C# 单例模式

    一、多线程不安全方式实现

 public sealed class SingleInstance
    {
        private static SingleInstance instance;
        private SingleInstance() { }
        public static SingleInstance Instance
        {
            get
            {
                if (null == instance)
                {
                    instance = new SingleInstance();
                }
                return instance;
            }
        }
    }

　　sealed表示SingleInstance不能被继承。其实构造函数私有化已经达到了这个效果，私有的构造函数不能被继承。为了可读性，可以加个sealed。

不安全的单例指的是在多线程环境下可能有多个线程同时进入if语句，创建了多次单例对象。

   二、安全的单例模式

  public sealed class SingleInstance
    {
        private static volatile SingleInstance instance;
        private static readonly object obj = new object();
        private SingleInstance() { }
        public static SingleInstance Instance
        {
            get
            {
                if (null == instance)
                {
                    lock (obj)
                    {
                        if (null == instance)
                        {
                            instance = new SingleInstance();
                        }
                    }

                }
                return instance;
            }
        }
    }

　加锁保护，在多线程下可以确保实例值被创建一次。缺点是每次获取单例，都要进行判断，涉及到的锁和解锁比较耗资源。

三、只读属性式

 public sealed class SingleInstance
    {
        private static readonly SingleInstance instance = new SingleInstance();
        private SingleInstance() { }
        public static SingleInstance Instance
        {
            get
            {
                return instance;
            }
        }
    }

　　 借助readonly属性，instance只被初始化一次，同样达到了单例的效果。在Main函数执行第一句话之前，instance其实已经被赋值了，并不是预期的 只有到访问Instance变量时才创建对象。

如下代码：

 class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Begin");
            var temp = SingleInstance.instance; ;
        }
    }

    public sealed class SingleInstance
    {
        public static readonly SingleInstance instance = new SingleInstance();
        private SingleInstance()
        {
            Console.WriteLine("初始化初始化！");
        }
        public static SingleInstance Instance
        {
            get { return instance; }
        }
    }

　　输出：

在执行第一句代码之前，实例已经被初始化。

解决方法是在SingleInstance中加上静态构造函数。

 public sealed class SingleInstance
    {
        public static readonly SingleInstance instance = new SingleInstance();
        static SingleInstance() { }
        private SingleInstance()
        {
            Console.WriteLine("初始化初始化！");
        }
        public static SingleInstance Instance
        {
            get { return instance; }
        }
    }

　　在运行输出：

   

四、使用Lazy

 public sealed class SingleInstance
    {
        private static readonly Lazy<SingleInstance> instance = new Lazy<SingleInstance>(() => new SingleInstance());
        private SingleInstance(){}
        public static SingleInstance Instance
        {
            get
            {
                return instance.Value;
            }
        }
    }

　　Lazy默认是线程安全的。MSDN描述如下：

 Will the lazily initialized object be accessed from more than one thread? If so, the Lazy<T> object might create it on any thread. You can use one of the simple constructors whose default behavior is to create a thread-safe Lazy<T> object, so that only one instance of the lazily instantiated object is created no matter how many threads try to access it. To create a Lazy<T> object that is not thread safe, you must use a constructor that enables you to specify no thread safety.

   

五、泛型单例

    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Begin");
            mySingle.Instance.age = 500;
            Console.WriteLine(mySingle.Instance.age);
        }
    }

    public abstract class SingleInstance<T>
    {
        private static readonly Lazy<T> _instance = new Lazy<T>(() =>
            {
                var ctors = typeof(T).GetConstructors(BindingFlags.Instance| BindingFlags.NonPublic| BindingFlags.Public);
                if (ctors.Count() != 1)
                    throw new InvalidOperationException(String.Format("Type {0} must have exactly one constructor.", typeof(T)));
                var ctor = ctors.SingleOrDefault(c => c.GetParameters().Count() == 0 && c.IsPrivate);
                if (ctor == null)
                    throw new InvalidOperationException(String.Format("The constructor for {0} must be private and take no parameters.", typeof(T)));
                return (T)ctor.Invoke(null);
            });
        public static T Instance
        {
          get{ return _instance.Value;}
        }
    }

    public class mySingle : SingleInstance<mySingle>
    {
        private mySingle() { }
        public int age;
    }