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:
Use the AsyncWaitHandle property of the IAsyncResult returned by the asynchronous operation's BeginOperationName method. This approach is demonstrated in this topic.
Call the asynchronous operation's EndOperationName method. For an example that demonstrates this approach, see Blocking Application Execution by Ending an Async Operation.
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
