Looking around the web, I've seen this simple pattern that implements a (thread-safe) singleton (in C#).
public sealed class MySingleton
{
private static readonly MySingleton _instance = new MySingleton();
private string[] _data = new string[10];
// Private constructor, so no outsiders have access.
private MySingleton()
{
// Initialize _data member here
}
// Static method to provide access to instance
public static MySingleton Instance
{
get { return _instance; }
}
public string GetStringCount
{
return _data.Length;
}
}
I understand that the _instance member needs to be declared as static because it is accessed from the static Instance() method.
But should other members be declared as static? By definition the singleton exists only once so the members can also only exist once (for the single instance), just as static variables exist only once.
Should I declare _data as static?
Will there be any functional difference?
Any performance difference?
Any other reason to prefer static or non-static?
If you have a Singleton you have one instance of a class. The class members must not be static (except for the Instance property backing field). If you have more than one statics in your Singleton you have actually created not one Singleton but many Singletons. A general advice is to use the static keyword only when absolutely necessary.
It is cleaner to store your singelton data inside your singleton instance as non static class members.
If you choose to use a singleton over a static class (see this and this), then I think it makes more sense to have instance members instead of static members.
Related
What if, I replaced private constructor with a static constructor in singleton Design pattern?
public sealed class Singleton
{
private static Singleton instance=null;
private Singleton()
{
}
public static Singleton Instance
{
get
{
if (instance==null)
{
instance = new Singleton();
}
return instance;
}
}
}
Static constructor will be called only once and I couldn't find any difference in the implementation. Can we replace private with static constructor?
All that the private constructor is really doing in this case is preventing anything outside of the class from instantiating an instance of class Singleton, which is almost certainly intentional as a singleton should only have a single instance.
Static class constructors are run once for a type, at an unknown time, before the type, or any of it's static members, is to be utilized. Static fields are initialized before the static constructor would be run.
So, I suppose you could replace the constructor with a static one, but that would then give you the implicit parameter-less constructor on the Singleton Type, which would allow anyone to instantiate an instance, which is likely at odds with why you are using the singleton pattern in the first place. It also wouldn't change anything about how your class was being constructed, really, so why do it?
Take the following class as an example:
public class Test { }
Under the covers, because there is no declared constructor, the C# compiler implicitly adds a parameterless, public constructor to the class, allowing consumers to create an instance.
public class Program {
public static void Main() {
var test = new Test();
}
}
This is all fine and good if you want to be able to make instances of your class. The singleton pattern intends to only provide a single instance of a type to the consumers. We could add this static instance to our test type like so:
public class Test { public static Test Instance {get;} = new Test(); }
and we would be able to get this static instance like so:
public class Program {
public static void Main() {
var test = Test.Instance; // good
var other = new Test(); // less than ideal
}
}
So we are providing access to our singleton object through it's instance field, as expected, but we can still create instances of the singleton type, which is less good, as it goes against the purpose of a singleton (namely, having only a single shared instance.)
So we add a private, parameterless constructor to the type.
public class Test {
private Test() {}
public static Test Instance {get;} = new Test();
}
Adding a constructor to a type will cause the C# compiler not to add an implicit public parameter-less constructor. Making it private allows it to be accessed within the class scope, which is used for instantiating our instance property, and prevents anything else from instantiating the object. The end result being:
public class Program {
public static void Main() {
var test = Test.Instance; // good
var other = new Test(); // Compile time error
}
}
Your singleton object now prevents other instances of the class from being instantiated, and the only way to use it is through the instance property as intended.
In simple terms, if you remove the private constructor, then anyone will be able to create a new instance of Singleton:
// With the private constructor, the compiler will prevent this code from working.
// Without it, the code becomes legal.
var newInstance = new Singleton();
And if anyone can instantiate Singleton as above, then you no longer have a singleton.
Another cleaner way to do it is to use readonly on you private instance.
