WaitOrTimerCallback Delegate
[ This article is for Windows Phone 8 developers. If you’re developing for Windows 10, see the latest documentation. ]
Represents a method to be called when a WaitHandle is signaled or times out.
Assembly: mscorlib (in mscorlib.dll)
Parameters
- state
- Type: System::Object
An object that contains information to be used by the callback method every time it executes.
- timedOut
- Type: System::Boolean
true to indicate that the WaitHandle timed out; false to indicate that it was signaled.
WaitOrTimerCallback represents a callback method that you want to execute when a registered wait handle times out or is signaled. Create the delegate by passing your callback method to the WaitOrTimerCallback constructor. Your method must have the signature shown in the Syntax section.
Create the registered wait handle by passing the WaitOrTimerCallback delegate and a WaitHandle to the ThreadPool::RegisterWaitForSingleObject method. Your callback method executes every time the WaitHandle times out or is signaled.
 Note: |
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Visual Basic and C# users can omit the WaitOrTimerCallback constructor, because the compilers infer the delegate type automatically and supply the correct constructor. In Visual Basic, use the AddressOf operator when passing the callback method to RegisterWaitForSingleObject. |
If you want to pass information to your callback method, create an object that contains the necessary information and pass it to RegisterWaitForSingleObject when you create the registered wait handle. Every time your callback method executes, the state parameter contains this object.
For more information about how to use callback methods to synchronize thread pool threads, see The Managed Thread Pool.
The following example shows how to use the WaitOrTimerCallback delegate to represent a callback method that is executed when a wait handle is signaled.
The example also shows how to use the RegisterWaitForSingleObject method to execute a specified callback method when a specified wait handle is signaled. In this example, the callback method is WaitProc and the wait handle is an AutoResetEvent.
The example defines a TaskInfo class to hold the information that is passed to the callback when it executes. The example creates a TaskInfo object and assigns it some string data. The RegisteredWaitHandle that is returned by the RegisterWaitForSingleObject method is assigned to the Handle field of the TaskInfo object, so that the callback method has access to the RegisteredWaitHandle.
In addition to specifying TaskInfo as the object to pass to the callback method, the call to the RegisterWaitForSingleObject method specifies the AutoResetEvent that the task will wait for, a WaitOrTimerCallback delegate that represents the WaitProc callback method, a one-second time-out interval, and multiple callbacks.
When the main thread signals the AutoResetEvent by calling its Set method, the WaitOrTimerCallback delegate is invoked. The WaitProc method tests RegisteredWaitHandle to determine whether a time-out occurred. If the callback was invoked because the wait handle was signaled, the WaitProc method unregisters the RegisteredWaitHandle, stopping additional callbacks. In the case of a time-out, the task continues waiting.
The WaitProc method displays its output in a TextBlock on the UI thread. To access the TextBlock from the callback thread, the WaitProc method uses the Dispatcher property to obtain a Dispatcher object for the TextBlock, and then uses the Dispatcher::BeginInvoke method to make the cross-thread call.