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.netmutexwaithandle

What does the exit context mean for a WaitHandle.WaitOne mean?

I'm trying to use a mutex to protect access to some hardware from multiple threads, but I'm confused as to what the exitContext parameter means / does:

public virtual bool WaitOne (
    int millisecondsTimeout,
    bool exitContext
)

The docs say:

exitContext - true to exit the synchronization domain for the context before the wait (if in a synchronized context), and reacquire it afterward; otherwise, false.

...but what does that actually mean and what are the consequences of setting it to either true or false? I've set it to true for now and the code appears to work, but I'm nervous that I don't fully understand what it's up to under the hood!

Solution

  • It also explains further down the page under Remarks that:

    Notes on Exiting the Context

    The exitContext parameter has no effect unless the WaitOne method is called from inside a nondefault managed context. This can happen if your thread is inside a call to an instance of a class derived from ContextBoundObject. Even if you are currently executing a method on a class that does not derive from ContextBoundObject, like String, you can be in a nondefault context if a ContextBoundObject is on your stack in the current application domain.

    When your code is executing in a nondefault context, specifying true for exitContext causes the thread to exit the nondefault managed context (that is, to transition to the default context) before executing the WaitOne method. The thread returns to the original nondefault context after the call to the WaitOne method completes.

    This can be useful when the context-bound class has SynchronizationAttribute. In that case, all calls to members of the class are automatically synchronized, and the synchronization domain is the entire body of code for the class. If code in the call stack of a member calls the WaitOne method and specifies true for exitContext, the thread exits the synchronization domain, allowing a thread that is blocked on a call to any member of the object to proceed. When the WaitOne method returns, the thread that made the call must wait to reenter the synchronization domain.