This is less code and also thread safe. The CLR takes care of everything for you, no need for lock , check for null and stuff.
public sealed class Singleton
{
private static readonly Singleton _instance = new Singleton();
public static Singleton Instance {
get {
return _instance;
}
}
private Singleton()
{
}
}
Then simply test:
[TestMethod]
public void IsSingleton()
{
Assert.AreSame(Singleton.Instance, Singleton.Instance);
}
EDIT:
example using lock
public sealed class Singleton
{
private static readonly object _lock = new object();
private static Singleton instance = new Singleton();
public static Singleton Instance
{
get
{
lock(_lock)
{
if (instance==null)
{
instance = new Singleton();
}
return instance;
}
}
}
private Singleton()
{
}
}
In simplest terms, if you remove private, the default public constructor will get exposed. Then outsiders will be allowed to use new Singleton(); and make number of instances of Singleton class. So no Singleton Pattern will be there.
Additionally this classic implementation of Singleton pattern (private constructor + static getInstance() with either lazy-loading or eager loading) is so evil. In modern day you must switch to a Dependency-Injection framework instead.
This should work just fine. You could also make the class static and generic so you can store whatever kind of value in instance you want it to hold. This would facilitate the separation of concerns, keeping the singleton pattern and the class that it will contain separate.
This question already has answers here:
Difference between static class and singleton pattern?
(41 answers)
Closed 8 years ago.
here is two piece of class code one is for Singleton and other one is for static class. i like to understand in programming when one should use static class and when one should use Singleton class?
both are used to hold the global object as a result we can access those data from any where of the program when it is running. scope is broad for both....the life time of application.
1) i really do not find any article which can guide me when i should use static class and when Singleton class should be good choice. i have seen people manage db connection using Singleton class.
2) what is the main difference between Singleton class & static class ?
public sealed class Singleton
{
private static Singleton instance = null;
private static readonly object padlock = new object();
Singleton()
{
}
public static Singleton Instance
{
get
{
lock (padlock)
{
if (instance == null)
{
instance = new Singleton();
}
return instance;
}
}
}
}
public static class TestStatic //: ITestSingleton
{
public static void doAction(string args)
{
Console.WriteLine("Test Static :: " + args);
}
}
Use the static modifier to declare a static member, which belongs to the type itself rather than to a specific object. The static modifier can be used with classes, fields, methods, properties, operators, events, and constructors, but it cannot be used with indexers, destructors, or types other than classes.
Use singleton when you need a class that has only one instance, and you need to provide a global point of access to the instance
A singleton is basiccly an entry point to a single instance of a class. The instance of that class can be passed to another method as a reference.
A static class doesn't have this behaviour (only static methods are allowed).
I would like to get an instance of a static class, but I can’t seem to do this without implementing a singleton wrapper on a non-static class– is this possible, or am I missing something?
public class MyInstanceTester
{
public MyInstanceTester()
{
//this is how i get a reference to a singleton now
MyClass instance1 = MyClass.Instance();
//this is what is would like to do (if only the compiler would let me)
MyStaticClass instance2 = MyStaticClass.Instance();
}
}
public class MyClass
{
private static MyClass _myInstance;
static MyClass()
{
_myInstance = new MyClass();
}
public static MyClass Instance()
{
return _myInstance;
}
}
public static class MyStaticClass
{
public static MyStaticClass Instance
{
get
{
return this;
}
}
}
There is no such thing as an instance of a static class. The singleton pattern simply returns the same instance of a class to repeated requests.
You may be getting confused by:
private static MyClass _myInstance;
This simply means that there will be a single instance of that particular object among all objects instantiated of the type that have _myInstance as a member.
A few notes:
The this keyword is not valid in a static member
If you have a static class then all members have to be static and so this will never be valid
A Singleton class cannot be a static class
Singletons declare a single static member to help ensure that only a single instance of that class exists
Note that a static reference to an object does not make the object static. Only the reference is static
Further reading: Jon Skeet has an excellent write up on implemeting Singletons in C# In Depth. I would suggest reading and studying this article until you grok it. It is quite good.
There is no reason to return a instance to a static class ( If the class is static there is no instance ).
You can access the class from everywhere, why returning a instance to it? I can't imagine any reason to do this.
