c# 单例模式的实现

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

    public static Singleton GetInstance()
    {
      return instance;
    }
  }

public class Singleton
  {
    private static Singleton instance = null;
    private Singleton() { }

    public static Singleton GetInstance()
    {
      if (instance == null)
        instance = new Singleton();
      return instance;
    }
  }

public class Singleton
  {
    private static Singleton instance = null;
    private static readonly object obj = new object();
    private Singleton() { }

    public static Singleton GetInstance()
    {
      //双重锁定
      if (instance == null)//只有为null需要实例化处理时才进行加锁，提高性能避免不必要的等待
      {
        lock (obj)
        {
          if (instance == null)//避免其他线程等待锁释放期间有线程已经实例化，从而造成多个实例
            instance = new Singleton();
        }
      }
      return instance;
    }
  }

public abstract class Singleton<T> where T:class,new()
  {
    private static readonly T instance = new T();
    protected Singleton() { }

    public static T GetSingleton()//获取单例
    {
      return instance;
    }
  }
  public class Person : Singleton<Person> { }

static void Main(string[] args)
    {
      Person p1 = Person.GetSingleton();
      Person p2 = Singleton<Person>.GetSingleton();

      if (object.ReferenceEquals(p1, p2))
      {
        Console.WriteLine("两个对象是同一实例");
      }
      Console.ReadKey();
    }