IAsyncResult.AsyncWaitHandle Property (System)

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Property Value

Examples

The following example demonstrates how to use the AsyncWaitHandle property to get a WaitHandle , and how to wait for an asynchronous call on a delegate. The WaitHandle is signaled when the asynchronous call completes, and you can wait for it by calling the WaitOne method.
The example consists of two classes: the class that contains the method that is called asynchronously, and the class that contains the Main method that makes the call.
For more information and more examples of calling methods asynchronously by using delegates, see Calling Synchronous Methods Asynchronously .

   using namespace System;
using namespace System::Threading;
using namespace System::Runtime::InteropServices; 
namespace Examples {
namespace AdvancedProgramming {
namespace AsynchronousOperations
    public ref class AsyncDemo 
    public:
        // The method to be executed asynchronously.
        String^ TestMethod(int callDuration, [OutAttribute] int% threadId) 
            Console::WriteLine("Test method begins.");
            Thread::Sleep(callDuration);
            threadId = Thread::CurrentThread->ManagedThreadId;
            return String::Format("My call time was {0}.", callDuration);
    // The delegate must have the same signature as the method
    // it will call asynchronously.
    public delegate String^ AsyncMethodCaller(int callDuration, [OutAttribute] int% threadId);
   
    using System;
using System.Threading;
namespace Examples.AdvancedProgramming.AsynchronousOperations
    public class AsyncDemo
        // The method to be executed asynchronously.
        public string TestMethod(int callDuration, out int threadId)
            Console.WriteLine("Test method begins.");
            Thread.Sleep(callDuration);
            threadId = Thread.CurrentThread.ManagedThreadId;
            return String.Format("My call time was {0}.", callDuration.ToString());
    // The delegate must have the same signature as the method
    // it will call asynchronously.
    public delegate string AsyncMethodCaller(int callDuration, out int threadId);
    
     Imports System.Threading
Imports System.Runtime.InteropServices 
Namespace Examples.AdvancedProgramming.AsynchronousOperations
    Public Class AsyncDemo 
        ' The method to be executed asynchronously.
        Public Function TestMethod(ByVal callDuration As Integer, _
                <Out> ByRef threadId As Integer) As String
            Console.WriteLine("Test method begins.")
            Thread.Sleep(callDuration)
            threadId = Thread.CurrentThread.ManagedThreadId()
            return String.Format("My call time was {0}.", callDuration.ToString())
        End Function
    End Class
    ' The delegate must have the same signature as the method
    ' it will call asynchronously.
    Public Delegate Function AsyncMethodCaller(ByVal callDuration As Integer, _
        <Out> ByRef threadId As Integer) As String
End Namespace
     
      #using <TestMethod.dll>
using namespace System;
using namespace System::Threading;
using namespace Examples::AdvancedProgramming::AsynchronousOperations;
void main() 
    // The asynchronous method puts the thread id here.
    int threadId;
    // Create an instance of the test class.
    AsyncDemo^ ad = gcnew AsyncDemo();
    // Create the delegate.
    AsyncMethodCaller^ caller = gcnew AsyncMethodCaller(ad, &AsyncDemo::TestMethod);
    // Initiate the asychronous call.
    IAsyncResult^ result = caller->BeginInvoke(3000, 
        threadId, nullptr, nullptr);
    Thread::Sleep(0);
    Console::WriteLine("Main thread {0} does some work.",
        Thread::CurrentThread->ManagedThreadId);
    // Wait for the WaitHandle to become signaled.
    result->AsyncWaitHandle->WaitOne();
    // Perform additional processing here.
    // Call EndInvoke to retrieve the results.
    String^ returnValue = caller->EndInvoke(threadId, result);
    // Close the wait handle.
    result->AsyncWaitHandle->Close();
    Console::WriteLine("The call executed on thread {0}, with return value \"{1}\".",
        threadId, returnValue);
/* This example produces output similar to the following:
Main thread 1 does some work.
Test method begins.
The call executed on thread 3, with return value "My call time was 3000.".
      
