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Monitors

Monitor objects expose the ability to synchronize access to a region of code by taking and releasing a lock on a particular object using the Monitor.Enter, Monitor.TryEnter, and Monitor.Exit methods. Once you have a lock on a code region, you can use the Monitor.Wait, Monitor.Pulse, and Monitor.PulseAll methods. Wait releases the lock if it is held and waits to be notified. When Wait is notified, it returns and obtains the lock again. Both Pulse and PulseAll signal for the next thread in the wait queue to proceed.

Typically, you enter the monitor by calling the Monitor.Enter or Monitor.TryEnter method immediately before executing the code in a try block. You then exit the monitor by calling the Monitor.Exit method in a finally block. This ensures that the lock is released even if an exception occurs.

The Visual Basic SyncLock and C# lock statements use Monitor.Enter to take the lock and Monitor.Exit to release it. The advantage of using the language statements is that everything in the lock or SyncLock block is included in a Try statement. The Try statement has a Finally block to guarantee that the lock is released.

Monitor locks objects (that is, reference types), not value types. While you can pass a value type to Enter and Exit, it is boxed separately for each call. Since each call creates a separate object, Enter never blocks, and the code it is supposedly protecting is not really synchronized. In addition, the object passed to Exit is different from the object passed to Enter, so Monitor throws SynchronizationLockException exception with the message "Object synchronization method was called from an unsynchronized block of code."

The following example illustrates this problem. It launches ten tasks, each of which just sleeps for 250 milliseconds. Each task then updates a counter variable, nTasks, which is intended to count the number of tasks that actually launched and executed. Because nTasks is a global variable that can be updated by multiple tasks simultaneously, a monitor is used to protect it from simultaneous modification by multiple tasks. However, as the output from the example shows, each of the tasks throws a SynchronizationLockException exception.

Each task throws a SynchronizationLockException exception because the nTasks variable is boxed before the call to the Monitor.Enter method in each task. In other words, each method call is passed a separate variable that is independent of the others. nTasks is boxed again in the call to the Monitor.Exit method. Once again, this creates ten new boxed variables, which are independent of each other, nTasks, and the ten boxed variables created in the call to the Monitor.Enter method. The exception is thrown, then, because our code is attempting to release a lock on a newly created variable that was not previously locked.

Although you can box a value type variable before calling Enter and Exit, as shown in the following example, and pass the same boxed object to both methods, there is no advantage to doing this. Changes to the unboxed variable are not reflected in the boxed copy, and there is no way to change the value of the boxed copy.

int x = 1;
object o = x;

Monitor.Enter(o);
try
{
    // Code that needs to be protected by the monitor.
}
finally
{
    // Always use Finally to ensure that you exit the Monitor.
    Monitor.Exit(o);
}

It is important to note the distinction between the use of the Monitor and WaitHandle objects. Monitor objects are purely managed, fully portable, and might be more efficient in terms of operating-system resource requirements. WaitHandle objects represent operating-system waitable objects, are useful for synchronizing between managed and unmanaged code, and expose some advanced operating-system features like the ability to wait on many objects at once.

The following example demonstrates the combined use of the Monitor class (implemented with the lock and SyncLock compiler statements), the Interlocked class, and the AutoResetEvent class.

using System;
using System.Threading;

// Note: The class whose internal public member is the synchronizing 
// method is not public; none of the client code takes a lock on the 
// Resource object.The member of the nonpublic class takes the lock on 
// itself. Written this way, malicious code cannot take a lock on 
// a public object. 
class SyncResource
{
    public void Access(Int32 threadNum)
    {
        // Uses Monitor class to enforce synchronization. 
        lock (this)
        {
            // Synchronized: Despite the next conditional, each thread 
            // waits on its predecessor. 
            if (threadNum % 2 == 0)
            {
                Thread.Sleep(2000);
            }
            Console.WriteLine("Start Synched Resource access (Thread={0})", threadNum);
            Thread.Sleep(200);
            Console.WriteLine("Stop Synched Resource access  (Thread={0})", threadNum);
        }
    }
}

// Without the lock, the method is called in the order in which threads reach it. 
class UnSyncResource
{
    public void Access(Int32 threadNum)
    {
        // Does not use Monitor class to enforce synchronization. 
        // The next call throws the thread order. 
        if (threadNum % 2 == 0)
        {
            Thread.Sleep(2000);
        }
        Console.WriteLine("Start UnSynched Resource access (Thread={0})", threadNum);
        Thread.Sleep(200);
        Console.WriteLine("Stop UnSynched Resource access  (Thread={0})", threadNum);
    }
}

public class App
{
    static Int32 numAsyncOps = 5;
    static AutoResetEvent asyncOpsAreDone = new AutoResetEvent(false);
    static SyncResource SyncRes = new SyncResource();
    static UnSyncResource UnSyncRes = new UnSyncResource();

    public static void Main()
    {
        for (Int32 threadNum = 0; threadNum < 5; threadNum++)
        {
            ThreadPool.QueueUserWorkItem(new WaitCallback(SyncUpdateResource), threadNum);
        }

        // Wait until this WaitHandle is signaled.
        asyncOpsAreDone.WaitOne();
        Console.WriteLine("\t\nAll synchronized operations have completed.\t\n");

        // Reset the thread count for unsynchronized calls.
        numAsyncOps = 5;

        for (Int32 threadNum = 0; threadNum < 5; threadNum++)
        {
            ThreadPool.QueueUserWorkItem(new WaitCallback(UnSyncUpdateResource), threadNum);
        }

        // Wait until this WaitHandle is signaled.
        asyncOpsAreDone.WaitOne();
        Console.WriteLine("\t\nAll unsynchronized thread operations have completed.");
    }

    // The callback method's signature MUST match that of a 
    // System.Threading.TimerCallback delegate (it takes an Object 
    // parameter and returns void). 
    static void SyncUpdateResource(Object state)
    {
        // This calls the internal synchronized method, passing 
        // a thread number.
        SyncRes.Access((Int32) state);

        // Count down the number of methods that the threads have called. 
        // This must be synchronized, however; you cannot know which thread 
        // will access the value **before** another thread's incremented 
        // value has been stored into the variable. 
        if (Interlocked.Decrement(ref numAsyncOps) == 0)
        {
            // Announce to Main that in fact all thread calls are done.
            asyncOpsAreDone.Set();
        }
    }

    // The callback method's signature MUST match that of a 
    // System.Threading.TimerCallback delegate (it takes an Object 
    // parameter and returns void). 
    static void UnSyncUpdateResource(Object state)
    {
        // This calls the unsynchronized method, passing a thread number.
        UnSyncRes.Access((Int32) state);

        // Count down the number of methods that the threads have called. 
        // This must be synchronized, however; you cannot know which thread 
        // will access the value **before** another thread's incremented 
        // value has been stored into the variable. 
        if (Interlocked.Decrement(ref numAsyncOps) == 0)
        {
            // Announce to Main that in fact all thread calls are done.
            asyncOpsAreDone.Set();
        }
    }
}
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