I'm wondering what the differences are between these two singleton classes. My professor showed us the last one said it was more flexible and better in it's performance. I'm also wondering why my professors version has a private constructor, can someone clearify why that is?
Version 1 - my version (standard I believe):
public class Singleton {
public static Singleton GetInstance() {
return Singleton.instance ?? (Singleton.instance = new Singleton());
}
Version 2 - flexible and high-performance:
public class Singleton {
private Singleton() {} // Why this?
private static class SingletonHolder() {
public static readonly Singleton instance = new Singleton();
}
public static Singleton getInstance() {
return SingletonHolder.instance;
}
}
In c#, Private Constructor is a special instance constructor and it is used in a classes that contains only static members. If a class contains one or more private constructors and no public constructors, then the other classes are not allowed to create an instance for that particular class except nested classes.
constructor is private, Because we don't want any outer class create an instance of this class.
and about performance, in version 1 you have to check: if instance of class is empty then create an instance, But in version 2 you have a static constructor with a readonly field, that mean the instance will be initialized when class call for the first time and for the next call static constructor never called again.
For more explanation about singleton pattern, you can follow this link
The constructor is private to prevent others from instantiating the class explicitly, thereby strictly enforcing the singleton pattern.
As for the nested class, it makes it lazy and thread-safe without requiring locks. Flexibility and High-Performance are semantics on top of those primary benefits. (I mean, if you really want high performance you should be coding C or C++, for real). Jon Skeet explains it here. https://csharpindepth.com/articles/Singleton (Fifth version)
In this day and age, Version 1 isn't exactly standard, most of the libraries use the System.Lazy<T> wrapper which manages lazyness without the boilerplate of nested classes.
Related
I know other threads exist regarding this question but I couldn't find satisfactory answer for this. Why singleton class needs to be sealed in C# ? (To clarify my question)Does making it sealed really help it achieving goal of singleton (only one instance) ?
I have added more clarification about what I am wanting to know and trying to find reason for, hope readers understand what I am trying to discuss.
I am talking about MSDN article's thread safe solution :
https://msdn.microsoft.com/en-us/library/ff650316.aspx.
In this article first they give solution which is not thread safe, then they give another solution using static initialization to make it thread safe and in that solution they make class: sealed
As per MSDN's explanation -- The class is marked sealed to prevent derivation, which could add instances. For a discussion of the pros and cons of marking a class sealed, see [Sells03].
When I tried to figure why they used Sealed in static initialization approach, only thing I could figure is: as they wanted to prevent additional instantiation they made it sealed (this only prevents instantiation done through derivation, nested sub class derivation could create additional instances but by using Sealed it doesn't prevent nested class from creating additional instances.) to achieve goal of singleton. If using sealed doesn't prevent additional instantiation through other ways then why use it ?
Below is the MSDN's thread safe solution:
public sealed class Singleton
{
private static readonly Singleton instance = new Singleton();
private Singleton(){}
public static Singleton Instance
{
get
{
return instance;
}
}
}
My question is does MSDN's thread safe solution achieve goal of singleton by making it sealed? They made class sealed to prevent instantiation through derivation but that doesn't stop instantiation done through other ways. See my example below:
public sealed class Singleton
{
private static readonly Singleton instance = new Singleton();
private Singleton() { }
public static Singleton Instance
{
get
{
return instance;
}
}
public class nestedSingleton
{
public nestedSingleton()
{
Singleton moreThanOneObject = new Singleton();
}
}
}
Simply put: if anyone can derive from it, there can be multiple instances. So you definitely need some way of preventing arbitrary derivation.
Now you can simply rely on a private constructor to prevent subclassing instead (and you need the private constructor anyway), but the only reason for making a Singleton class unsealed would be if you wanted a nested type to be the implementation, e.g.
public class Singleton
{
public static Singleton Instance { get; }
= (DateTime.Now.Seconds & 1) == 0
? (Singleton) new EvenSingleton() : new OddSingleton();
private Singleton() {}
private class OddSingleton : Singleton {}
private class EvenSingleton : Singleton {}
}
This is properly a singleton - but it's a pretty unusual situation. It's vastly more common not to need a nested type to be the implementation, at which point it's clearer IMO to seal the singleton to make it clearer that you don't intend there to be any subclasses.
Basically it's communication as much as anything - a sealed class clearly prohibits derivation, whereas just having the private constructor (but no nested types) makes it more implicit. I think being explicit is clearer.
(Also, in earlier versions of C#, there are some bizarre ways you can get subclasses to compile with mutually recursive constructor calls so you never end up with a base constructor initializer. This has now been fixed, fortunately.)
To respond to a question addition:
Sealed keyword doesnt even make sense in this case if MSDN added it to prevent creating more instances of singleton by Nested sub classes.
In the example you've given, the nested class isn't a subclass. It looks like this:
public class nestedSingleton
whereas a nested subclass would look like this:
public class nestedSingleton : Singleton
(except ideally following .NET naming conventions).
