I extended a singleton class that gets the reference to the singleton object with the following call
SingletonClass.singleton
Now I want that every time I call the singleton variable on my CustomSingletonClass that I get a reference to a CustomSingletonClass and not SingletonClass.
Right now I'm using a typecast to achieve that ((CustomSingletonClass)CustomSingletonClass.singleton)
Do I have to override the singleton property somehow?
Usually singleton use static properties to get the instance of the object (seems like yours is SingletonClass.singleton).
As it is a static call, you can't override this porperty in an inherited object because the call will always be done on the current type, not on the instance (overriding static is just nonsense).
You could try to change the way it works by adding a setter to your singleton and setting the desired instance before any call.
public class A
{
private static A _instance;
public static A Instance
{
get
{
if (_instance == null)
{
_instance = new A();
}
return _instance;
}
set { _instance = value; }
}
}
public class B : A
{
}
A.Instance = new B();
Related
I am having a singleton class in my code.
In my main function, I created an object of that class.
Then, I tried to create the clone for that object.
And, it gave me "StackOverflowException".
My code looks like:
namespace SingletonApplication
{
class A : ICloneable
{
private static readonly A A1 = new A();
public A A2 { get { return A1; } }
public object Clone()
{
var obj = ((ICloneable)A1).Clone();
return obj;
}
}
class Program
{
static void Main(string[] args)
{
A obj1 = new A();
A obj2 = (A)(obj1.Clone());
Console.WriteLine(object.ReferenceEquals(obj1.A2, obj2.A2));
Console.ReadKey();
}
}
}
Error:
Singleton, by definition, is meant to be a class with only one instance across entire application.
The StackOverflowException you get is caused by Clone menthod which keeps calling itself.
The requirments are contradictory ones:
Singleton can have at most one instance by its own definition
Clone() method is supposed to produce a clone, a new (== second) instance
Probably a better solution is to return a fake clone (i.e. itself)
// sealed: do not inherit from me (I'm a Singleton) and create a second instance
sealed class A : ICloneable
{
private static readonly A A1 = new A();
//private constructor: do not create instances (I'm a Sinleton)
private A() {}
public A A2 { get { return A1; } }
// We can't create a new clone but can return an existing instance
public object Clone()
{
// Cloned Singleton? Let it be itself
return this;
}
}
Instead of recursively calling Clone on the same object over and over again, you should actually do something to clone that object.
Usually you create a new instance and copy all relevant properties over. Like this:
A newA = new A();
newA.X = this.X;
return newA;
However, there is nothing to copy in your code. The readonly field will stay as it is. I wonder why you need a copy of a singleton since that defeats the purpose of the design pattern. And by the way, you implementation of singleton is quite unique. I advise to read up on singletons and follow some samples there.
The error is caused because You let Clone call itself. resulting in an endless loop
And This is not a singleton. A singleton should look like this
class A
{
private static A instance;
private A() { }
public static A Instance
{
get
{
if (instance == null)
{
instance = new A();
}
return instance;
}
}
}
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.
it may be basic question
to have a singleton in multi-threaded environment we can use a lock. Please refer the code snippet. But why do we need double-checked locking in singleton pattern? And more what does double-checked locking means?
class singleton
{
private static singleton instance = null;
private static singleton() { }
private static object objectlock = new object();
public static singleton Instance
{
get
{
lock (objectlock) //single - check lock
{
if (instance == null)
{
instance = new singleton();
}
return instance;
}
}
}
}
Jon Skeet explains this in detail.
Locks are expensive.
If the object already exists, there's no point in taking out a lock.
Thus, you have a first check outside the lock.
However, even if the object didn't exist before you took the look, another thread may have created it between the if condition and the lock statement.
Therefore, you need to check again inside the lock.
However, the best way to write a singleton is to use a static constructor:
public sealed class Singleton
{
private Singleton()
{
}
public static Singleton Instance { get { return Nested.instance; } }
private class Nested
{
// Explicit static constructor to tell C# compiler
// not to mark type as beforefieldinit
static Nested()
{
}
internal static readonly Singleton instance = new Singleton();
}
}
And with .Net 4.x and newer you should defer to the Lazy class when possible as this pattern is used with the Initialization And Publication option. (note: the inverse is available as well where creation isn't thread safe but the publication of the instance is via the Publication option)
Multithreaded Singleton : The best approach to use double check locking
public sealed class Singleton
{
private static volatile Singleton _instance;
private static readonly object InstanceLoker= new Object();
private Singleton() {}
public static Singleton Instance
{
get
{
if (_instance == null)
{
lock (InstanceLoker)
{
if (_instance == null)
_instance = new Singleton();
}
}
return _instance;
}
}
}
The "best" way I know is this:
public class MySingleton {
// object for synchronization
private static readonly object syncRoot = new object();
// the singleton instance
private static MySingleton #default;
public static MySingleton Default {
get {
// geting singleton instance without locking
var result = #default;
// if result is NOT null, no additional action is required
if ( object.ReferenceEquals(result, null) ){
// lock the synchronization object
lock(syncRoot) {
// geting singleton instanc in lock - because
// the value of #default field could be changed
result = #default;
// checking for NULL
if ( object.ReferenceEquals(result, null) ) {
// if result is NULL, create new singleton instance
result = new MySingleton();
// set the default instance
#default = result;
}
}
}
// return singleton instance
return result;
}
}
}
If you create the object in the field initialiser, you don't need the lock:
class singleton
{
private static singleton instance = new singleton();
private static singleton() { }
public static singleton Instance
{
get { return instance; }
}
}
Also - bear in mind that the lock is only controlling the creation of the object, the object would still need to be thread-safe if you're using it in multiple threads.
