Monitor.Wait Method (Object, Int32, Boolean)
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. This method also specifies whether the synchronization domain for the context (if in a synchronized context) is exited before the wait and reacquired afterward.
Assembly: mscorlib (in mscorlib.dll)
'Declaration Public Shared Function Wait ( _ obj As Object, _ millisecondsTimeout As Integer, _ exitContext As Boolean _ ) As Boolean
- Type: System.Object
The object on which to wait.
- Type: System.Int32
The number of milliseconds to wait before the thread enters the ready queue.
- Type: System.Boolean
true to exit and reacquire the synchronization domain for the context (if in a synchronized context) before the wait; otherwise, false.
Return ValueType: 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.
The obj parameter is Nothing.
Wait is not invoked from within a synchronized block of code.
The thread that invokes Wait is later interrupted from the waiting state. This happens when another thread calls this thread's Interrupt method.
The value of the millisecondsTimeout parameter is negative, and is not equal to Infinite.
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 and enters the waiting queue. At this point, the next thread in the ready queue (if there is one) is allowed to take control of the lock. 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 millisecondsTimeout 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.
If Infinite is specified for the millisecondsTimeout parameter, this method blocks indefinitely unless the holder of the lock calls Pulse or PulseAll. If millisecondsTimeout equals 0, 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.
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.
Notes on Exiting the Context
The exitContext parameter has no effect unless the Wait method is called from inside a nondefault managed context. This can happen if your thread is inside a call to an instance of a class derived from ContextBoundObject. Even if you are currently executing a method on a class that is not derived from ContextBoundObject, like String, you can be in a nondefault context if a ContextBoundObject is on your stack in the current application domain.
When your code is executing in a nondefault context, specifying true for exitContext causes the thread to exit the nondefault managed context (that is, to transition to the default context) before executing the Wait method. It returns to the original nondefault context after the call to the Wait method completes.
This can be useful when the context-bound class has the SynchronizationAttribute attribute applied. In that case, all calls to members of the class are automatically synchronized, and the synchronization domain is the entire body of code for the class. If code in the call stack of a member calls the Wait method and specifies true for exitContext, the thread exits the synchronization domain, allowing a thread that is blocked on a call to any member of the object to proceed. When the Wait method returns, the thread that made the call must wait to reenter the synchronization domain.