C# 单例线程安全

Question

我有一个类似这样的单例类

public class Singleton
{
    private static Singleton m_instance;
    private Timer m_timer;
    private static List<CustomObject> m_cacheObjects;

    private Singleton()
    {    
        m_cacheObjects = new List<CustomObject>();
        m_timer= new Timer(MyTimerCallBack, 
                           null, 
                           TimeSpan.FromSeconds(60), 
                           TimeSpan.FromSeconds(60));           
    }

    public static Singleton Instance
    {
        get
        {
            if (m_instance == null)
            {
                m_instance = new Singleton();
            }
            return m_instance;
        }
    }

    private void MyTimerCallBack(object state)
    {
        //******** Update the list by interval here ******************

        m_cacheObjects = UpdateTheList();
    }

    public void CallMe()
    {
        foreach (CustomObject obj in m_cacheObjects)
        {
            // do something here based on obj

            // The question is, does the m_cacheObjects is thread safe??
            // what happen if the m_cacheObjects is changed
            // during the loop interation?
        }
    }
}

CallMe 方法将由网络服务调用:

  [WebMethod]
    public void CallMeWebService()
    {
        Singleton.Instance.CallMe();
    }

问题:1) m_cacheObjects 是线程安全的吗？如果在循环交互期间(在 CallMe() 中)更改了 m_cacheObjects(由于计时器)，会发生什么情况？

2) 调用Webservice CallMeWebService()时是否会创建一个新线程？

Answer 1

1:不，静态列表不是自动线程安全的；您必须手动保护 m_cacheObjects

2:这是一个实现细节；乍一看，它似乎将自己公开为一个同步方法，但如何完全取决于它

实际上，您的静态初始化也不是线程安全的；我可以暴力破解使用两个不同 Singleton 实例的场景。产生它需要重复，但它会发生。

坦率地说，除非你有充分的理由不这样做，否则最简单但最安全的单例模式就是:

private static readonly Singleton m_instance = new Singleton();