Article
11/18/2011

Monitor Class

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Provides a mechanism that synchronizes access to objects.

Inheritance Hierarchy

System.Object
System.Threading.Monitor

Namespace: System.Threading
Assembly: mscorlib (in mscorlib.dll)

Syntax

'Declaration
<ComVisibleAttribute(True)> _
Public NotInheritable Class Monitor

[ComVisibleAttribute(true)]
public static class Monitor

The Monitor type exposes the following members.

Methods

	Name	Description
	Enter(Object)	Acquires an exclusive lock on the specified object.
	Enter(Object, Boolean%)	Acquires an exclusive lock on the specified object, and atomically sets a value that indicates whether the lock was taken.
	Exit	Releases an exclusive lock on the specified object.
	Pulse	Notifies a thread in the waiting queue of a change in the locked object's state.
	PulseAll	Notifies all waiting threads of a change in the object's state.
	TryEnter(Object)	Attempts to acquire an exclusive lock on the specified object.
	TryEnter(Object, Boolean%)	Attempts to acquire an exclusive lock on the specified object, and atomically sets a value that indicates whether the lock was taken.
	TryEnter(Object, Int32)	Attempts, for the specified number of milliseconds, to acquire an exclusive lock on the specified object.
	TryEnter(Object, TimeSpan)	Attempts, for the specified amount of time, to acquire an exclusive lock on the specified object.
	TryEnter(Object, Int32, Boolean%)	Attempts, for the specified number of milliseconds, to acquire an exclusive lock on the specified object, and atomically sets a value that indicates whether the lock was taken.
	TryEnter(Object, TimeSpan, Boolean%)	Attempts, for the specified amount of time, to acquire an exclusive lock on the specified object, and atomically sets a value that indicates whether the lock was taken.
	Wait(Object)	Releases the lock on an object and blocks the current thread until it reacquires the lock.
	Wait(Object, Int32)	Releases the lock on an object and blocks the current thread until it reacquires the lock. If the specified time-out interval elapses, the thread enters the ready queue.
	Wait(Object, TimeSpan)	Releases the lock on an object and blocks the current thread until it reacquires the lock. If the specified time-out interval elapses, the thread enters the ready queue.

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Remarks

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.

Note:
Use Monitor to lock objects (that is, reference types), not value types. For details, see Enter and the conceptual topic Monitors.

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	Description
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.
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.
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.
Exit	Releases the lock on an object. This action also marks the end of a critical section protected by the locked object.

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. 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 Enter(Object, Boolean%) method overload and the Exit method in a try…finally (Try…Finally in Visual Basic) to ensure that the monitor is released.

Beginning with Silverlight 4, there are two sets of overloads for the Enter and TryEnter methods. One set of overloads has a ref (ByRef in Visual Basic) Boolean parameter that is atomically set to true if the lock is acquired, even if an exception is thrown when acquiring the lock. Use these overloads if it is critical to release the lock in all cases, even when the resources the lock is protecting might not be in a consistent state.

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.

Version Information

Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0

XNA Framework

Supported in: Xbox 360, Windows Phone OS 7.0

Platforms

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.

Thread Safety

This type is thread safe.

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