public class FileStorage
{
private static FileStorage _instance;
public static FileStorage instance
{
get
{
return (_instance != null) ? _instance : _instance = new FileStorage();
}
}
//public string GetAddress...
}
can i use that piece of code for a simple singleton solution? the code will execute on single thread.
Does the return, return a new FileStorage()?
what does exactly be returned here?
var foo = (bee = new[]{/*immagination...*/})
will foo be a boolean value? or effectively it's a reference to bee?
Yes, this would work as you intended. However, even if you are working with a single-threaded application, keep in mind that this method is not thread-safe.
Still, to avoid going on a bug hunt when you need to use threads in your app, it is usually better to ensure thread safety precautionarily.
For example:
public class FileStorage
{
private static readonly object _singletonLock = new object();
private static FileStorage _instance = null;
public static FileStorage Instance
{
get
{
lock (_singletonLock)
{
return _instance ??= new FileStorage();
}
}
}
}
This code snippet makes use of the ??= operator (assign if null) and also encapsulates the getter method of the Instance property in a lock-statement to ensure thread safety.
??= is much more compact and easier to read and has the same effect as your ternary operator.
Related
Im trying to test my singleton object in C# but somehow not satisfied on how I brute force instantiating the object(using parallel foreach).
Is there a right way/better way to test it?
I don't understand what you mean with "parallel foreach". A singleton is implemented like this:
public class MyClass
{
private static MyClass _instance;
private MyClass()
{
//Do Stuff
}
public static MyClass GetInstance()
{
if(_instance == null)
_instance = new MyClass();
return _instance;
}
}
Another way, instead of a Method is a Property:
private static readonly object LockObject = new object();
private static MyClass _instance;
public static MyClass Instance
{
get
{
lock (LockObject)
{
return _instance ?? (_instance = new MyClass());
}
}
}
While I prefer the first Method, cause it's much easier to implement, even for beginners, the Property is also a good way
Singleton class can have only one instance, If it is initialized multiple times then that means you have not correctly implemented the Singleton Design.
You can check the instance.Hashcode() value, it must remain same wherever you are using the instance of that Singleton class.
I was asked to write Singleton in the interview today. I wrote the below, please note, I used "property set" method to set and then I returned the instance using "get" method. But I see in internet that most places they use only get, meaning, what I did below is wrong? Sorry I dont have VS ide with me to verify it now, so posting it here.
Also, some used sealed class including with private constructor. Why sealed with private cons?
public class Singleton
{
private static readonly Singleton instance;
private Singleton() {}
public static Singleton Instance
{
set
{
if(instance == null){
instance = new Singleton();
}
}
get
{
return instance;
}
}
}
My advice is to try to compile and run the code yourself. It's by far the easiest way to understand how it works.
If you would try to build your code you would get the following error :
Error CS0198 A static readonly field cannot be assigned to (except in a static constructor or a variable initializer)
In other words, you should instantiate your instance inside the constructor.
Regarding your question, a private constructor is needed to prevent access from outside your class and also it is enough to make sure that other classes cannot inherit from your class. You don't really need the sealed.
You can find a really good summary regarding the singleton pattern # https://csharpindepth.com/articles/singleton
#Learner Since its a interview question and mostly in India they ask to write to Psuedo code to evaluate the candidate coding skills, I try to fit myself in the candidate shoes to give the answer.
