Articolo
11/18/2011

Monitor.Wait Method (Object, TimeSpan)

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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.

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

Syntax

'Declaration
Public Shared Function Wait ( _
    obj As Object, _
    timeout As TimeSpan _
) As Boolean

public static bool Wait(
    Object obj,
    TimeSpan timeout
)

Parameters

obj
Type: System.Object
The object on which to wait.

timeout
Type: System.TimeSpan
A TimeSpan representing the amount of time to wait before the thread enters the ready queue.

Return Value

Type: System.Boolean
true if the lock was reacquired before the specified time elapsed; false if the lock was reacquired after the specified time elapsed. The method does not return until the lock is reacquired.

Exceptions

Exception	Condition
ArgumentNullException	The obj parameter is nulla null reference (Nothing in Visual Basic).
SynchronizationLockException	The calling thread does not own the lock for the specified object.
ArgumentOutOfRangeException	The value of the timeout parameter in milliseconds is negative and does not represent Infinite (–1 millisecond), or is greater than MaxValue.

Remarks

This method does not return until it reacquires an exclusive lock on the obj parameter.

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 needs to wait for a state change that will occur as a result of another thread's operations.

The time-out ensures that the current thread does not block indefinitely if another thread releases the lock without first calling the Pulse or PulseAll method. It also moves the thread to the ready queue, bypassing other threads ahead of it in the wait queue, so that it can reacquire the lock sooner. The thread can test the return value of the Wait method to determine whether it reacquired the lock prior to the time-out. The thread can evaluate the conditions that caused it to enter the wait, and if necessary call the Wait method again.

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. The thread that invoked Wait remains in the waiting queue until either a thread that holds the lock invokes PulseAll, or it is the next in the queue and a thread that holds the lock invokes Pulse. However, if timeout elapses before another thread invokes this object's Pulse or PulseAll method, the original thread is moved to the ready queue in order to regain the lock.

Note:
If a TimeSpan representing –1 millisecond is specified for the timeout parameter, this method blocks indefinitely unless the holder of the lock calls Pulse or PulseAll. If timeout is 0 milliseconds, the thread that calls Wait releases the lock and then immediately enters the ready queue in order to regain the lock.

The caller executes Wait once, regardless of the number of times Enter has been invoked for the specified object. Conceptually, the Wait method stores the number of times the caller invoked Enter on the object and invokes Exit 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 Enter as many times as necessary to restore the saved Enter count for the caller. Calling Wait releases the lock for the specified object only; if the caller is the owner of locks on other objects, these locks are not released.

Note:
A synchronized object holds several references, including a reference to the thread that currently holds the lock, a reference to the ready queue, which contains the threads that are ready to obtain the lock, and a reference to the waiting queue, which contains the threads that are waiting for notification of a change in the object's state.

The Pulse, PulseAll, and Wait methods must be invoked from within a synchronized block of code.

The remarks for the Pulse method explain what happens if Pulse is called when no threads are waiting.

Version Notes

Silverlight for Windows Phone

When a user navigates away from a Windows Phone application, the application is typically put into a dormant state. When the user returns to a dormant application, the application automatically resumes. If the application is put into a dormant state while this API is being used, the API will not complete as expected. Applications should be designed to handle this possibility. For more information about the Windows Phone execution model, see Execution Model for Windows Phone.

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.

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