• C# 单例模式


    一、多线程不安全方式实现

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    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

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    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

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    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:

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    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。

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    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

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    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

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    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
        {
          getreturn _instance.Value;}
        }
    }
     
    public class mySingle : SingleInstance<mySingle>
    {
        private mySingle() { }
        public int age;
    }

    https://www.cnblogs.com/lh218/p/4713599.html

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  • 原文地址:https://www.cnblogs.com/asdyzh/p/9968858.html
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