Well design patterns has evolved over a period of time with advancements in the programming language and Singleton is not a exception. There are many ways that we can create a Singleton class using C#. I would like to showcase few of the flavors that I can able to recollect
1. Plain vanilla Singleton without Thread-Safety
public sealed class Singleton
{
private Singleton()
{
}
private static Singleton instance = null;
public static Singleton Instance
{
get
{
if (instance == null)
{
instance = new Singleton();
}
return instance;
}
}
}
2. Singleton with Thread Saftey
public sealed class Singleton_ThreadLock
{
Singleton_ThreadLock()
{
}
private static readonly object padlock = new object();
private static Singleton_ThreadLock instance = null;
public static Singleton_ThreadLock Instance
{
get
{
// Uses the lock to avoid another resource to create the instance in parallel
lock (padlock)
{
if (instance == null)
{
instance = new Singleton_ThreadLock();
}
return instance;
}
}
}
}
3. Singleton - Double Thread Safe
public sealed class Singleton_DoubleThreadSafe
{
Singleton_DoubleThreadSafe()
{
}
private static readonly object padlock = new object();
private static Singleton_DoubleThreadSafe instance = null;
public static Singleton_DoubleThreadSafe Instance
{
get
{
if (instance == null)
{
lock (padlock)
{
if (instance == null)
{
instance = new Singleton_DoubleThreadSafe();
}
}
}
return instance;
}
}
}
4. Singleton - Early Initialization
public sealed class Singleton_EarlyInitialization
{
private static readonly Singleton_EarlyInitialization instance = new Singleton_EarlyInitialization();
// Explicit static constructor to tell C# compiler
// not to mark type as beforefieldinit
static Singleton_EarlyInitialization()
{
}
private Singleton_EarlyInitialization()
{
}
public static Singleton_EarlyInitialization Instance
{
get
{
return instance;
}
}
}
5. Singleton - Lazy Initialization using .Net 4.0+ Framework
public sealed class Singleton
{
private Singleton()
{
}
private static readonly Lazy<Singleton> lazy = new Lazy<Singleton>(() => new Singleton());
public static Singleton Instance
{
get
{
return lazy.Value;
}
}
}
Caveats:
Well there are few people who create instance of the class using reflection (I did in one of my framework) but his can also be avoided. There are few samples in net that can show how to avoid it
Its always best practice to make the Singleton class as sealed as it will restrict developers from inheriting the class.
There are lots of IOC's in the market that can create Singleton instance of a normal class without following the above Singleton implementation.
Assume a class SomeClass with private static field like this. The access to this field is synchronized using lock.
private static SomeClass _instance
private static object _sync = new object();
public static SomeClass Instance
{
get
{
lock (_sync)
{
if (_instance == null)
{
_instance = Create();
}
return _instance;
}
}
}
When another code from different thread will try to set the value of this variable to e.g. null using reflection, will the lock prevent this and let the reflection call wait until the lock was released?
E.g. something like this:
Type type = typeof(SomeClass);
string fieldName = "_instance";
object value = null;
FieldInfo field = type.GetField(fieldName, true);
field.SetValue(null, value);
No, lock will not prevent any access that does not go through locking the same resource. Since reflection will not go through lock, you will get race conditions.
Here is (slightly different from your code but nontheless doing same thing) what I mean→
void SetOne(){
lock (_sync){
critical_element = SOME_VALUE;
}
}
void SetTwo(){
critical_element = SOME_ANOTHER_VALUE;
}
Above definitely has race conditions.
Here is my understanding behind the OP's question. I think OP wants to use Singleton pattern and here is a very nice and thread safe implementation. You do not need to deal with locks either. However, some bad users might still set the backing field using reflection.
public sealed class Singleton
{
private static readonly Lazy<Singleton> lazy = new Lazy<Singleton>(() => new Singleton());
public static Singleton Instance { get { return lazy.Value; } }
private Singleton(){}
}
I have a singleton defined like this:
public partial class MoonDataManager
{
static MoonDataManager _singletonInstance;
public static MoonDataManager SingletonInstance
{
get
{
return _singletonInstance;
}
private set
{
_singletonInstance = value;
}
}
I have a function that safely creates the instance:
public static async Task<MoonDataManager> CreateSingletonAsync()
{
_singletonInstance = new MoonDataManager();
Should I:
return _singletonInstance; (field)
or
return SingletonInstance; (property)
I'm concerned with Garbage Collection, especially in iOS or Android within Xamarin.
Also if there are naming patterns for this in C# let me know if I deviated from a standard.
Update:
Now I think I really got myself stuck with threading and async methods. Here are the objects and their goals:
MoonDataManager : Run the RegisterTable<Models.IssuerKey> once per table. This is a generic method that essentially runs (new MobileServiceSQLiteStore).DefineTable<T>()
OfflineStore : This is a MobileServiceSQLiteStore.
