为什么锁(这个){...}不好？

在MSDN文档说,

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

如果可以公开访问实例,那就是"一个问题".我想知道为什么？是因为锁定的持有时间超过了必要的时间吗？还是有一些更阴险的原因？

this在锁定语句中使用它是一种糟糕的形式,因为通常无法控制其他人可能锁定该对象.

为了正确规划并行操作,应特别注意考虑可能的死锁情况,并且具有未知数量的锁定入口点会妨碍这一点.例如,任何具有对象引用的人都可以在没有对象设计者/创建者知道的情况下锁定它.这增加了多线程解决方案的复杂性,并可能影响其正确性.

私有字段通常是更好的选择,因为编译器将对其强制实施访问限制,并且它将封装锁定机制.使用this通过将部分锁定实现暴露给公众来违反封装.this除非记录在案,否则您还不清楚是否会获得锁定.即使这样,依靠文档来防止问题也是次优的.

最后,存在一种常见的误解,即lock(this)实际修改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.这是错误的.作为参数传递的对象lock仅用作键.如果已经锁定了该锁,则无法进行锁定; 否则,允许锁定.

这就是为什么使用字符串作为lock语句中的键是不好的,因为它们是不可变的并且在应用程序的各个部分之间共享/访问.您应该使用私有变量,Object实例将很好地使用.

以下面的C#代码为例.

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'.

lock(typeof(SomeObject))