Singleton Pattern单例模式

https://csharpindepth.com/Articles/Singleton

懒汉，饿汉，以及双层锁之外

Fourth version - not quite as lazy, but 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;
            }
        }
    }

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. I now have an article with more details about this issue . Also note that it affects performance, as discussed near the bottom of the page.

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.) 

Fifth version - fully lazy instantiation

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()
            {
            }
            }

publicsealedclassSingleton { privatestaticreadonly Singleton instance = new Singleton(); // Explicit static constructor to tell C# compiler// not to mark type as beforefieldinitstatic Singleton() { } private Singleton() { } publicstatic Singleton Instance { get { return instance; } } }