MobileClient : This is a MobileServiceClient.
MoonDataManager Dependencies: The MoonDataManager requires OfflineStore and MobileClient to finish initialization. Specifically, it does a MobileServiceClient.SyncContext.InitializeAsync(OfflineStore)
I'm not sure how to make sense of this spaghetti of dependencies... or how to make the code look nice, and be thread safe.
Here is the new iteration of the code:
private readonly Lazy<MobileServiceClient> lazyMobileClient =
new Lazy<MobileServiceClient>(() => new MobileServiceClient(Constants.ApplicationURL), true); // true for thread safety
public MobileServiceClient MobileClient { get { return lazyMobileClient.Value; } }
private readonly Lazy< MobileServiceSQLiteStore> offlineDB =
new Lazy<MobileServiceSQLiteStore>(() => new MobileServiceSQLiteStore(Constants.OfflineDBName), true ); // true for thread safety
private MobileServiceSQLiteStore OfflineStore { get { return offlineDB.Value; } }
private static readonly Lazy<MoonDataManager> lazy = new Lazy<MoonDataManager>(() => new MoonDataManager(), true); // true for thread safety
public static MoonDataManager Instance { get { return lazy.Value; } }
private MoonDataManager()
{
MoonDataManager.Instance.RegisterTable<Models.IssuerKey>();
// Initialize file sync
// todo: investigate FileSyncTriggerFactory overload.
//Was present on Mar 30, 2016 Channel9 https://channel9.msdn.com/events/Build/2016/P408
MoonDataManager.Instance.MobileClient.InitializeFileSyncContext
(new IssuerKeyFileSyncHandler(Instance), Instance.OfflineStore);
// NOTE THE ASYNC METHOD HERE (won't compile)
await MoonDataManager.Instance.MobileClient
.SyncContext.InitializeAsync(MoonDataManager.Instance.OfflineStore,
StoreTrackingOptions.NotifyLocalAndServerOperations);
}
For .NET 4 or higher, you can use the Lazy<T> and create it like this.
public sealed class Singleton
{
private static readonly Lazy<Singleton> lazy =
new Lazy<Singleton>(() => new Singleton(), true); // true for thread safety
public static Singleton Instance { get { return lazy.Value; } }
private Singleton()
{
}
}
It will be created only if it is accessed and only the first time and it is threadsafe.
The definition
static MoonDataManager _singletonInstance;
ensures that the instance of MoonDataManager is a GC root, and it will not be collected until the application domain ends, because it is a static value.
I'd return the private singleton and forego the auto property that you have.
public partial class MoonDataManager
{
private static readonly Lazy<MoonDataManager> _manager =
new Lazy<MoonDataManager>(() => new MoonDataManager());
public static MoonDataManager SingletonInstance => _manager.Value;
}
When MoonDataManager.Value is accessed for the first time, it is initialized using the Func<MoonDataManager> that was passed to the constructor for Lazy<T>. On subsequent accesses, the same instance is returned.
A Singleton creates itself the first time it's accessed, in a way that ensures only one instance will get created, even if a second thread tries to access it while it's still being instantiated
your CreateSingletonAsync() violates this, and looks like it'd allow for multi-thread nastiness
You want something like:
public static MoonDataManager SingletonInstance
{
get
{
if (_singletonInsatnce != null)
return _singletonInstance;
lock (lockobject)
{
// check for null again, as new one may have been created while a thread was waiting on the lock
if (_singletonInsatnce != null)
return _singletonInstance;
else
// create new one here.
}
}
// no setter, because by definition no other class can instantiate the singleton
}
All this is just to ensure that two threads asking for one object don't end up creating two objects, or the second thread getting a half-created object if the first thread's one is still being created.
NB: Singletons have become unfashionable.
NB: If you can be sure that you've got time to create your object before it's ever accessed, you can just use a static member and create it on application start.
Your question "should I return the property or field" doesn't make sense -- you're already returning the field from the property getter, which is standard practise. Where else are you wanting to return something?
