if in used from another assembly, internal
Marking the constructor private. Of course, this doesn't prevent the class from instantiating itself through a static method, for example...
More practically, what's the purpose of disallowing class instantiation. If it's to have a singleton, then a private constructor is appropriate. If it's to force subclassing, making the class abstract is better; if it's to have a class with utility methods, making it static is one way (then you can only have static methods).
I need to know how to prevent a class from being Instantiated in .net?
Your question is not clear.
Do you mean instantiated at runtime? Make the class abstract or static.
Do you mean that the constructor is not accessible in code? Make the constructor private. But note that someone could still use reflection to grab a handle on a constructor and instantiate an instance at runtime.
So, which do you mean?
If the question is:
How can you make your class not be instanced without having your class
be static or abstract?
Then the answer to this is to implement the singleton pattern, which in .NET 4+ this is done easily with:
public sealed class myClass
{
private static readonly Lazy<myClass> lazyInstance =
new Lazy<myClass>(() => new myClass());
public static Instance
{
get
{
return lazyInstance.Value;
}
}
private myClass()
{
// constructor logic here
}
}
The singleton pattern allows you to pass your class around as a reference to methods, while still ensuring that you have only a single instance of your class. It also makes testing much easier as you can have a ImyClass instance which myClass implements, this is very helpful when making mock objects.
Without .NET4 you can still implement the singleton pattern, for example:
private static readonly myClass instance = new myClass();
public static Instance
{
get
{
return instance;
}
}
// rest of code remains the same
Which doesn't have deferred loading until it's called, there's lots of other ways as well (I think about 6 different ways), but the above two are the most common ones.
In summary the question is likely asking if you know the singleton pattern and if you recognise it's importance over static classes for unit tests and mock objects.
As others have already pointed out, static fields, even those marked readonly can be set with reflection, in addition the private constructor can be called using reflection. If you need to prevent these, either you need to make the calling code run in a less trusted app-domain space, or you will need to implement your class as static.
Generally though, people don't bother with such levels of reflection to get around your constraints unless they 'really need to' for example, writing obscure / extreme fringe case unit tests.
Consider the following code
public sealed class Singleton
{
private static readonly Singleton instance = new Singleton();
public static Singleton Instance { get { return instance; } }
static Singleton() {}
private Singleton() {}
}
Question
1) Here what is the purpose of static constructor ? (I know static constructor will be called before the first instance of the class is created).But in the context of above code
can't i use it without the static constructor?
2) I heard that one of the advantages of singleton is that it can be extended into factory.
Since it is a sealed class ,how will you extend it into factory?can you give some example?
The static constructor ensures that the singleton really isn't constructed before it's used. If the static constructor isn't present, the CLR has a lot more leeway about when it runs the type initializer. See my article on beforefieldinit and my blog post about .NET 4 type initialization changes for more info.
As for turning a singleton into a factory - you'd really have to give more context. Your Instance property could choose whether to always return a reference to the same object or not, I suppose...
There is no need for a static constructor. SKEET!
I've never heard of that. You can't, obviously, if the class is sealed. But you could moderately easily rewrite it as a factory, as there is only one way to instantiate it--through a property. Change that into a method, and you have the starts of a factory. The method may return the same instance each time, or may return an instance from a pool, or do whatever it wants.
I wouldn't suggest you have a factory property, as properties shouldn't be doing too much work.