I'm going through some code, and I've come across something that appears rather strange to me. Here's a short example of what's being done.
namespace some.namespace
{
public abstract class ClassName
{
private static ClassName _instance = null;
private static ClassName Instance
{
get
{
return _instance;
}
set
{
_instance = value;
}
}
public ClassName ()
{
if(Instance != null)
{
return;
}
Instance = this;
}
}
}
Is there good reason for this that I'm missing? Throughout the code, there are references to ClassName.Instance....
This is a pattern called Singleton.
In software engineering, the singleton pattern is a design pattern
that restricts the instantiation of a class to one object. This is
useful when exactly one object is needed to coordinate actions across
the system.
From wikipedia.
This is a very naive implementation of the Singleton pattern. See this for a deeper discussion of the topic.
Like others have already answered: this is a Singleton-Pattern. But it has some flaws.
It is not thread-safe.
It's constructor is public, this means it doesn't really serve the singleton principle: To ensure that there is only one instance of the class. You could simply create as many as you want.
The class is not sealed. This means you could inherit from this class and happily build instances from the sub-class.
See C# in Depth about Singletons for a complete look at this topic.
Related
So, with the brand new C# 6 we got those neat auto-property initializers. I thought I might as well take advantage of these to make more concise singletons than ever. Apparently someone else got that idea, too.
public sealed class Singleton
{
public static Singleton Instance { get; } = new Singleton();
private Singleton() { /* some initialization code */ }
}
My questions are:
How thread-safe it is?
How lazy it is, or when the instance is actually created? (not a priority, but it would be good for future reference)
Is it a good idea overall?
(it might look similar to this question, but it's not)
Your code will be expanded to the following:
public sealed class Singleton
{
private static readonly Singleton <Instance>k__BackingField = new Singleton();
public static Singleton Instance { get { return <Instance>k__BackingField; } }
private Singleton() { /* some initialization code */ }
}
(<Instance>k__BackingField is the unspeakable name of the compiler-generated field.)
So, the properties of your code will be exactly the same as of the code above. Namely, this pattern is thread-safe and it can be a good idea, depending on circumstances.
Assuming you're not accessing any other static members of this type before accessing Instance, then the exact degree of laziness is up to the runtime. Commonly, it will be something like "the instance is created the first time a method that could access Instance is JIT-compiled", but you don't have any guarantee about this.
If you want to make sure that the instance is created just before Instance is accessed for the first time, add an empty static constructor to your class. (This can have a small negative effect on performance, but that probably won't matter to you.)
Since most of this is not really specific to C# 6, a good source of further information would be Jon Skeet's articles about singletons and static constructors/type initializers.
I have some code as shown below. I guess it is Singleton pattern. Why do I need a static constructor. Also what is the advantages of this? Thanks for your reply ...
public sealed class Myclass
{
static Myclass()
{
Myclass.Application = new Myclass();
}
protected Myclass()
{
}
static Myclass _application;
public static Myclass Application
{
get { return Myclass._application; }
protected set { Myclass._application = value; }
}
string _name;
public string Name
{
get { return _name}
protected set { _name= value; }
}
}
To start with, this class is somewhat odd for having a protected constructor. It's not a fatal flaw given that it's sealed, but it's distinctly odd.
There's a potential difference in timing between this code and the nearly-equivalent use of a static variable initializer:
static readonly Myclass _application = new Myclass();
(There's no need for a setter in this case, of course.)
You can't do that with an automatically implemented property though.
Using static initialization in some form gets you "free" thread-safety - you don't need to do any locking in order to get lazy initialization.
You may find my singleton implementation article interesting for more options.
Usage of a type ctor here is a guarantee that singleton instance will be initialized once. It's more simple that double-checked lock pattern, when implementing lazy singleton initialization, but it has disadvantage the same reason - singleton creation may be very expensive, and singleton may never be used during app lifetime.
There is no immediate advantage in the static constructor as opposed to a lazy-instantiated on get approach, other than thread safety as pointed out by Jon Skeet's answer. This may or may not be relevant in your situation, though you don't specify anything. It just makes the code look different, but is going to result in the same functionality.
The "advantage" of the singleton pattern is that is allows easy access to a single instance of a class, basically a sort of "globally" accessible instance of the class.
I say "advantage" as there are many discussions about the Singleton pattern being an anti-pattern. I am on the fence. In a small application this can function OK and most of the proposed alternative solutions involve Dependency Injection frameworks (often sometimes with the life-span of "Singleton"!), which may be impractical on smaller apps.
Just a note, having a sealed class with protected members is pointless - sealed classes cannot be inherited.
If you write in this way you can use auto property and do not actually implement it.
Say like this:
public sealed class Myclass
{
static Myclass()
{
Myclass.Application = new Myclass();
}
.....
public static Myclass Application {get;set;}
...
}
Basically, there is no any practcal advantage if not like this one: code-style.
You have to create the instance of the class somewhere, and that can either be in a static constructor, or in the property that gets the instance.
Anyhow, it's not a good code example that you have found.
It has protected members even though it's sealed, which only makes the code confusing. They should be private.
If you look at the Name property you will notice that it's impossible to set it, so it will always be null.
static constructor it gets called during the loading of the assembly...
