System.Threading.Monitor Class

public sealed class Monitor

Base Types

Object
  Monitor

Assembly

mscorlib

Library

BCL

Summary

Provides a mechanism that synchronizes access to objects.

Description

The Monitor class controls access to objects by granting a single thread a lock for an object. 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 the lock for the object. Additionally, the Monitor class can be used to ensure that no other thread can 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.

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 taken by threads that access synchronized objects:

Action	Description
Enter	Acquires a lock for an object. Also marks the beginning of a critical section. No other thread can enter the critical section unless they are executing the instructions in the critical section using a different locked object. [Note: See the System.Threading.Monitor.Enter(System.Object) and System.Threading.Monitor.TryEnter(System.Object) methods.]
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 (see below) are used to notify waiting threads about changes to an object's state. [Note: See System.Threading.Monitor.Wait(System.Object,System.Int32,System.Boolean) .]
Pulse (signal)	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 can 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. [Note: See System.Threading.Monitor.Pulse(System.Object) and System.Threading.Monitor.PulseAll(System.Object) .]
Exit	Releases the lock on an object. Also marks the end of a critical section protected by the locked object. [Note: See System.Threading.Monitor.Exit(System.Object) .]

The System.Threading.Monitor.Enter(System.Object) and System.Threading.Monitor.Exit(System.Object) methods are used 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 System.Threading.Monitor.Enter(System.Object) method guarantees that only a single thread can execute the enclosed code with the locked object. 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, the instance method invokes the System.Threading.Monitor.Enter(System.Object) and corresponding System.Threading.Monitor.Exit(System.Object) methods using itself (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 object of the current instance as the locked object.

[Note: The functionality provided by the System.Threading.Monitor.Enter(System.Object) and System.Threading.Monitor.Exit(System.Object) methods is identical to that provided by the C# lock statement.

If a critical section spans an entire method, the locking facility described above can be achieved by placing the MethodImplAttribute on the method, and specifying the System.Runtime.CompilerServices.MethodImplOptions.Synchronized option. Using this attribute, the System.Threading.Monitor.Enter(System.Object) and System.Threading.Monitor.Exit(System.Object) 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 C# lock statement) instead of the attribute.

While it is possible for the System.Threading.Monitor.Enter(System.Object) and System.Threading.Monitor.Exit(System.Object) statements that lock and release a given object to cross member and/or class boundaries, this practice is strongly discouraged.

]

See Also

System.Threading Namespace

Members

Monitor Methods

Monitor.Enter Method
Monitor.Exit Method
Monitor.Pulse Method
Monitor.PulseAll Method
Monitor.TryEnter(System.Object) Method
Monitor.TryEnter(System.Object, int) Method
Monitor.TryEnter(System.Object, System.TimeSpan) Method
Monitor.Wait(System.Object, int) Method
Monitor.Wait(System.Object, System.TimeSpan) Method
Monitor.Wait(System.Object) Method

obj

The Object on which to acquire the lock.

Exceptions

Exception Type	Condition
ArgumentNullException	obj is null .

Description

This method acquires an exclusive lock on obj .

A caller of this method is required to invoke System.Threading.Monitor.Exit(System.Object) once for each System.Threading.Monitor.Enter(System.Object) invoked.

The caller of this method is blocked if another thread has obtained the lock by calling System.Threading.Monitor.Enter(System.Object) and specifying the same object. The caller is not blocked if the current thread holds the lock. The same thread can invoke System.Threading.Monitor.Enter(System.Object) more than once (and it will not block); however, an equal number of System.Threading.Monitor.Exit(System.Object) calls are required to be invoked before other threads waiting on the object will unblock.

[Note: Invoking this member is identical to using the C# lock statement.]

See Also

System.Threading.Monitor Class, System.Threading Namespace

Releases an exclusive lock on the specified Object .

Parameters

obj

The Object on which to release the lock.

Exceptions

Exception Type	Condition
ArgumentNullException	obj is null .
SynchronizationLockException	The current thread does not own the lock for the specified object.

Description

This method releases an exclusive lock on obj. The caller is required to own the lock on obj .

If the caller owns the lock on the specified object, and has made an equal number of System.Threading.Monitor.Exit(System.Object) and System.Threading.Monitor.Enter(System.Object) calls for the object, then the lock is released. If the caller has not invoked System.Threading.Monitor.Exit(System.Object) as many times as System.Threading.Monitor.Enter(System.Object) , the lock is not released.

