Sunday, August 14, 2011

Thread safe singleton Pattern


.NET is multithreaded meaning that more than one thread can execute our Singleton concurrently. This means that we never know—and don't usually care—that our thread may be starting and stopping with other threads running in the gaps.
The singleton pattern is one of the best-known patterns in software engineering. Essentially, a singleton is a class which only allows a single instance of itself to be created, and usually gives simple access to that instance.
Singleton is lazily - i.e. that the instance isn't created until it is first needed.
The four common characteristics of a singleton are

  • A single constructor, which is private and parameterless. This prevents other classes from instantiating it (which would be a violation of the pattern). Note that it also prevents subclassing - if a singleton can be subclassed once, it can be subclassed twice, and if each of those subclasses can create an instance, the pattern is violated. The factory pattern can be used if you need a single instance of a base type, but the exact type isn't known until runtime.
  • The class is sealed. This is unnecessary, strictly speaking, due to the above point, but may help the JIT to optimise things more.
  • A static variable which holds a reference to the single created instance, if any.
  • A public static means of getting the reference to the single created instance, creating one if necessary.


Note that all of these implementations also use a public static property Instance as the means of accessing the instance. In all cases, the property could easily be converted to a method, with no impact on thread-safety or performance.

Singleton Implementation

public sealed class Singleton
{
    static Singleton instance=null;
    Singleton()
    {
    }

    public static Singleton Instance
    {
       get
        {
            if (instance==null)
            {
                instance = new Singleton();
            }
            return instance;
        }
    }
}

Please note that the above implementation is not thread safe. Means to say that if Two different threads could both have evaluated the test if (instance==null) and found it to be true, then both create instances, which violates the singleton pattern. Note that in fact the instance may already have been created before the expression is evaluated, but the memory model doesn't guarantee that the new value of instance will be seen by other threads unless suitable memory barriers have been passed.

Thread safe Singleton implementation.
public sealed class Singleton
{
    static Singleton instance=null;
    static readonly object padlock = new object();
 
    Singleton()
    {
    }
 
    public static Singleton Instance
    {
        get
        {
            lock (padlock)
            {
                if (instance==null)
                {
                    instance = new Singleton();
                }
                return instance;
            }
        }
    }
}

This implementation is thread-safe. The thread takes out a lock on a shared object, and then checks whether or not the instance has been created before creating the instance. This takes care of the memory barrier issue (as locking makes sure that all reads occur logically after the lock acquire, and unlocking makes sure that all writes occur logically before the lock release) and ensures that only one thread will create an instance (as only one thread can be in that part of the code at a time - by the time the second thread enters it,the first thread will have created the instance, so the expression will evaluate to false). Unfortunately, performance suffers as a lock is acquired every time the instance is requested.
Note that instead of locking on typeof(Singleton) as some versions of this implementation do, I lock on the value of a static variable which is private to the class. Locking on objects which other classes can access and lock on (such as the type) risks performance issues and even deadlocks. This is a general style preference of mine - wherever possible, only lock on objects specifically created for the purpose of locking, or which document that they are to be locked on for specific purposes (e.g. for waiting/pulsing a queue). Usually such objects should be private to the class they are used in. This helps to make writing thread-safe applications significantly easier.


thread-safety using double-check locking
public sealed class Singleton
{
    static Singleton instance=null;
    static readonly object padlock = new object();
 
    Singleton()
    {
    }
 
    public static Singleton Instance
    {
        get
        {
            if (instance==null)
            {
                lock (padlock)
                {
                    if (instance==null)
                    {
                        instance = new Singleton();
                    }
                }
            }
            return instance;
        }
    }
}

implementation attempts to be thread-safe without the necessity of taking out a lock every time. Unfortunately, there are four downsides to the pattern:
  • It's easy to get wrong. The pattern needs to be pretty much exactly as above - any significant changes are likely to impact either performance or correctness.
  • It still doesn't perform as well as the later implementations.
thread-safe without using locks
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;
        }
    }
}
As you can see, this is really is extremely simple - but why is it thread-safe and how lazy is it? Well, static constructors in C# are specified to execute only when an instance of the class is created or a static member is referenced, and to execute only once per AppDomain. Given that this check for the type being newly constructed needs to be executed whatever else happens, it will be faster than adding extra checking as in the previous examples. There are a couple of wrinkles, however:
  • It's not as lazy as the other implementations. In particular, if you have static members other than Instance, the first reference to those members will involve creating the instance. This is corrected in the next implementation.
  • There are complications if one static constructor invokes another which invokes the first again. Look in the .NET specifications (currently section 9.5.3 of partition II) for more details about the exact nature of type initializers - they're unlikely to bite you, but it's worth being aware of the consequences of static constructors which refer to each other in a cycle.
  • The laziness of type initializers is only guaranteed by .NET when the type isn't marked with a special flag called beforefieldinit. Unfortunately, the C# compiler (as provided in the .NET 1.1 runtime, at least) marks all types which don't have a static constructor (i.e. a block which looks like a constructor but is marked static) as beforefieldinit.
One shortcut you can take with this implementation (and only this one) is to just make instance a public static readonly variable, and get rid of the property entirely. This makes the basic skeleton code absolutely tiny! Many people, however, prefer to have a property in case further action is needed in future, and JIT inlining is likely to make the performance identical. (Note that the static constructor itself is still required if you require laziness.)
fully lazy instantiation
public sealed class Singleton
{
    Singleton()
    {
    }

    public static Singleton Instance
    {
        get
        {
            return Nested.instance;
        }
    }
   
    class Nested
    {
        // Explicit static constructor to tell C# compiler
        // not to mark type as beforefieldinit
        static Nested()
        {
        }

        internal static readonly Singleton instance = new Singleton();
    }
}


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