c# - 为什么 lock(this) {...} 不好？

Question

MSDN documentation说

public class SomeObject
{
  public void SomeOperation()
  {
    lock(this)
    {
      //Access instance variables
    }
  }
}

是“如果可以公开访问实例的问题”。我想知道为什么？是因为锁的持有时间比必要的要长吗？还是有一些更阴险的原因？

Answer 1

在锁定语句中使用 this 是一种错误的形式，因为通常您无法控制还有谁可能锁定该对象。

为了正确规划并行操作，应特别注意考虑可能出现的死锁情况，而未知数量的锁入口点会阻碍这一点。例如，任何引用该对象的人都可以在对象设计者/创建者不知道的情况下锁定它。这增加了多线程解决方案的复杂性，并可能影响它们的正确性。

私有(private)字段通常是更好的选择，因为编译器会对其实现访问限制，并且会封装锁定机制。使用 this 会向公众公开部分锁定实现，从而违反了封装。除非已记录，否则您也不清楚您将获得 this 上的锁。即便如此，依靠文档来预防问题也不是最佳选择。

最后，有一个常见的误解，即 lock(this) 实际上修改了作为参数传递的对象，并以某种方式使其成为只读或不可访问的。这是错误的。作为参数传递给 lock 的对象仅用作 key。如果该键已经被锁定，则无法进行锁定；否则，允许锁定。

这就是为什么在 lock 语句中使用字符串作为键是不好的，因为它们是不可变的，并且可以跨应用程序的各个部分共享/访问。您应该改用私有(private)变量，Object 实例会很好。

运行以下 C# 代码作为示例。

public class Person
{
    public int Age { get; set;  }
    public string Name { get; set; }

    public void LockThis()
    {
        lock (this)
        {
            System.Threading.Thread.Sleep(10000);
        }
    }
}

class Program
{
    static void Main(string[] args)
    {
        var nancy = new Person {Name = "Nancy Drew", Age = 15};
        var a = new Thread(nancy.LockThis);
        a.Start();
        var b = new Thread(Timewarp);
        b.Start(nancy);
        Thread.Sleep(10);
        var anotherNancy = new Person { Name = "Nancy Drew", Age = 50 };
        var c = new Thread(NameChange);
        c.Start(anotherNancy);
        a.Join();
        Console.ReadLine();
    }

    static void Timewarp(object subject)
    {
        var person = subject as Person;
        if (person == null) throw new ArgumentNullException("subject");
        // A lock does not make the object read-only.
        lock (person.Name)
        {
            while (person.Age <= 23)
            {
                // There will be a lock on 'person' due to the LockThis method running in another thread
                if (Monitor.TryEnter(person, 10) == false)
                {
                    Console.WriteLine("'this' person is locked!");
                }
                else Monitor.Exit(person);
                person.Age++;
                if(person.Age == 18)
                {
                    // Changing the 'person.Name' value doesn't change the lock...
                    person.Name = "Nancy Smith";
                }
                Console.WriteLine("{0} is {1} years old.", person.Name, person.Age);
            }
        }
    }

    static void NameChange(object subject)
    {
        var person = subject as Person;
        if (person == null) throw new ArgumentNullException("subject");
        // You should avoid locking on strings, since they are immutable.
        if (Monitor.TryEnter(person.Name, 30) == false)
        {
            Console.WriteLine("Failed to obtain lock on 50 year old Nancy, because Timewarp(object) locked on string \"Nancy Drew\".");
        }
        else Monitor.Exit(person.Name);

        if (Monitor.TryEnter("Nancy Drew", 30) == false)
        {
            Console.WriteLine("Failed to obtain lock using 'Nancy Drew' literal, locked by 'person.Name' since both are the same object thanks to inlining!");
        }
        else Monitor.Exit("Nancy Drew");
        if (Monitor.TryEnter(person.Name, 10000))
        {
            string oldName = person.Name;
            person.Name = "Nancy Callahan";
            Console.WriteLine("Name changed from '{0}' to '{1}'.", oldName, person.Name);
        }
        else Monitor.Exit(person.Name);
    }
}

控制台输出

'this' person is locked!
Nancy Drew is 16 years old.
'this' person is locked!
Nancy Drew is 17 years old.
Failed to obtain lock on 50 year old Nancy, because Timewarp(object) locked on string "Nancy Drew".
'this' person is locked!
Nancy Smith is 18 years old.
'this' person is locked!
Nancy Smith is 19 years old.
'this' person is locked!
Nancy Smith is 20 years old.
Failed to obtain lock using 'Nancy Drew' literal, locked by 'person.Name' since both are the same object thanks to inlining!
'this' person is locked!
Nancy Smith is 21 years old.
'this' person is locked!
Nancy Smith is 22 years old.
'this' person is locked!
Nancy Smith is 23 years old.
'this' person is locked!
Nancy Smith is 24 years old.
Name changed from 'Nancy Drew' to 'Nancy Callahan'.