It does not have to be
Implementing Singleton in C#
This is not safe on multithreads but a singleton
public class Singleton
{
private static Singleton instance;
private Singleton() {}
public static Singleton Instance
{
get
{
if (instance == null)
{
instance = new Singleton();
}
return instance;
}
}
}
Making it sealed would make it not possible to inherit it (eliminating a possible cause for several instances of a singleton)
The reason is very tied to implementation details.
For example, see the following simple and wrong singleton implementation in C#:
public class A
{
private static readonly A _instance;
static A()
{
_instance = new A();
}
public static A Instance => _instance;
public string Name { get; set; }
}
Now let's derive it into B:
public class B : A
{
public string Surname { get; set; }
}
What would happen if you call B.Current static property?
// ERROR! Surname isn't a property of A
string surname = B.Instance.Surname;
One of main drawbacks of not sealing a singleton class is that static members are still accessible on derived classes. That is, it's very confusing that you can access Instance property on B but it returns the singleton instance of A.
You can't take this risk and one simple and yet powerful way of avoiding confusion is sealing A. Since A is a singleton implementation, you don't want to be inheritable, because derived classes can't expand the singleton.
If you're agree with the point of avoiding the described situation, a more convenient implementation of singleton could be:
// SEALED
public sealed class A
{
private static readonly A _instance;
// Avoid that other code excepting A class members
// can instantiate A
private A() {}
static A()
{
_instance = new A();
}
public static A Instance => _instance;
public string Name { get; set; }
}
About the criticism of using a static read-only field instead of lazy-loading the single instance...
I've also checked that some seem to be against my proposal of instantiating the single instance as part of static constructor or using a field initializer. I could also simplify the implementation as follows:
public sealed class A
{
private A() {}
private static readonly A _instance = new A();
public static A Instance => _instance;
public string Name { get; set; }
}
And, as far as I know, it provides more advantages than instantiating the single A instance in the Instance static property getter:
...
private static A _instance;
public static A Instance
{
get
{
if(_instance == null) _instance = new A();
return _instance;
}
}
See this other Q&A to understand that static constructors are thread-safe: Is the C# static constructor thread safe?
In response to my answer's criticism about my sample singleton implementation
In some comment, Jon Skeet has said:
A singleton doesn't just provide a single instance - it ensure that
it's the only instance ever created. See
en.wikipedia.org/wiki/Singleton_pattern "In software engineering, the
singleton pattern is a design pattern that restricts the instantiation
of a class to one object." Your code doesn't do that.
It's not a good argument to invalidate my sample implementation. For example, most inversion of control containers let you configure the life-cycle to singleton of a given component and its implementation.
The whole implementation is a regular public class with no constraint preventing you from even configuring the same implementation for other component, or you can even instantiate the implementation with new (i.e. new A()) wherever you want.
But the so-called component is a singleton, because you've configured that it will be a single instance of the whole component across the entire AppDomain.
Now let's see the Implementation section on Singleton pattern (Wikipedia):
Implementation of a singleton pattern must satisfy the single instance
and global access principles. It requires a mechanism to access the
singleton class member without creating a class object and a mechanism
to persist the value of class members among class objects. The
singleton pattern is implemented by creating a class with a method
that creates a new instance of the class if one does not exist. If an
instance already exists, it simply returns a reference to that object.
To make sure that the object cannot be instantiated any other way, the
constructor is made private. Note the distinction between a simple
static instance of a class and a singleton: although a singleton can
be implemented as a static instance, it can also be lazily
constructed, requiring no memory or resources until needed.
Comparing this implementation description with my sample implementation, I would say that the following implementation is a singleton:
public sealed class A
{
private A() {}
private static readonly A _instance = new A();
public static A Instance => _instance;
public string Name { get; set; }
}
You can't publicly instantiate it.
It can be only a single instance within a given AppDomain.
It provides global access to itself.
It implements a method to create the single instance, but it's using C# specific syntax and syntactic sugar.
It's lazily instantiated when the static Instance property is first instantiated. This is how static constructors and static class field initializers work in C#.
Now let's analyze the inversion of control container case:
Usually components are defined by interfaces and abstract classes. Few times are concrete classes. Thus, you can't instantiate an abstract class or interface.
There's no Instance static property, but the implementation description on Wikipedia says: [...] It requires a mechanism to access the singleton class member without creating a class object and a mechanism to persist the value of class members among class objects. The fact that you can implement this with an Instance static property of singleton class is just an implementation detail on some programming languages. The pattern description is fine with how inversion of control containers work. Either something like service locator anti-pattern or just dependency injection, are mechanisms that provide the single instance.
With inversion of control containers you can access singletons globally. For example, Container.Resolve<ISomeComponentConfiguredAsSingleton>(). BTW, excepting on few cases, service locator is considered an anti-pattern.
Most inversion of control containers can provide custom factories where you can define how the component implementation will be instantiated.
Based on previous point, you can easily implement a factory with lazy-loading.
In summary, I would say that in my particular case I've already left my ortodoxy and I assume that design patterns are abstract definitions to implement solutions to common problems and most are and should be language-agnostic. As long as my code works as the description of a given design pattern for me is enough to consider I'm implementing the whole design pattern. You can even implement singleton without calling your class WhateverSingleton.
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.