You should return the private instance. You can read more about the singleton pattern on MSDN. The standard singleton implementation is as follows:
public class Singleton
{
private static Singleton instance;
private Singleton() {}
public static Singleton Instance
{
get
{
if (instance == null)
{
instance = new Singleton();
}
return instance;
}
}
}
Although, normally, you don't have a setter for the property. This pattern has already previously been discussed on SO.
I have a function which does "migration" from an old format to a new format. I need this to occur in the constructor of my object, but not the static constructor because an argument is needed. How can I have a piece of code only execute exactly one time?
For some context:
class Foo
{
public Foo(string bar)
{
ShouldOnlyExecuteOnce(bar);
}
}
and then usage could be (with each line a different thread)
var f = new Foo("bar");
var fb = new Foo("meh");
etc
How can I properly guard the "ShouldOnlyExecuteOnce" method?
Because this is a sort of "migration" type function, I want for the first object created to "win" and get to migrate the old data into this new object. Later objects that get constructed should not attempt to execute this function, even if their arguments are different
You could use double check locking.
class Foo
{
private static bool ShouldOnlyExecuteOnceExecuted = false;
private static readonly object Locker = new object();
public Foo(string bar)
{
SetShouldOnlyExecuteOnce(bar);
}
private void SetShouldOnlyExecuteOnce(string bar)
{
if(!ShouldOnlyExecuteOnceExecuted)
{
lock(Locker)
{
if(!ShouldOnlyExecuteOnceExecuted)
{
ShouldOnlyExecuteOnce(bar);
ShouldOnlyExecuteOnceExecuted = true;
}
}
}
}
}
I would generally recommend against implementing subtle mechanisms such as double-checked locking, especially when you already have them implemented in the BCL. In this case:
public class SafeInitializater
{
private bool _initialized;
private object _dummy;
private object _syncLock;
public void InitializeOnce(Action initializer)
{
LazyInitializer.EnsureInitialized(ref _dummy, ref _initialized, ref _syncLock,
() => {
initializer();
return null;
});
}
}
Usage sample:
var initializer = new SafeInitializater(); //or you could make this static somewhere
var t1 = Task.Run(() =>
{
Console.WriteLine($"Task {Task.CurrentId} entering the race");
initializer.InitializeOnce(() => Console.WriteLine($"Task {Task.CurrentId} won!"));
});
var t2 = Task.Run(() =>
{
Console.WriteLine($"Task {Task.CurrentId} entering the race");
initializer.InitializeOnce(() => Console.WriteLine($"Task {Task.CurrentId} won!"));
});
await Task.WhenAll(t1, t2);
You need a mutex. This is exactly the kind of usecase mutexes are designed for.
http://www.dotnetperls.com/mutex
you should read about monitors, semaphors and the sigelton design-pattern
What is the Mutex and semaphore In c#? where we need to implement?
http://www.c-sharpcorner.com/UploadFile/1d42da/threading-with-semaphore-in-C-Sharp/
I have no idea, but I'd guess/try:
1 - A static class acting as a wrapper and calls your method in the static constructor
2 - Some IoC container?
3 - Singleton?
4 - Lock?
5 - All of above?
I agree with P.Brian that this seems like a singleton pattern. You can see a very good article on it here http://csharpindepth.com/articles/general/singleton.aspx which breaks it down nicely. Generally the best solution is #4;
public sealed class Singleton
{
private static readonly Singleton instance = new Singleton();
// Explicit static constructor to tell C# compiler
// not to mark type as beforefieldinit
static Singleton()
{
}
private Singleton()
{
}
public static Singleton Instance
{
get
{
return instance;
}
}
}
A Singleton pattern should work.
Parrotting Jon Skeet: http://csharpindepth.com/articles/general/singleton.aspx
public sealed class Singleton
{
private static readonly Singleton instance = new Singleton();
// Explicit static constructor to tell C# compiler
// not to mark type as beforefieldinit
static Singleton()
{
}
private Singleton()
{
}
public static Singleton Instance
{
get
{
return instance;
}
}
}
Singleton's are constructed one time. Although, the part about passing in a new string value could present an issue. Is this value ignored on all consecutive calls?