C# 單例模式

    一、多線程不安全方式實現

 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不能被繼承。其實構造函數私有化已經達到了這個效果，私有的構造函數不能被繼承。為了可讀性，可以加個sealed。

不安全的單例指的是在多線程環境下可能有多個線程同時進入if語句，創建了多次單例對象。

   二、安全的單例模式

  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;
            }
        }
    }

　加鎖保護，在多線程下可以確保實例值被創建一次。缺點是每次獲取單例，都要進行判斷，涉及到的鎖和解鎖比較耗資源。

三、只讀屬性式

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

　　 借助readonly屬性，instance只被初始化一次，同樣達到了單例的效果。在Main函數執行第一句話之前，instance其實已經被賦值了，並不是預期的 只有到訪問Instance變量時才創建對象。

如下代碼：

 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; }
        }
    }

　　輸出：

在執行第一句代碼之前，實例已經被初始化。

解決方法是在SingleInstance中加上靜態構造函數。

 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; }
        }
    }

　　在運行輸出：

   

 

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默認是線程安全的。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、泛型單例

    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;
    }