The Monitor class controls access to objects by granting a lock for an object to a single thread. Object locks provide the ability to restrict access to a block of code, commonly called a critical section. While a thread owns the lock for an object, no other thread can acquire that lock. You can also use Monitor to ensure that no other thread is allowed to access a section of application code being executed by the lock owner, unless the other thread is executing the code using a different locked object.
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Use
Monitor to lock objects (that is, reference types), not value types. For details, see Enter and the conceptual topic Monitors.
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Monitor has the following features:
It is associated with an object on demand.
It is unbound, which means it can be called directly from any context.
An instance of the Monitor class cannot be created.
The following information is maintained for each synchronized object:
A reference to the thread that currently holds the lock.
A reference to a ready queue, which contains the threads that are ready to obtain the lock.
A reference to a waiting queue, which contains the threads that are waiting for notification of a change in the state of the locked object.
The following table describes the actions that can be taken by threads that access synchronized objects:
Action
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Description
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Enter, TryEnter |
Acquires a lock for an object. This action also marks the beginning of a critical section. No other thread can enter the critical section unless it is executing the instructions in the critical section using a different locked object.
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Wait |
Releases the lock on an object in order to permit other threads to lock and access the object. The calling thread waits while another thread accesses the object. Pulse signals are used to notify waiting threads about changes to an object's state.
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Pulse (signal), PulseAll |
Sends a signal to one or more waiting threads. The signal notifies a waiting thread that the state of the locked object has changed, and the owner of the lock is ready to release the lock. The waiting thread is placed in the object's ready queue so that it might eventually receive the lock for the object. Once the thread has the lock, it can check the new state of the object to see if the required state has been reached.
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Exit |
Releases the lock on an object. This action also marks the end of a critical section protected by the locked object.
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Use the Enter and Exit methods to mark the beginning and end of a critical section. If the critical section is a set of contiguous instructions, then the lock acquired by the Enter method guarantees that only a single thread can execute the enclosed code with the locked object. In this case, it is recommended you place those instructions in a try block and place the Exit instruction in a finally block. This facility is typically used to synchronize access to a static or instance method of a class. If an instance method requires synchronized thread access, it invokes the Enter and corresponding Exit methods using the current instance as the object to lock. Since only one thread can hold the lock on the current instance, the method can only be executed by one thread at a time. Static methods are protected in a similar fashion using the Type of the current instance as the locked object. The functionality provided by the Enter and Exit methods is identical to that provided by the C# lock statement (SyncLock in Visual Basic), except that lock and SyncLock wrap the Exit method in a try…finally block (Try…Finally in Visual Basic) to ensure that the monitor is released.
If a critical section spans an entire method, the locking facility described above can be achieved by placing the System.Runtime.CompilerServices..::.MethodImplAttribute on the method, and specifying the Synchronized value in the constructor of MethodImplAttribute. Using this attribute, the Enter and Exit statements are not needed. Note that the attribute causes the current thread to hold the lock until the method returns; if the lock can be released sooner, use the Monitor class or the C# lock statement instead of the attribute.
While it is possible for the Enter and Exit statements that lock and release a given object to cross member or class boundaries or both, this practice is not recommended.
When selecting an object on which to synchronize, you should lock only on private or internal objects. Locking on external objects might result in deadlocks, because unrelated code could choose the same objects to lock on for different purposes.