Static class usage
To use a static class just write it like below:
MyStaticClass.MyMethod();
Int32 callCount = MyStaticClass.CallCount;
As you can see it doesn't even make sense to declare a variable because this would just look like this:
MyStaticClass msc = MyStaticClass.Instance();
msc.MyMethod();
Int32 callCount = msc.CallCount;
If you want to have a shorter name you just can use:
using MSC = MyNamespace.MyStaticClass;
From your comments I assume your solution would be:
Make your class non-static. (Just keep the methods static.)
Your terminology is wrong. Please read the MSDN article on the static keyword.
A static member cannot be referenced through an instance. Instead, it
is referenced through the type name.
A singleton is a class that only allows a single instance of itself. A common implimentation of this in C# is:
public class MyClass
{
private MyClass _value = null;
public MyClass Value {
get { return _value ?? (_value = new MyClass()); }
}
}
The major problem is here:
public static class MyStaticClass
{
public static MyStaticClass Instance
{
get
{
return this; //compile time error!
}
}
}
this refers to an instance of a class which does not make sense in a static class as there can be no instance of one. This by itself should make you realize that there is a fundamental error in what you are asking: "I would like to get an instance of a static class". You can not return an instance of a static class as a static class by definition can not be instantiated.
The singleton pattern just makes sure that you always return the same instance of a class. But said class can never be static.
Is this singleton implementation correct and thread-safe?
class Class
{
public static readonly Class Instance;
static Class()
{
Instance = new Class();
}
private Class() {}
}
Technically, your version should work. However, I would not recommend exposing a public field within your Singleton class, and prefer using a Property (with a getter only). This will help future-proof your API if you need to make changes later. I also recommend sealing any singleton implementation, as subclassing a singleton class is almost always a bad idea and problematic.
I would, personally, use the following in C#, if you're targetting .NET 3.5 or earlier:
public sealed class Singleton
{
static readonly Singleton instance = new Singleton();
public static Singleton Instance
{
get
{
return instance;
}
}
static Singleton() { }
private Singleton() { }
}
If you're using .NET 4, you can make this even easier for yourself via Lazy<T>:
public sealed class Singleton
{
private static readonly Lazy<Singleton> instance = new Lazy<Singleton>( () => new Singleton() );
private Singleton() {}
public static Singleton Instance { get { return instance.Value; } }
}
The .NET 4 version also has the advantage of being fully lazy - even if your Singleton class has other static methods which are used prior to the access of the "Instance" property. You can do a fully-lazy .NET 3.5- version, as well, by using a private, nested class. Jon Skeet demonstrated this on his blog.
Yes. I would also make the class 'sealed' to avoid any future confusion.
Good discussion of how to do that is here:
http://www.yoda.arachsys.com/csharp/singleton.html
You should do the initialization in the variable declaration:
public static readonly Class Instance = new Class();
I was reading up on singleton class design in C# on this great resource and decided to go with alternative 4:
public sealed class Singleton1
{
static readonly Singleton1 _instance = new Singleton1();
static Singleton1()
{
}
Singleton1()
{
}
public static Singleton1 Instance
{
get
{
return _instance;
}
}
}
Now I wonder if this can be rewritten using auto properties like this?
public sealed class Singleton2
{
static Singleton2()
{
Instance = new Singleton2();
}
Singleton2()
{
}
public static Singleton2 Instance { get; private set; }
}
If its only a matter of readability I definitely prefer the second version, but I want to get it right.
It would work correctly, but it is not really the same.
Even if the property is marked as private, it is not read-only (e.g. assignable in the constructor only). Therefore you should use the first variant, which expresses the intent of having a variable (reference) which does not change after its initial assignment.
This is not equivalent, as the backing field of the autoproperty will not be readonly.
The backing field of the property is not readonly but as this is a private property it is not that great an issue. But the big advantage for the first implementation is that you can drop the static constructor, but for the second one you use a static constructor. A static constructor can add a performance hit (see http://msdn.microsoft.com/en-us/library/ms182275(v=VS.100).aspx )
Another thing to consider here is that initializing a property is more work than just initializing a field.
The static constructor in Singleton2 calls the set method on the property, which then sets the value of the backing field. I have no idea if or how this affects thread safety but it's another difference.