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WaitOrTimerCallback Delegate

Represents a method to be called when a WaitHandle is signaled or times out.

[Visual Basic]
<Serializable>
Public Delegate Sub WaitOrTimerCallback( _
   ByVal state As Object, _
   ByVal timedOut As Boolean _
)
[C#]
[Serializable]
public delegate void WaitOrTimerCallback(
   object state,
   bool timedOut
);
[C++]
[Serializable]
public __gc __delegate void WaitOrTimerCallback(
   Object* state,
   bool timedOut
);

[JScript] In JScript, you can use the delegates in the .NET Framework, but you cannot define your own.

Parameters [Visual Basic, C#, C++]

The declaration of your callback method must have the same parameters as the WaitOrTimerCallback delegate declaration.

state
An object containing information to be used by the callback method each time it executes.
timedOut
true if the WaitHandle timed out; false if it was signaled.

Remarks

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 here.

Create the registered wait handle by passing the WaitOrTimerCallback delegate and a WaitHandle to ThreadPool.RegisterWaitForSingleObject. Your callback method executes each time the WaitHandle times out or is signaled.

[Visual Basic] Note   Visual Basic users can omit the WaitOrTimerCallback constructor, and simply use the AddressOf operator when passing the callback method to RegisterWaitForSingleObject. Visual Basic automatically calls the correct delegate constructor.

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. Each time your callback method executes, the state parameter contains this object.

For more information about using callback methods to synchronize thread pool threads, see Thread Pooling.

Example

[Visual Basic] 
Imports System
Imports System.Threading

' TaskInfo contains data that will be passed to the callback
' method.
Public Class TaskInfo
    public Handle As RegisteredWaitHandle = Nothing
    public OtherInfo As String = "default"
End Class

Public Class Example
    Public Shared Sub Main()
        ' The main thread uses AutoResetEvent to signal the
        ' registered wait handle, which executes the callback
        ' method.
        Dim ev As New AutoResetEvent(false)

        Dim ti As New TaskInfo()
        ti.OtherInfo = "First task"
        ' The TaskInfo for the task includes the registered wait
        ' handle returned by RegisterWaitForSingleObject.  This
        ' allows the wait to be terminated when the object has
        ' been signaled once (see WaitProc).
        ti.Handle = ThreadPool.RegisterWaitForSingleObject( _
            ev, _
            New WaitOrTimerCallback(AddressOf WaitProc), _
            ti, _
            1000, _
            false _
        )

        ' The main thread waits about three seconds, to demonstrate 
        ' the time-outs on the queued task, and then signals.
        Thread.Sleep(3100)
        Console.WriteLine("Main thread signals.")
        ev.Set()

        ' The main thread sleeps, which should give the callback
        ' method time to execute.  If you comment out this line, the
        ' program usually ends before the ThreadPool thread can execute.
        Thread.Sleep(1000)
        ' If you start a thread yourself, you can wait for it to end
        ' by calling Thread.Join.  This option is not available with 
        ' thread pool threads.
    End Sub
   
    ' The callback method executes when the registered wait times out,
    ' or when the WaitHandle (in this case AutoResetEvent) is signaled.
    ' WaitProc unregisters the WaitHandle the first time the event is 
    ' signaled.
    Public Shared Sub WaitProc(state As Object, timedOut As Boolean)
        ' The state object must be cast to the correct type, because the
        ' signature of the WaitOrTimerCallback delegate specifies type
        ' Object.
        Dim ti As TaskInfo = CType(state, TaskInfo)

        Dim cause As String = "TIMED OUT"
        If Not timedOut Then
            cause = "SIGNALED"
            ' If the callback method executes because the WaitHandle is
            ' signaled, stop future execution of the callback method
            ' by unregistering the WaitHandle.
            If Not ti.Handle Is Nothing Then
                ti.Handle.Unregister(Nothing)
            End If
        End If 

        Console.WriteLine("WaitProc( {0} ) executes on thread {1}; cause = {2}.", _
            ti.OtherInfo, _
            Thread.CurrentThread.GetHashCode().ToString(), _
            cause _
        )
    End Sub
End Class

[C#] 
using System;
using System.Threading;

// TaskInfo contains data that will be passed to the callback
// method.
public class TaskInfo {
    public RegisteredWaitHandle Handle = null;
    public string OtherInfo = "default";
}

public class Example {
    public static void Main(string[] args) {
        // The main thread uses AutoResetEvent to signal the
        // registered wait handle, which executes the callback
        // method.
        AutoResetEvent ev = new AutoResetEvent(false);

        TaskInfo ti = new TaskInfo();
        ti.OtherInfo = "First task";
        // The TaskInfo for the task includes the registered wait
        // handle returned by RegisterWaitForSingleObject.  This
        // allows the wait to be terminated when the object has
        // been signaled once (see WaitProc).
        ti.Handle = ThreadPool.RegisterWaitForSingleObject(
            ev,
            new WaitOrTimerCallback(WaitProc),
            ti,
            1000,
            false
        );

