看看MSDN主题线程同步(c#编程指南)

Generally, it is best to avoid locking on a public type, or on object instances beyond the control of your application. For example, lock(this) can be problematic if the instance can be accessed publicly, because code beyond your control may lock on the object as well. This could create deadlock situations where two or more threads wait for the release of the same object. Locking on a public data type, as opposed to an object, can cause problems for the same reason. Locking on literal strings is especially risky because literal strings are interned by the common language runtime (CLR). This means that there is one instance of any given string literal for the entire program, the exact same object represents the literal in all running application domains, on all threads. As a result, a lock placed on a string with the same contents anywhere in the application process locks all instances of that string in the application. As a result, it is best to lock a private or protected member that is not interned. Some classes provide members specifically for locking. The Array type, for example, provides SyncRoot. Many collection types provide a SyncRoot member as well.

.．.同样的论点也适用于这个结构:

lock(typeof(SomeObject))

关于它有一篇非常好的文章http://bytes.com/topic/c-sharp/answers/249277-dont-lock-type-objects，作者是Rico Mariani，他是Microsoft®.NET运行时的性能架构师

摘录:

这里的基本问题是您不拥有类型对象，而您 不知道还有谁能拿到。总的来说，这是一个非常糟糕的主意 依赖于锁定一个不是你创建的对象，也不知道还有谁 可能是存取。这样做会导致僵局。最安全的方法是 只锁定私有对象。

因为任何可以看到类实例的代码块也可以锁定该引用。您希望隐藏(封装)锁定对象，以便只有需要引用它的代码才能引用它。关键字this指向当前类实例，因此任何数量的东西都可以引用它，并可以使用它来进行线程同步。

需要明确的是，这很糟糕，因为其他一些代码块可能会使用类实例来锁定，并且可能会阻止您的代码获得及时的锁定，或者可能会产生其他线程同步问题。最好的情况是:没有其他方法使用对您的类的引用来锁定。中间情况:某些东西使用对你的类的引用来锁，这导致了性能问题。最坏的情况:某些东西使用你的类的引用来进行锁，这会导致非常糟糕、非常微妙、非常难以调试的问题。

以下是不建议使用的原因。

短版: 考虑下面的代码片段:

object foo = new Object(); 
object bar = foo; 

lock(foo)
{
  lock(bar){}
}  

这里，foo和bar引用的是导致死锁的同一个对象实例。这是现实中可能发生的事情的简化版本。

长版: 为了根据下面的代码片段更详细地解释它，假设您编写了一个类(在本例中为SomeClass)，并且类的使用者(名为“John”的编码器)希望获得类实例(在本例中为someObject)上的锁。他遇到死锁是因为他在实例someObject上获得了一个锁，在这个锁中他调用了该实例的一个方法(SomeMethod())，该方法在内部获得了同一个实例上的锁。

I could have written the following example with or without Task/Thread and the gist of deadlock still remains the same. To prevent bizarre situation where the main Thread finishes while its children are still running, I used .Wait(). However, in long-running-tasks or situation where a code-snippet executes more frequently, you would definitely see the same behavior. Although John applied a bad practice of using an instance of a class as a lock-object, but we (as the developer of a classlibrary SomeClass) should deter such situation simple by not using this as a lock-object in our class. Instead, we should declare a simple private field and use that as our lock-object. using System; using System.Threading; using System.Threading.Tasks; class SomeClass { public void SomeMethod() { //NOTE: Locks over an object that is already locked by the caller. // Hence, the following code-block never executes. lock (this) { Console.WriteLine("Hi"); } } } public class Program { public static void Main() { SomeClass o = new SomeClass(); lock (o) { Task.Run(() => o.SomeMethod()).Wait(); } Console.WriteLine("Finish"); } }

想象一下，你的办公室有一位技术娴熟的秘书，这是部门的共享资源。偶尔，你会因为有任务而冲向他们，只希望你的另一个同事还没有认领他们。通常你只需要等很短的一段时间。

因为关心就是分享，你的经理决定顾客也可以直接使用秘书。但这有一个副作用:当您为该客户工作时，客户甚至可能会认领它们，并且您还需要他们执行部分任务。死锁发生，因为声明不再是层次结构。如果从一开始就不允许客户索赔，这完全可以避免。

lock(this) is bad as we've seen. An outside object might lock on the object and since you don't control who's using the class, anyone can lock on it... Which is the exact example as described above. Again, the solution is to limit exposure of the object. However, if you have a private, protected or internal class you could already control who is locking on your object, because you're sure that you've written your code yourself. So the message here is: don't expose it as public. Also, ensuring that a lock is used in similar scenario's avoids deadlocks.

与此完全相反的是锁定整个应用领域共享的资源——这是最坏的情况。这就像把你的秘书放在外面，让外面的人去认领一样。结果是彻底的混乱——或者就源代码而言:这是一个坏主意;把它扔掉，重新开始。我们怎么做呢?

正如这里大多数人指出的那样，类型在应用领域是共享的。但是我们可以使用更好的东西:字符串。原因是字符串被池化了。换句话说，如果你有两个字符串在一个应用域中有相同的内容，它们有可能有完全相同的指针。由于指针被用作锁键，基本上你得到的是“为未定义的行为做好准备”的同义词。

类似地，你不应该锁定WCF对象HttpContext。目前,线程。当前，单例(一般)等。避免这一切的最简单方法是什么?private [static] object myLock = new object();