Monitor.TryEnter(object) 和 Monitor.TryEnter(object, ref bool) 之间存在什么重要区别？

发布于 2024-09-18 12:38:39 字数 1207 浏览 15 评论 0原文

看来这些代码片段的行为应该相同：

1: Monitor.TryEnter(object)

if (Monitor.TryEnter(lockObject))
{
    try
    {
        DoSomething();
    }
    finally
    {
        Monitor.Exit(lockObject);
    }
}

2: Monitor.TryEnter(object, ref bool) - 在 .NET 4.0 中引入，

bool lockAcquired;
try
{
    Monitor.TryEnter(lockObject, ref lockAcquired);
    if (lockAcquired)
    {
        DoSomething();
    }
}
finally
{
    if (lockAcquired)
    {
        Monitor.Exit(lockObject);
    }
}

我从 MSDN 文档中看到采用 ref bool 参数的重载：

如果由于以下原因未获得锁定抛出异常，变量为 lockTaken 参数指定此方法结束后为 false。这允许程序确定，在所有情况，是否有必要释放锁。

但文档还指出，仅采用 object 参数的重载不会引发除 ArgumentNullException 之外的异常。因此，如果上面的代码片段 1 中抛出异常，那可能只是因为 lockObject 为 null，在这种情况下没有锁无论如何，都会被获取（并且 TryEnter 会返回 false），因此不需要 Monitor.Exit 调用。

显然，他们不会无缘无故地引入这种过载。那么 Monitor.TryEnter(object, ref bool) 方法旨在解决什么场景？

原文

It seems that these code snippets ought to behave identically:

1: Monitor.TryEnter(object)

if (Monitor.TryEnter(lockObject))
{
    try
    {
        DoSomething();
    }
    finally
    {
        Monitor.Exit(lockObject);
    }
}

2: Monitor.TryEnter(object, ref bool) - introduced in .NET 4.0

bool lockAcquired;
try
{
    Monitor.TryEnter(lockObject, ref lockAcquired);
    if (lockAcquired)
    {
        DoSomething();
    }
}
finally
{
    if (lockAcquired)
    {
        Monitor.Exit(lockObject);
    }
}

I see from the MSDN documentation on the overload taking a ref bool parameter:

If the lock was not taken because an
exception was thrown, the variable
specified for the lockTaken parameter
is false after this method ends. This
allows the program to determine, in
all cases, whether it is necessary to
release the lock.

But the documentation also states that the overload taking only the object parameter throws no exceptions other than ArgumentNullException. So it seems like if an exception were thrown in code snippet 1 above, it could only be because lockObject is null, in which case no lock was taken (and TryEnter would've returned false) anyway, so the Monitor.Exit call would not be needed.

Clearly they would not have introduced this overload for no reason. So what scenario is the Monitor.TryEnter(object, ref bool) method intended to address?

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