[Note: If the lock is released and there are other threads in the ready queue for the object, one of the threads will acquire the lock. If there are other threads in the waiting queue waiting to acquire the lock, they are not automatically moved to the ready queue when the owner of the lock calls System.Threading.Monitor.Exit(System.Object). To move one or more waiting threads into the ready queue, call System.Threading.Monitor.Pulse(System.Object) or System.Threading.Monitor.PulseAll(System.Object) prior to invoking System.Threading.Monitor.Exit(System.Object).]

See Also

System.Threading.Monitor Class, System.Threading Namespace

obj

The Object a thread might be waiting for.

Exceptions

Exception Type	Condition
ArgumentNullException	obj is null .
SynchronizationLockException	The calling thread does not own the lock for the specified object.

Description

The thread that currently owns the lock on the specified object invokes this method to signal the next thread in line for the lock (in the queue of threads waiting to acquire the lock on the object). Upon receiving the pulse, the waiting thread is moved to the ready queue. When the thread that invoked Pulse releases the lock, the next thread in the ready queue (which is not necessarily the thread that was pulsed) acquires the lock.

[Note: To signal a waiting object using Pulse , you must be the current owner of the lock.

To signal multiple threads, use the System.Threading.Monitor.PulseAll(System.Object) method.

]

See Also

System.Threading.Monitor Class, System.Threading Namespace

obj

The Object that one or more threads might be waiting for.

Exceptions

Exception Type	Condition
ArgumentNullException	obj is null .
SynchronizationLockException	The calling thread does not own the lock for the specified object.

Description

The thread that currently owns the lock on the specified object invokes this method to signal all threads waiting to acquire the lock on the object. After the signal is sent, the waiting threads are moved to the ready queue. When the thread that invoked PulseAll releases the lock, the next thread in the ready queue acquires the lock.

[Note: To signal waiting objects using PulseAll , you must be the current owner of the lock.

To signal a single thread, use the System.Threading.Monitor.Pulse(System.Object) method.

]

See Also

System.Threading.Monitor Class, System.Threading Namespace

obj

The Object on which to acquire the lock.

Return Value

true if the current thread acquired the lock; otherwise, false .

Exceptions

Exception Type	Condition
ArgumentNullException	obj is null .

Description

If successful, this method acquires an exclusive lock on obj. This method returns immediately, whether or not the lock is available.

This method is equivalent to System.Threading.Monitor.TryEnter(System.Object) (obj, 0).

See Also

System.Threading.Monitor Class, System.Threading Namespace

obj

The Object on which to acquire the lock.

millisecondsTimeout

A Int32 containing the maximum number of milliseconds to wait for the lock.

Return Value

true if the current thread acquired the lock; otherwise, false .

Exceptions

Exception Type	Condition
ArgumentNullException	obj is null .
ArgumentOutOfRangeException	millisecondsTimeout is negative, and not equal to System.Threading.Timeout.Infinite .

Description

If successful, this method acquires an exclusive lock on obj.

If millisecondsTimeout equals System.Threading.Timeout.Infinite, this method is equivalent to System.Threading.Monitor.Enter(System.Object) (obj). If millisecondsTimeout equals zero, this method is equivalent to System.Threading.Monitor.TryEnter(System.Object) (obj).

See Also

System.Threading.Monitor Class, System.Threading Namespace

obj

The Object on which to acquire the lock.

timeout

A TimeSpan set to the maximum amount of time to wait for the lock.

Return Value

true if the current thread acquires the lock; otherwise, false .

Exceptions

Exception Type	Condition
ArgumentNullException	obj is null .
ArgumentOutOfRangeException	The value of timeout in milliseconds is negative and is not equal to System.Threading.Timeout.Infinite , or is greater than System.Int32.MaxValue .

Description

If successful, this method acquires an exclusive lock on obj.

If the value of timeout converted to milliseconds equals System.Threading.Timeout.Infinite, this method is equivalent to System.Threading.Monitor.Enter(System.Object) (obj). If the value of timeout equals zero, this method is equivalent to System.Threading.Monitor.TryEnter(System.Object) (obj).

See Also

System.Threading.Monitor Class, System.Threading Namespace

obj

The Object on which to wait.

millisecondsTimeout

A Int32 containing the maximum number of milliseconds to wait before this method returns.

Return Value

true if the lock was reacquired before the specified time elapsed; otherwise, false .

