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AsyncCallback Delegate

References a method to be called when a corresponding asynchronous operation completes.

Namespace:  System
Assembly:  mscorlib (in mscorlib.dll)

<SerializableAttribute> _
<ComVisibleAttribute(True)> _
Public Delegate Sub AsyncCallback ( _
	ar As IAsyncResult _
Dim instance As New AsyncCallback(AddressOf HandlerMethod)


Type: System.IAsyncResult

The result of the asynchronous operation.

Use an AsyncCallback delegate to process the results of an asynchronous operation in a separate thread. The AsyncCallback delegate represents a callback method that is called when the asynchronous operation completes. The callback method takes an IAsyncResult parameter, which is subsequently used to obtain the results of the asynchronous operation.

For more information about asynchronous programming, see Using an AsyncCallback Delegate to End an Asynchronous Operation and Using an AsyncCallback Delegate and State Object in Asynchronous Programming Design Patterns.

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.

'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  

Imports System
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
                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.
                    ' 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


            ' 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)
                    ' 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
                        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)
                ' Get the results. 
                Dim host as IPHostEntry = Dns.EndGetHostEntry(result)
            ' Store the exception message. 
            Catch e as SocketException
                ' Decrement the request counter in a thread-safe manner.
            End Try 
        End Sub 
    End Class 
End Namespace

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5, 3.0, 2.0, 1.1, 1.0

.NET Compact Framework

Supported in: 3.5, 2.0, 1.0

XNA Framework

Supported in: 3.0, 2.0, 1.0