EDIT:
thanks jon I was wrong...
now i understand static constructors in C# are specified to execute only when an instance of the class is created or a static member is referenced..
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.
I was working in the Microsoft.Ink dll recently using C# and was debugging a problem (which is not related to this) I noticed, that when I was debugging it, ink objects had a strokes object, which had an ink object, which had.... etc.
This confused me, as I was under the assumption you could not do this (I come from a C++ Background)
But I ignored it, solved the problem, and moved on. Today, I run into a similar problem, as I look at a class which had a private member which was the same class as itself.
public sealed class Factory
{
private static Factory instance = new Factory();
}
How is that even possible? I can now call instance.instance.instance.instance...etc. This, as you can imagine, hurts my mortal brain, and I'm sure it can't be good on the computer either. How does the compiler deal with this? And Just how deep does the rabbit hole go?
Because it's static and therefore there is only one copy of the variable instance within the AppDomain.
What you're thinking of is this:
public class Foo
{
private Foo lol = new Foo();
}
Notice, everything here is instance, not static.
As the commenters noted (long ago), this is valid syntactically, but would result in a StackOverflowException being thrown, as the assignment requires construction, and construction creates a new assignment. One triggers the other in a cycle that ends when the call stack reaches its maximum length.
In OP's example, assignment requires construction, but the assignment is triggered by the static constructor, not the instance constructor. The static constructor only executes once within an AppDomain, in order to initialize the class' Type. It isn't triggered by instance construction, and so (in OP's example) won't result in a stack overflow.
it's not necessarily recursive by nature. think of a linked list. or a tree.
class Directory
{
string name;
Directory parentDirectory;
}
It's just allows objects of that class to have an internal reference to another object of that class.
This is a software pattern known as "Singleton".
Some people frown upon the use of the pattern for more reasons than just stated in the question but for better or for worse it is a common pattern in the .NET Framework. You will find Singleton Properties (or fields) on classes that are meant to be instantiated only once. Think of a static Instance property as a global hook upon which to hang an object.
Since this is a class, and not a struct, when you declare a field that is the class, you are only defining a reference to a class. This allows you to keep having references, provided you assign them.
In your case, you're reference allocates a new class, but it is static, so it's only going to do it one time, no matter how many classes you create. The instance constructor runs the first time Factory is used, and will call a single non-static constructor. Doing instance.instance.instance is not allowed, since instance is static. You cannot access a static variable from a member - you need to do Factory.instance.
However, you ~could~ make instance non-static, and have it be a reference to some other "Factory" class, or even a reference to this. In that case, you could chain instance.instance.instance - but it will just follow the references as long as you've set them. Everything works, no problems.
There will only ever be one instance of 'instance' because it is static. The only way you should be able to access it is by calling Factory.instance.
string text = Factory.instance.ToString(); // legal
string text2 = Factory.instance.instance.ToString(); // compiler error
I think you should ask the other way around: Why shouldn't this be possible? Factory is just a type like any type which gets resolved by the compiler.
As most of the answers here point out that this is working only because Factory is a static field, I have added the following sample. Please note that this is a very primitive sample of a chained list (you probably wouldn't implement it that way for various reasons, but I didn't come up with a better example yet). In this example, ChainedListItem is a container for an element of a single-linked list, which contains a field of the very same type to point to the next item in the list. The list has an (empty) head element and the last element is marked by having an empty _nextItem field:
public class ChainedListItem<T>
{
private ChainedListItem<T> _nextItem;
T _content;
public ChainedListItem<T> NextItem
{
get { return _nextItem; }
set { _nextItem = value; }
}
public T Content
{
get { return _content; }
set { _content = value; }
}
public ChainedListItem<T> Add(T content)
{
_nextItem = new ChainedListItem<T>();
_nextItem.Content = content;
return _nextItem;
}
public void Dump()
{
ChainedListItem<T> current = this;
while ((current = current.NextItem) != null)
{
Console.WriteLine(current._content);
}
}
}
class Program
{
static void Main(string[] args)
{
ChainedListItem<int> chainedList = new ChainedListItem<int>();
chainedList.Add(1).Add(2).Add(3);
chainedList.Dump();
}
}
The "rabbit hole" goes as deep as your stack space allows you to make another call to the constructor of the type. If you try to go deeper than that, you will get a stackoverflow exception as with any other recursion.
By the way, the code that you wrote in your answer is showing a very basic implementation of a Singleton which is actually based on having a (private) static member of the same type as the surrounding type.
And, last but not least, such constructs are also perfectly fine in C++.
It is a singleton. Meaning there is really only one instance of the class.
Is that the entire class? Typically in C# you will see a singleton like
public class SomeClass
{
static readonly SomeClass instance = new SomeClass();
public static SomeClass Instance
{
get { return instance; }
}
static SomeClass()
{
}
SomeClass()
{
}
}
I'm not sure how you would even access the instance since it is private. The only thing this would be useful for is a Singleton implementation, but if that is the case you are mission the public property exposing the instance.
This is done all the time is most OO languages. instance is a static member of Factory. There is nothing unusual about this code. It is standard Factory pattern. Do you also have a problem with code like this?
x = x + 1;
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.