Using an AsyncCallback Delegate to End an Asynchronous Operation

Applications that can do other work while waiting for the results of an asynchronous operation should not block waiting until the operation completes. Use one of the following options to continue executing instructions while waiting for an asynchronous operation to complete:

• Use an AsyncCallback delegate to process the results of the asynchronous operation in a separate thread. This approach is demonstrated in this topic.

• Use the IsCompleted property of the IAsyncResult returned by the asynchronous operation's BeginOperationName method to determine whether the operation has completed. For an example that demonstrates this approach, see Polling for the Status of an Asynchronous Operation.

Example

The following code example demonstrates using asynchronous methods in the Dns class to retrieve Domain Name System (DNS) information for user-specified computers. This example creates an AsyncCallback delegate that references the ProcessDnsInformation method. This method is called once for each asynchronous request for DNS information.

Note that the user-specified host is passed to the BeginGetHostByName Object parameter. For an example that demonstrates defining and using a more complex state object, see Using an AsyncCallback Delegate and State Object.

/*
The following example demonstrates using asynchronous methods to
get Domain Name System information for the specified host computers.
This example uses a delegate to obtain the results of each asynchronous
operation.
*/
using System;
using System.Net;
using System.Net.Sockets;
using System.Threading;
using System.Collections.Specialized;
using System.Collections;
namespace Examples.AdvancedProgramming.AsynchronousOperations
{
 public class UseDelegateForAsyncCallback
 {
 static int requestCounter;
 static ArrayList hostData = new ArrayList();
 static StringCollection hostNames = new StringCollection();
 static void UpdateUserInterface()
 {
 // Print a message to indicate that the application
 // is still working on the remaining requests.
 Console.WriteLine($"{requestCounter} requests remaining.");
 }
 public static void Main()
 {
 // Create the delegate that will process the results of the
 // asynchronous request.
 AsyncCallback callBack = new AsyncCallback(ProcessDnsInformation);
 string host;
 do
 {
 Console.Write(" Enter the name of a host computer or <enter> to finish: ");
 host = Console.ReadLine();
 if (host.Length > 0)
 {
 // Increment the request counter in a thread safe manner.
 Interlocked.Increment(ref requestCounter);
 // Start the asynchronous request for DNS information.
 Dns.BeginGetHostEntry(host, callBack, host);
 }
 } while (host.Length > 0);
 // The user has entered all of the host names for lookup.
 // Now wait until the threads complete.
 while (requestCounter > 0)
 {
 UpdateUserInterface();
 }
 // Display the results.
 for (int i = 0; i< hostNames.Count; i++)
 {
 object data = hostData [i];
 string message = data as string;
 // A SocketException was thrown.
 if (message != null)
 {
 Console.WriteLine($"Request for {hostNames[i]} returned message: {message}");
 continue;
 }
 // Get the results.
 IPHostEntry h = (IPHostEntry) data;
 string[] aliases = h.Aliases;
 IPAddress[] addresses = h.AddressList;
 if (aliases.Length > 0)
 {
 Console.WriteLine($"Aliases for {hostNames[i]}");
 for (int j = 0; j < aliases.Length; j++)
 {
 Console.WriteLine($"{aliases[j]}");
 }
 }
 if (addresses.Length > 0)
 {
 Console.WriteLine($"Addresses for {hostNames[i]}");
 for (int k = 0; k < addresses.Length; k++)
 {
 Console.WriteLine($"{addresses[k].ToString()}");
 }
 }
 }
 }
 // The following method is called when each asynchronous operation completes.
 static void ProcessDnsInformation(IAsyncResult result)
 {
 string hostName = (string) result.AsyncState;
 hostNames.Add(hostName);
 try
 {
 // Get the results.
 IPHostEntry host = Dns.EndGetHostEntry(result);
 hostData.Add(host);
 }
 // Store the exception message.
 catch (SocketException e)
 {
 hostData.Add(e.Message);
 }
 finally
 {
 // Decrement the request counter in a thread-safe manner.
 Interlocked.Decrement(ref requestCounter);
 }
 }
 }
}

'The following example demonstrates using asynchronous methods to
'get Domain Name System information for the specified host computers.
'This example uses a delegate to obtain the results of each asynchronous 
'operation.
Imports System.Net
Imports System.Net.Sockets
Imports System.Threading
Imports System.Collections.Specialized
Imports System.Collections
Namespace Examples.AdvancedProgramming.AsynchronousOperations
 Public Class UseDelegateForAsyncCallback
 Dim Shared requestCounter as Integer
 Dim Shared hostData as ArrayList = new ArrayList()
 Dim Shared hostNames as StringCollection = new StringCollection()
 Shared Sub UpdateUserInterface()
 ' Print a message to indicate that the application
 ' is still working on the remaining requests.
 Console.WriteLine("{0} requests remaining.", requestCounter)
 End Sub
 Public Shared Sub Main()
 ' Create the delegate that will process the results of the 
 ' asynchronous request.
 Dim callBack as AsyncCallback
 Dim host as string
 Dim i, j, k as Integer
 callback = AddressOf ProcessDnsInformation
 Do
 Console.Write(" Enter the name of a host computer or <enter> to finish: ")
 host = Console.ReadLine()
 If host.Length > 0
 ' Increment the request counter in a thread safe manner.
 Interlocked.Increment(requestCounter)
 ' Start the asynchronous request for DNS information.
 Dns.BeginGetHostEntry(host, callBack, host)
 End If
 Loop While (host.Length > 0)
 ' The user has entered all of the host names for lookup.
 ' Now wait until the threads complete.
 Do While requestCounter > 0
 UpdateUserInterface()
 Loop
 ' Display the results.
 For i = 0 To hostNames.Count - 1
 Dim dataObject as Object = hostData(i)
 Dim message as String
 ' Was a SocketException was thrown?
 If TypeOf dataObject is String
 message = CType(dataObject, String)
 Console.WriteLine("Request for {0} returned message: {1}", _
 hostNames(i), message)
 Else
 ' Get the results.
 Dim h as IPHostEntry = CType(dataObject, IPHostEntry)
 Dim aliases() as String = h.Aliases
 Dim addresses() as IPAddress = h.AddressList
 If aliases.Length > 0
 Console.WriteLine("Aliases for 0}", hostNames(i))
 For j = 0 To aliases.Length - 1
 Console.WriteLine("{0}", aliases(j))
 Next j
 End If
 If addresses.Length > 0
 Console.WriteLine("Addresses for {0}", hostNames(i))
 For k = 0 To addresses.Length - 1
 Console.WriteLine("{0}", addresses(k).ToString())
 Next k
 End If
 End If
 Next i
 End Sub
 ' The following method is called when each asynchronous operation completes.
 Shared Sub ProcessDnsInformation(result as IAsyncResult)
 Dim hostName as String = CType(result.AsyncState, String)
 hostNames.Add(hostName)
 Try
 ' Get the results.
 Dim host as IPHostEntry = Dns.EndGetHostEntry(result)
 hostData.Add(host)
 ' Store the exception message.
 Catch e as SocketException
 hostData.Add(e.Message)
 Finally
 ' Decrement the request counter in a thread-safe manner.
 Interlocked.Decrement(requestCounter)
 End Try
 End Sub
 End Class
End Namespace

See also

• Event-based Asynchronous Pattern (EAP)
• Event-based Asynchronous Pattern Overview
• Calling Asynchronous Methods Using IAsyncResult
• Using an AsyncCallback Delegate and State Object