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.
Related
I've seen many people write singleton like this
public class Singleton
{
private static Singleton _instance = null;
public static Singleton Instance
{
get
{
if (_instance == null)
{
_instance = new Singleton();
}
return _instance;
}
}
}
What is the difference from this code
public class Singleton
{
private static Singleton _instance = new Singleton();
public static Singleton Instance
{
get { return _instance; }
}
}
Nowaday(C# 6), we have Getter-only auto-properties, is this difference from above(I prefer to write it like this)
public class Singleton
{
public static Singleton Instance { get; } = new Singleton();
}
From what I've known, static field is guaranteed to be ready before I access it for the first time, so it's nothing different, only thing that different is in the first case, I will know when instance is created.
Is there anything more than this or I misunderstand everything?
It is called "Lazy" which postpones the creation of value to first request.
Create the object at the very beginning.
Simplified version of 2.
Or, you can simply use "Lazy" class which many people neglect.
public class Singleton
{
private static Lazy<Singleton> instance = new Lazy<Singleton>();
public static Singleton Instance => instance.Value;
}
"Lazy" is good for large programs.
Reduce startup time since creation is postponed.
Save resource if eventually class is not used.
When I tried to use 2 different versions of the same class , they act actually the same.
I searched but can't find a satisfied answer for this question
What are the differences between Singleton and static property below 2 example, it is about initialization time ? and how can i observe differences ?
Edit : I don't ask about differences static class and singleton. Both of them non static, only difference is, first one initialized in Instance property, second one initialized directly
public sealed class Singleton
{
private static Singleton instance;
private Singleton()
{
}
public static Singleton Instance
{
get
{
if (instance == null)
{
instance = new Singleton();
}
return instance;
}
}
}
public sealed class Singleton2
{
private static Singleton2 instance = new Singleton2();
private Singleton2()
{
}
public static Singleton2 Instance
{
get
{
return instance;
}
}
}
Your first singleton implementation isn't thread safe; if multiple threads try to create a singleton at the same time it's possible for two instances to end up being created. Your second implementation doesn't have that flaw.
Other than that, they're functionally the same.
The instance of the Singleton class will be created the first time it is requested by the Singleton.Instance method.
The instance of Singleton2 will be created on initialization of its type which can be caused by several mechanisms. It will certainly be created before accessing any property or method on the Singleton2 class
Let's say I have the following singleton:
public sealed class singleton
{
// fields, creation, etc.
private static IList<SomeObjectType> _objects = new List<SomeObjectType>();
private ManualResetEvent _updateCompleted = new ManualResetEvent(true);
private bool RefreshRequired()
{
//true if records in DB newer than the objects in the list
}
private void RefreshList()
{
_updateCompleted.Reset();
//code to refresh the list
_updateCompleted.Set();
}
public IList<SomeObjectType> Objects
{
get
{
_updateCompleted.Wait();
if (RefreshRequired())
RefreshList();
return _objects;
}
}
..
This way I am trying to achieve that data stored in the list is always up to date before any client reads it. This mechanism is very simple, but it is working well so far. However, obviously it is not sufficient for multithreading scenarios.
If there were multiple threads accessing the Objects-Member, I wanted only the first one to check if data is up to date and, then update the List if necessary. While the refresh is in progress, all other threads should be forced to wait BEFORE even checking if an refresh is required.
I have read much ablut locks, BlockedCollections, and ManualResetEvents, but I am not sure about which concept to use.
Could you explain which one you would choose and how you would solve the described task?
Best answer i can suggest is found here: http://csharpindepth.com/Articles/General/Singleton.aspx
Since I'll get yelled at by the masses if only posting a link, here are some samples taken from the article to ponder. The article largely has to do with performance, and appropriateness so please read it's descriptions of these samples.
As far as your Refresh method, the others commented on that fairly well. It's just as important knowing how it's intended to be consumed.
Hopefully the article gives you some food for thought.
Simple thread safety...
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;
}
}
}
}
Thread-safe without using locks...
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;
}
}
}
Fully lazy instantiation...
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();
}
}
Using .NET 4's Lazy type...
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 always see singletons implemented like this:
public class Singleton
{
static Singleton instance;
static object obj = new object();
public static Singleton Instance
{
get
{
lock (obj)
{
if (instance == null)
{
instance = new Singleton();
}
return instance;
}
}
}
protected Singleton() { }
}
Is there's something wrong with implementing it like this:
public class Singleton
{
static readonly Singleton instance = new Singleton();
public static Singleton Instance
{
get { return instance; }
}
protected Singleton() { }
}
? It gets lazy initialized on the very same moment as in the first implementation so I wonder why this isn't a popular solution?
It should be also faster because there's no need for a condition, locking and the field is marked as readonly which will let the compiler to do some optimisations
Let's not talk about the singleton (anti)pattern itself, please
The CLR will initialize the field upon the first use of that (or any other static) field. It promises to do so in a thread-safe manner.
The difference between your code is that the first bit of code supports thread-safe lazy initialization where the second doesn't. This means that when your code never accesses Singleton.Instance of the first code, no new Singleton() will ever be created. For the second class it will, as soon as you access Instance or (directly or indirect) any other static member of that class. Worse even - it may be initialized before that because you lack a static constructor.
Favoring shorter and more readable code, since .NET 4 you can use Lazy<T> to significantly shorten the first code block:
public class Singleton
{
static readonly Lazy<Singleton> instance =
new Lazy<Singleton>(() => new Singleton());
public static Singleton Instance
{
get { return instance.Value; }
}
static Singleton() { }
private Singleton() { }
}
As Lazy<T> also promises thread-safety. This will ensure new Singleton() is only called once, and only when Singleton.Instance is actually used.
On MSDN I found two approaches to creating a singleton class:
public class Singleton {
private static Singleton instance;
private Singleton() {}
public static Singleton Instance {
get {
if (instance == null)
instance = new Singleton();
return instance;
}
}
}
and
public sealed class Singleton {
private static readonly Singleton instance = new Singleton();
private Singleton(){}
public static Singleton Instance {
get { return instance; }
}
}
My question is: can we just use a static constructor that will make for us this object before first use?
Can you use the static constructor, sure. I don't know why you'd want to use it over just using the second example you've shown, but you certainly could. It would be functionally identical to your second example, but just requiring more typing to get there.
Note that your first example cannot be safely used if the property is accessed from multiple threads, while the second is safe. Your first example would need to use a lock or other synchronization mechanism to prevent the possibility of multiple instances being created.