Blocking Application Execution Using an AsyncWaitHandle

Applications that cannot continue to do other work while waiting for the results of an asynchronous operation must block until the operation completes. Use one of the following options to block your application's main thread while waiting for an asynchronous operation to complete:

Applications that use one or more WaitHandle objects to block until an asynchronous operation is complete will typically call the BeginOperationName method, perform any work that can be done without the results of the operation, and then block until the asynchronous operation(s) completes. An application can block on a single operation by calling one of the WaitOne methods using the AsyncWaitHandle. To block while waiting for a set of asynchronous operations to complete, store the associated AsyncWaitHandle objects in an array and call one of the WaitAll methods. To block while waiting for any one of a set of asynchronous operations to complete, store the associated AsyncWaitHandle objects in an array and call one of the WaitAny methods.

Example

The following code example demonstrates using asynchronous methods in the DNS class to retrieve Domain Name System information for a user-specified computer. The example demonstrates blocking using the WaitHandle associated with the asynchronous operation. Note that null (Nothing in Visual Basic) is passed for the BeginGetHostByNamerequestCallback and stateObject parameters because these are not required when using this approach.

/*
The following example demonstrates using asynchronous methods to
get Domain Name System information for the specified host computer.

*/

using System;
using System.Net;
using System.Net.Sockets;
using System.Threading;

namespace Examples.AdvancedProgramming.AsynchronousOperations
{
    public class WaitUntilOperationCompletes
    {
        public static void Main(string[] args)
        {
            // Make sure the caller supplied a host name.
            if (args.Length == 0 || args[0].Length == 0)
            {
                // Print a message and exit.
                Console.WriteLine("You must specify the name of a host computer.");
                return;
            }
            // Start the asynchronous request for DNS information.
            IAsyncResult result = Dns.BeginGetHostEntry(args[0], null, null);
            Console.WriteLine("Processing request for information...");
            // Wait until the operation completes.
            result.AsyncWaitHandle.WaitOne();
            // The operation completed. Process the results.
            try
            {
                // Get the results.
                IPHostEntry host = Dns.EndGetHostEntry(result);
                string[] aliases = host.Aliases;
                IPAddress[] addresses = host.AddressList;
                if (aliases.Length > 0)
                {
                    Console.WriteLine("Aliases");
                    for (int i = 0; i < aliases.Length; i++)
                    {
                        Console.WriteLine("{0}", aliases[i]);
                    }
                }
                if (addresses.Length > 0)
                {
                    Console.WriteLine("Addresses");
                    for (int i = 0; i < addresses.Length; i++)
                    {
                        Console.WriteLine("{0}",addresses[i].ToString());
                    }
                }
            }
            catch (SocketException e)
            {
                Console.WriteLine("Exception occurred while processing the request: {0}",
                    e.Message);
            }
        }
    }
}
' The following example demonstrates using asynchronous methods to
' get Domain Name System information for the specified host computer.

Imports System.Net
Imports System.Net.Sockets
Imports System.Threading

namespace Examples.AdvancedProgramming.AsynchronousOperations
    Public Class WaitUntilOperationCompletes

        Public Shared Sub Main(args() as String)
            ' Make sure the caller supplied a host name.
            If (args.Length = 0)
                ' Print a message and exit.
                Console.WriteLine("You must specify the name of a host computer.")
                End
            End If
            ' Start the asynchronous request for DNS information.
            Dim result as IAsyncResult = Dns.BeginGetHostEntry(args(0), Nothing, Nothing)
            Console.WriteLine("Processing request for information...")
            ' Wait until the operation completes.
            result.AsyncWaitHandle.WaitOne()
            ' The operation completed. Process the results.
            Try
                ' Get the results.
                Dim host as IPHostEntry = Dns.EndGetHostEntry(result)
                Dim aliases() as String = host.Aliases
                Dim addresses() as IPAddress = host.AddressList
                Dim i as Integer
                If aliases.Length > 0
                    Console.WriteLine("Aliases")
                    For i = 0 To aliases.Length - 1
                        Console.WriteLine("{0}", aliases(i))
                    Next i
                End If
                If addresses.Length > 0
                    Console.WriteLine("Addresses")
                    For i = 0 To addresses.Length - 1
                        Console.WriteLine("{0}", addresses(i).ToString())
                    Next i
                End If
            Catch e as SocketException
                Console.WriteLine("An exception occurred while processing the request: {0}" _
                  , e.Message)
            End Try
        End Sub
    End Class
End Namespace

See also