       using System;
using System.Threading;
namespace Examples.AdvancedProgramming.AsynchronousOperations
    public class AsyncMain
        static void Main()
            // The asynchronous method puts the thread id here.
            int threadId;
            // Create an instance of the test class.
            AsyncDemo ad = new AsyncDemo();
            // Create the delegate.
            AsyncMethodCaller caller = new AsyncMethodCaller(ad.TestMethod);
            // Initiate the asychronous call.
            IAsyncResult result = caller.BeginInvoke(3000,
                out threadId, null, null);
            Thread.Sleep(0);
            Console.WriteLine("Main thread {0} does some work.",
                Thread.CurrentThread.ManagedThreadId);
            // Wait for the WaitHandle to become signaled.
            result.AsyncWaitHandle.WaitOne();
            // Perform additional processing here.
            // Call EndInvoke to retrieve the results.
            string returnValue = caller.EndInvoke(out threadId, result);
            // Close the wait handle.
            result.AsyncWaitHandle.Close();
            Console.WriteLine("The call executed on thread {0}, with return value \"{1}\".",
                threadId, returnValue);
/* This example produces output similar to the following:
Main thread 1 does some work.
Test method begins.
The call executed on thread 3, with return value "My call time was 3000.".
       
        Imports System.Threading
Imports System.Runtime.InteropServices 
Namespace Examples.AdvancedProgramming.AsynchronousOperations
    Public Class AsyncMain 
        Shared Sub Main() 
            ' The asynchronous method puts the thread id here.
            Dim threadId As Integer
            ' Create an instance of the test class.
            Dim ad As New AsyncDemo()
            ' Create the delegate.
            Dim caller As New AsyncMethodCaller(AddressOf ad.TestMethod)
            ' Initiate the asynchronous call.
            Dim result As IAsyncResult = caller.BeginInvoke(3000, _
                threadId, Nothing, Nothing)
            Thread.Sleep(0)
            Console.WriteLine("Main thread {0} does some work.", _
                Thread.CurrentThread.ManagedThreadId)
            ' Perform additional processing here and then
            ' wait for the WaitHandle to be signaled.
            result.AsyncWaitHandle.WaitOne()
            ' Call EndInvoke to retrieve the results.
            Dim returnValue As String = caller.EndInvoke(threadId, result)
            ' Close the wait handle.
            result.AsyncWaitHandle.Close()
            Console.WriteLine("The call executed on thread {0}, with return value ""{1}"".", _
                threadId, returnValue)
        End Sub
    End Class
End Namespace
'This example produces output similar to the following:
'Main thread 1 does some work.
'Test method begins.
'The call executed on thread 3, with return value "My call time was 3000.".
        
         Remarks
        
        
         The return value allows the client to wait for an asynchronous operation to complete instead of polling
         
          IsCompleted
         
         until the operation concludes. The return value can be used to perform a
         
          WaitOne
         
         ,
         
          WaitAny
         
         , or
         
          WaitAll
         
         operation.
        
        
         The common language runtime supplies a number of waitable objects, such as
         
          ManualResetEvent
         
         ,
         
          AutoResetEvent
         
         , and
         
          Mutex
         
         , all of which mirror Win32 synchronization primitives.
        
        
         Notes to Implementers
        
        
         The object that implements
         
          IAsyncResult
         
         does not need to create the
         
          WaitHandle
         
         until the
         
          AsyncWaitHandle
         
         property is read. It is the choice of the
         
          IAsyncResult
         
         implementer. However, if the implementer creates
         
          AsyncWaitHandle
         
         , it is the responsibility of the implementer to signal the
         
          WaitHandle
         
         that will terminate the wait at the appropriate time. For example,
         
          AsyncResult
         
         terminates the wait on behalf of the caller when an asynchronously invoked method returns. Once created,
         
          AsyncWaitHandle
         
         should be kept alive until the user calls the method that concludes the asynchronous operation. At that time the object behind
         
          AsyncWaitHandle
         
         can be discarded.
        
        
         Notes to Callers
        
        
         Clients that wait for the operation to complete (as opposed to polling) use this property to obtain a synchronization object to wait on.
        
        
         Note: When you use the
         
          BeginInvoke
         
         method of a delegate to call a method asynchronously and obtain a wait handle from the resulting
         
          IAsyncResult
         
         , we recommend that you close the wait handle as soon as you are finished using it, by calling the
         
          Close()
         
         method. If you simply release all references to the wait handle, system resources are freed when garbage collection reclaims the wait handle, but garbage collection works more efficiently when disposable objects are explicitly closed or disposed. For more information, see the
         
          AsyncWaitHandle
         
         property.
        
        
         Applies to