Singleton Alternative - is it equivalent?

问题

Quick question -

I know that the standard singleton pattern is as follows:

Original

public class Singleton1
{

    public static Singleton1 _Instance;
    public static Singleton1 Instance
    {
        get
        {
            if (_Instance == null)
            {
                _Instance = new Singleton1();
            }
            return _Instance;
        }
    }

    private Singleton1()
    {

    }
}

But it seems like this code is unnecessary. To me, you could accomplish the same thing with either of the following simple design patterns:

Version 2

public class Singleton2
{
    public static readonly Singleton2 Instance = new Singleton2();

    private Singleton2()
    {
    }
}

Version 3

public class Singleton3
{
    static Singleton3()
    {
    }
}

To me, it seems like version 2 is the superior method of doing this because it allows you to pass in parameters (or not) yet still have a finite number of instance. My application is fairly latency/performance sensitive - do any of these patterns have a performance gain?

It would seem that while it will longer to access each one the first time because the object is being created. Also, it would seem that the original one is ever so slightly slower because it must check to see whether its backing field is null every time something else accesses it.

Thanks in advanced!

回答1:

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()
    {
    }
}

Fast, clean, thread-safe.

回答2:

One problem with singletons implemented as static instances is that they make testing and mocking more difficult.

See this scenario:

public void BusinessLogicMethod()
{
    var initialValue = MySingleton.Instance.GetInitialValue();
    var processedValue = initialValue + specialSomething + businessLogic;
    MySingleton.Instance.SaveProcessedValue(processedValue);
}

Now, let's say I want to write a unit-test for this method. Ideally, I want to write a test that specifies input and output and tests only the business logic. But with a static singleton, the method's implementation is tied to the singleton's implementation. Can I set the InitialValue easily at the beginning of the test, or is it dependent on other factors/DB access/whatever?

However, if I use a non-static singleton, coupled with some dependency injection or service locator pattern, I can build my function like this:

public void BusinessLogicMethod()
{
    var singleton = ServiceLocator.Resolve<MySingleton>();
    var processedValue = singleton.InitialValue + specialSomething + businessLogic;
    singleton.SaveProcessedValue(processedValue);
}

and my test can go like this, using vaguely Moq-like mock syntax:

public void TestBusinessLogic()    
{
    MySingleton fakeSingleton = new Mock<MySingleton>();
    fakeSingleton.Setup(s => s.InitialValue).Returns(5);

    // Register the fake in the ServiceLocator
    ServiceLocator.Register<MySingleton>(fakeSingleton.Object);

    // Run
    MyBusinessMethod();

    // Assert        
    fakeSingleton.Verify (s => s.SaveProcessedValue()).Called(Exactly.Once);
}

without worrying about the REAL singleton implementation.

回答3:

Singleton2 is not the same as Singleton1 as the Instance is not "lazy" evaluated. In Singleton1, Instance is created only when it is accessed and from then on the same one is used. In SingleTon2, the Instance is initialized with the class and before being actually accessed.

回答4:

My favourite singleton implementation is this one: http://www.codeproject.com/Articles/14026/Generic-Singleton-Pattern-using-Reflection-in-C

Make sure your .ctor is not public, which is the most common mistake, then, it is safely/fully reusable.

(I need to have a close look at Peter Kiss' one which looks nice too)

回答5:

To answer your performance question, the time it takes to check whether the private field is null is negligible. Therefore I wouldn't be worrying about how it is implemented with regards to performance here.

来源：https://stackoverflow.com/questions/9451672/singleton-alternative-is-it-equivalent

标签

performance

c#-4.0

singleton