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C# 单例模式

时间:2018-11-16 13:59:57      阅读:231      评论:0      收藏:0      [点我收藏+]

标签:etc   创建   解决方法   opera   app   new   初始化   tps   int   

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:技术分享图片

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:

   技术分享图片

 

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

C# 单例模式

标签:etc   创建   解决方法   opera   app   new   初始化   tps   int   

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

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