When we try to execute the method of the singleton class using Parallel libraries. It doesn’t recognize the singleton behavior because of multi threading is executing in TPL which causes to failure of concept Singleton Pattern . To overcome this problem there is concept of the locking of the object so that at a time only one thread can access it.
But this is not efficient approach because involvement of lock checking creates unnecessary watches to the object. To avoid it we make use “Double locking checking”
This is a slight variance of this question: Possible to use a singleton with a non-default constructor in C#?
I have a class that takes parameters for it's constructor. I would like to make this singleton such that the parameters are taken upon initialising the singleton and thus would not need to be passed in each time the instance is retrieved.
My solution (which is not elegant) for this is to have a CreateInstance() static method that takes the parameters and constructs the singleton instance. I would then have another static method GetInstance() which would be parameterless to obtain the singleton instance. In code, I would then need to ensure the logic calls CreateInstance before any calls to GetInstance. However, I cannot enforce this at compile time. I can, however, check at runtime by throwing an exception in GetInstance if it is called before CreateInstance.
Is there anyway I can achieve this behaviour with compile time enforcement? Or at the very least, is there a better way of doing the same thing?
There is no way to do it at compile time, because that would be like asking the compiler "can you prove that code X is never executed before code Y is executed, in the presence of multiple threads?". It cannot be done.
As for the runtime behavior of your design, I think this is as good as it can ever be.
You can make it slightly better by exposing a Func<SingletonType> property in your singleton class. When someone asks for the singleton instance and the instance has not already been created, your class would call this "factory method" to construct the singleton. If the factory method is null, then you either throw an exception or (if applicable) construct using some default parameters.
What this does is essentially defer the construction of the singleton until it's actually needed for the first time, so it's some improvement. But the underlying principle is the same.
Update:
As LukeH points out, this is pretty much what Lazy<T> does (.NET 4 only). If possible, use that one instead of writing your own.
In a classic singleton, the real magic happens in static readonly which creates the instance as soon as it is used:
public class MySingleton
{
private static readonly _instance = new MySingleton();
private MySingleton() {}
public static MySingleton Instance
{
get
{
return _instance;
}
}
}
If you have parameters to pass to constructor, you have to implement locking yourself (note the double if sandwitching the lock):
public class MySingletonWithConstructor
{
private static _instance;
private static object _lock = new Object();
private MySingletonWithConstructor(string myArg)
{
// ... do whatever necessary
}
public static MySingletonWithConstructor Instance
{
get
{
if(_instance==null)
{
lock(_lock)
{
if(_instance==null) // double if to prevent race condition
{
_instance = new MySingletonWithConstructor("Something");
}
}
}
return _instance;
}
}
}
You could just have GetInstance() call the CreateInstance() method if the singleton object doesn't exist already.
I would do it similar to this. You may have to add locks or other things to ensure:
public class ClassA {
private static ClassA instance;
private int param;
private ClassA(int param) {
this.param = param;
}
public static ClassA getInstance() {
if (instance == null) {
throw new CustomException("Not yet initialised");
} else {
return instance;
}
}
public static void createInstance(int param) {
if (instance == null) {
instance = new ClassA(param);
}
}
}
In your GetInstance() method, why dont you just call CreateInstance if your value is null, then you have lazy initialisation..
Use CreateInstance() to be the loader of a Lazy<T> and have GetInstance return the Lazy.Value (you might want to create a static readonly field that is set to = thelazy.Value to ensure a single entry into CreateInstance())
We have a custom DLL implemented IHttpModule to handle the httpApplication_EndRequest, what I want to know is
The DLL has a class (not a static class) which has a static property is used to create an instance for a static variables/object reference defined inside the class.
Now, should I need to lock inside static property before creating an instance for the static object/variable?
Eg:-
public class SPEnvironment : IEnvironment
{
private static SPEnvironment _instance;
private static object _syncRoot = new object();
private SPEnvironment()
{
try {
.....
}
finally {
......
}
}
public static SPEnvironment Instance
{
get
{
if (_instance == null)
{
lock (_syncRoot)
{
if (_instance == null)
{
_instance = new SPEnvironment();
}
}
}
return _instance;
}
}
}
I will calling this from an another class, like below
SPEnvironment.Instance;
Is it the right way? or the lock should be removed?
The double null check with a lock in the middle is a good, thread-safe way of instantiating a singleton. However, you could save yourself a lot of code by just saying
public class SPEnvironment : IEnvironment
{
public static SPEnvironment Instance = new SPEnvironment();
private SPEnvironment()
{
try {
.....
}
finally {
......
}
}
}
The difference between the two is that this code instantiates the singleton the first time an object of that type is created where your code instantiates the singleton the first time SPEnvironment.Instance is accessed. In almost all cases, those are the same thing; in most of the remaining cases, it doesn't matter; but it is a subtle distinction that is worth understanding for that very rare edge case.