Exceptions

Exception Type	Condition
ArgumentNullException	obj is null .
SynchronizationLockException	The calling thread does not own the lock for the specified object.
ArgumentOutOfRangeException	The value of millisecondsTimeout is negative, and not equal to System.Threading.Timeout.Infinite .

Description

If successful, this method reacquires an exclusive lock on obj.

This method behaves identically to System.Threading.Monitor.Wait(System.Object,System.Int32,System.Boolean) (obj), except that it does not block indefinitely unless System.Threading.Timeout.Infinite is specified for millisecondsTimeout . Once the specified time has elapsed, this method returns a value that indicates whether the lock has been reacquired by the caller. If millisecondsTimeout equals 0, this method returns immediately.

[Note: This method is called when the caller is waiting for a change in the state of the object, which occurs as a result of another thread's operations on the object. For additional details, see System.Threading.Monitor.Wait(System.Object,System.Int32,System.Boolean) (obj).]

See Also

System.Threading.Monitor Class, System.Threading Namespace

obj

The Object on which to wait.

timeout

A TimeSpan set to the maximum amount of time to wait before this method returns.

Return Value

true if the lock was reacquired before the specified time elapsed; otherwise, false .

Exceptions

Exception Type	Condition
ArgumentNullException	obj is null .
SynchronizationLockException	The calling thread does not own the lock for the specified object.
ArgumentOutOfRangeException	If timeout is negative, and is not equal to System.Threading.Timeout.Infinite , or is greater than System.Int32.MaxValue.

Description

If successful, this method reacquires an exclusive lock on obj.

This method behaves identically to System.Threading.Monitor.Wait(System.Object,System.Int32,System.Boolean) (obj), except that it does not block indefinitely unless System.Threading.Timeout.Infinite milliseconds is specified for timeout . Once the specified time has elapsed, this method returns a value that indicates whether the lock has been reacquired by the caller. If timeout equals 0, this method returns immediately.

[Note: This method is called when the caller is waiting for a change in the state of the object, which occurs as a result of another thread's operations on the object. For additional details, see System.Threading.Monitor.Wait(System.Object,System.Int32,System.Boolean) (obj ).]

See Also

System.Threading.Monitor Class, System.Threading Namespace

obj

The Object on which to wait.

Return Value

true if the call returned because the caller reacquired the lock for the specified object. This method does not return if the lock is not reacquired.

Exceptions

Exception Type	Condition
ArgumentNullException	obj is null .
SynchronizationLockException	The calling thread does not own the lock for the specified object.

Description

This method reacquires an exclusive lock on obj.

The thread that currently owns the lock on the specified object invokes this method in order to release the object so that another thread can access it. The caller is blocked while waiting to reacquire the lock. This method is called when the caller is waiting for a change in the state of the object, which occurs as a result of another thread's operations on the object.

When a thread calls Wait , it releases the lock on the object and enters the object's waiting queue. The next thread in the object's ready queue (if there is one) acquires the lock and has exclusive use of the object. All threads that call Wait remain in the waiting queue until they receive a signal via System.Threading.Monitor.Pulse(System.Object) or System.Threading.Monitor.PulseAll(System.Object) sent by the owner of the lock. If Pulse is sent, only the thread at the head of the waiting queue is affected. If PulseAll is sent, all threads that are waiting for the object are affected. When the signal is received, one or more threads leave the waiting queue and enter the ready queue. A thread in the ready queue is permitted to reacquire the lock.

This method returns when the calling thread reacquires the lock on the object. Note that this method blocks indefinitely if the holder of the lock does not call System.Threading.Monitor.Pulse(System.Object) or System.Threading.Monitor.PulseAll(System.Object).

The caller executes System.Threading.Monitor.Wait(System.Object,System.Int32,System.Boolean) once, regardless of the number of times System.Threading.Monitor.Enter(System.Object) has been invoked for the specified object. Conceptually, the System.Threading.Monitor.Wait(System.Object,System.Int32,System.Boolean) method stores the number of times the caller invoked System.Threading.Monitor.Enter(System.Object) on the object and invokes System.Threading.Monitor.Exit(System.Object) as many times as necessary to fully release the locked object. The caller then blocks while waiting to reacquire the object. When the caller reacquires the lock, the system calls System.Threading.Monitor.Enter(System.Object) as many times as necessary to restore the saved Enter count for the caller.

Calling System.Threading.Monitor.Wait(System.Object,System.Int32,System.Boolean) releases the lock for the specified object only; if the caller is the owner of locks on other objects, these locks are not released.

See Also

System.Threading.Monitor Class, System.Threading Namespace