        // The main thread waits three seconds, to demonstrate the
        // time-outs on the queued thread, and then signals.
        Thread.Sleep(3100);
        Console.WriteLine("Main thread signals.");
        ev.Set();

        // The main thread sleeps, which should give the callback
        // method time to execute.  If you comment out this line, the
        // program usually ends before the ThreadPool thread can execute.
        Thread.Sleep(1000);
        // If you start a thread yourself, you can wait for it to end
        // by calling Thread.Join.  This option is not available with 
        // thread pool threads.
    }
   
    // The callback method executes when the registered wait times out,
    // or when the WaitHandle (in this case AutoResetEvent) is signaled.
    // WaitProc unregisters the WaitHandle the first time the event is 
    // signaled.
    public static void WaitProc(object state, bool timedOut) {
        // The state object must be cast to the correct type, because the
        // signature of the WaitOrTimerCallback delegate specifies type
        // Object.
        TaskInfo ti = (TaskInfo) state;

        string cause = "TIMED OUT";
        if (!timedOut) {
            cause = "SIGNALED";
            // If the callback method executes because the WaitHandle is
            // signaled, stop future execution of the callback method
            // by unregistering the WaitHandle.
            if (ti.Handle != null)
                ti.Handle.Unregister(null);
        } 

        Console.WriteLine("WaitProc( {0} ) executes on thread {1}; cause = {2}.",
            ti.OtherInfo, 
            Thread.CurrentThread.GetHashCode().ToString(), 
            cause
        );
    }
}

[C++] 
#using <mscorlib.dll>

using namespace System;
using namespace System::Threading;

// TaskInfo contains data that will be passed to the callback
// method.
public __gc class TaskInfo 
{
public:
    TaskInfo()
    {
        Handle = 0;
        OtherInfo = S"default";
    }

    RegisteredWaitHandle* Handle;;
    String* OtherInfo;
};


__gc class Example
{
    // The callback method executes when the registered wait times out,
    // or when the WaitHandle (in this case AutoResetEvent) is signaled.
    // WaitProc unregisters the WaitHandle the first time the event is 
    // signaled.
public:
    static void WaitProc(Object* state, bool timedOut) 
    {
        // The state Object must be cast to the correct type, because the
        // signature of the WaitOrTimerCallback delegate specifies type
        // Object.
        TaskInfo* ti = static_cast<TaskInfo*>(state);

        String* cause = S"TIMED OUT";
        if (!timedOut) {
            cause = S"SIGNALED";
            // If the callback method executes because the WaitHandle is
            // signaled, stop future execution of the callback method
            // by unregistering the WaitHandle.
            if (ti->Handle != 0)
                ti->Handle->Unregister(0);
        } 

        Console::WriteLine(S"WaitProc( {0}) executes on thread {1}; cause = {2}.",
            ti->OtherInfo, __box(Thread::CurrentThread->GetHashCode()), cause);
    }
};

int main() 
{
    // The main thread uses AutoResetEvent to signal the
    // registered wait handle, which executes the callback
    // method.
    AutoResetEvent* ev = new AutoResetEvent(false);

    TaskInfo* ti = new TaskInfo();
    ti->OtherInfo = S"First task";
    // The TaskInfo for the task includes the registered wait
    // handle returned by RegisterWaitForSingleObject.  This
    // allows the wait to be terminated when the object has
    // been signaled once (see WaitProc).
    ti->Handle = ThreadPool::RegisterWaitForSingleObject(ev,
        new WaitOrTimerCallback(0, Example::WaitProc),
        ti,
        1000,
        false);

    // The main thread waits three seconds, to demonstrate the
    // time-outs on the queued thread, and then signals.
    Thread::Sleep(3100);
    Console::WriteLine(S"Main thread signals.");
    ev->Set();

    // The main thread sleeps, which should give the callback
    // method time to execute.  If you comment out this line, the
    // program usually ends before the ThreadPool thread can execute.
    Thread::Sleep(1000);
    // If you start a thread yourself, you can wait for it to end
    // by calling Thread::Join.  This option is not available with 
    // thread pool threads.
    return 0;
}

[JScript] No example is available for JScript. To view a Visual Basic, C#, or C++ example, click the Language Filter button Language Filter in the upper-left corner of the page.

Requirements

Namespace: System.Threading

Platforms: Windows 98, Windows NT 4.0, Windows Millennium Edition, Windows 2000, Windows XP Home Edition, Windows XP Professional, Windows Server 2003 family

Assembly: Mscorlib (in Mscorlib.dll)

See Also

System.Threading Namespace | Threads and Threading | Thread Pooling

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