Monitor.TryEnter / Monitor.Exit and SynchronizationLockException

问题

Is it possible to detect if the same thread trying to release the lock? We have many places in code that looks like:

try
{
    try
    {
       if(!Monitor.TryEnter(obj, 2000)) 
       { 
            throw new Exception("can not lock"); 
       }
    }
    finally
    {
       Monitor.Exit(obj);
    }
}
catch
{
    //Log
}

The above code very simplified, and actually Enter and Exit statement located in custom object (lock manager).

The problem, that in that structure, we have SynchronizationLockException when trying to "Exit", since it looks like the thread that not succeed to lock, tries to release in finally.

So the question, is how I can know if the thread who making Monitor.Exit is the same thread who did Monitor.Enter?
I thought that I can use CurrentThread.Id to sync enter and exit, but I'm not sure if it "safe" enough.

回答1:

As you think that to put the calling of Monitor.Exit in try-catch was 'durty'(dirty?), here's a very simple idea trying to 'take the durty away'. Lock is reentrant for the same thread and if one thread acquired successfully, before it releases, attempt from another thread will fail. So that you can consider something like:

public void Exit(object key) {
    if(!IsActive) {
        return;
    }

    if(LockDictionary.ContainsKey(key)) {
        var syncObject=LockDictionary[key];

        if(Monitor.TryEnter(syncObject.SyncObject, 0)) {
            SetLockExit(syncObject);
            Monitor.Exit(syncObject.SyncObject);
            Monitor.Exit(syncObject.SyncObject);
        }
    }
}

We call Monitor.Exit twice because we lock it twice, one in the code outer, and one just here.

回答2:

So the question, is how I can know if the thread who making Monitor.Exit is the same thread who did Monitor.Enter?

You can't, easily, as far as I'm aware. You can't find out which thread owns a monitor.

However, this is just a coding issue - you should change your code so that it doesn't even attempt to release the monitor when it shouldn't. So your code above could be rewritten as:

if (!Monitor.TryEnter(obj, 2000))
{
    throw new Exception(...);
}
try
{
    // Presumably other code
}
finally
{
     Monitor.Exit(obj);
}

Or even better, if you're using .NET 4, use the overload of TryEnter which accepts an ret parameter:

bool gotMonitor = false;
try
{
    Monitor.TryEnter(obj, ref gotMonitor);
    if (!gotMonitor)
    {
        throw new Exception(...);
    }
    // Presumably other code
}
finally
{
    if (gotMonitor)
    {
        Monitor.Exit(obj);
    }
}

回答3:

I know this is an older question, but here's my answer anyway. I would move the try-finally construct inside the if:

try
{
    if(Monitor.TryEnter(obj, 2000))
    {
        try
        {
            // code here
        }
        finally
        {
            Monitor.Exit(obj);
        }
    }
    else
    {
        throw new Exception("Can't acquire lock");
    }
}
catch
{
    // log
}

来源：https://stackoverflow.com/questions/14089640/monitor-tryenter-monitor-exit-and-synchronizationlockexception