Blocking Application Execution by Ending an Asynchronous Operation

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:

• Call the asynchronous operations EndOperationName method. This approach is demonstrated in this topic.

• Use the AsyncWaitHandle property of the IAsyncResult returned by the asynchronous operation's BeginOperationName method. For an example that demonstrates this approach, see Blocking Application Execution Using an AsyncWaitHandle.

Applications that use the EndOperationName method 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 call EndOperationName.

Example

The following code example demonstrates using asynchronous methods in the Dns class to retrieve Domain Name System information for a user-specified computer. Note that null (Nothing in Visual Basic) is passed for the BeginGetHostByName requestCallback and stateObject parameters because these arguments 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.

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

Namespace Examples.AdvancedProgramming.AsynchronousOperations
    Public Class BlockUntilOperationCompletes
        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.
            ' This example does not use a delegate or user-supplied object
            ' so the last two arguments are Nothing.
            Dim result as IAsyncResult = Dns.BeginGetHostEntry(args(0), Nothing, Nothing)
            Console.WriteLine("Processing your request for information...")
            ' Do any additional work that can be done here.
            Try 
                ' EndGetHostByName blocks until the process completes.
                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

/*
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;

namespace Examples.AdvancedProgramming.AsynchronousOperations
{
    public class BlockUntilOperationCompletes
    {
        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.
            // This example does not use a delegate or user-supplied object
            // so the last two arguments are null.
            IAsyncResult result = Dns.BeginGetHostEntry(args[0], null, null);
            Console.WriteLine("Processing your request for information...");
            // Do any additional work that can be done here.
            try 
            {
                // EndGetHostByName blocks until the process completes.
                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("An exception occurred while processing the request: {0}", e.Message);
            }
        }
    }
}

See Also

Concepts

Event-based Asynchronous Pattern Overview

Other Resources

Asynchronous Programming Design Patterns