// 不要用这种方式
public sealed class Singleton
{
  private static Singleton instance=null;//声明自己本身的静态实例
  private Singleton(){}//私有构造
  public static Singleton Instance() //提供全局访问点
  {
      if (instance==null)//实例不存在则创建
      {
        instance = new Singleton();
      }
      return instance;
  }
}

public sealed class Singleton
{
  private static Singleton instance = null;
  private static readonly object padlock = new object();//定义一个标识确保线程同步
  Singleton(){}
  public static Singleton Instance()
  {
      lock (padlock)//线程到达时加锁 运行完之后解锁 当遇到加锁线程就会挂起等待解锁
      {
        if (instance == null)
        {
          instance = new Singleton();
        }
        return instance;
      }
  }
}

public sealed class Singleton
{
  private static Singleton instance = null;
  private static readonly object padlock = new object();
  Singleton(){}
  public static Singleton Instance
  {
    get
    {
      if (instance == null)//外层的if语句块，这使得每个线程欲获取实例时不必每次都得加锁，因为只有实例为空时（即需要创建一个实例），才需加锁创建
      {
        lock (padlock)
        {
          if (instance == null)
          {
            instance = new Singleton();
          }
        }
      }
      return instance;
    }
  }
}

public sealed class Singleton
{
  private static readonly Singleton instance = new Singleton();

  // 显示的static 构造函数
  //静态构造函数抑制了beforefieldinit 特性(访问成员之前就执行静态函数)
  static Singleton(){}
  private Singleton(){}
  public static Singleton Instance
  {
    get
    {
      return instance;
    }
  }
}

public sealed class Singleton
{
  private Singleton(){}
  public static Singleton Instance { get { return Nested.instance; }}   
//嵌套类
  private class Nested
  {
    //抑制beforefieldinit特性
    static Nested(){}
    internal static readonly Singleton instance = new Singleton();
  }
}

public sealed class Singleton
{
//使用.NET4 Lazy<T>
  private static readonly Lazy<Singleton> lazy =new Lazy<Singleton>(() => new Singleton());
  public static Singleton Instance { get { return lazy.Value; } }
  private Singleton() {}
}

public sealed class Singleton
{
    private static readonly Singleton instance=new Singleton();//直接实例化
    private Singleton(){}
    public static Singleton Instance()
    {
        return instance;
    }
}