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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:

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. 

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
            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

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