Question

I am trying to asynchronously resolve an IP address to host name using C# and then add the result to a ListBox. The asynchronous operation works alright but i can not add the resolved name to a listbox because another thread owns it. How can i bypass this issue and add the resolved name to a listbox.

here is what i’ve done so far

public static ManualResetEvent GetHostEntryFinished = new ManualResetEvent(false);            

            private void AsyncDNSResolver(string IPString)
        {
        GetHostEntryFinished.Reset();

        IPHostEntry host = new IPHostEntry();
        host.AddressList = new IPAddress[] { IPAddress.Parse(IPString) };
        Dns.BeginGetHostEntry(host.AddressList[0], new AsyncCallback(GetHostEntryCallback), host);

        GetHostEntryFinished.WaitOne();

    }

    public static void GetHostEntryCallback(IAsyncResult ar)
    {
        IPHostEntry host = (IPHostEntry)ar.AsyncState;
        while (!ar.IsCompleted) ;
        host = Dns.EndGetHostEntry(ar);
       lsvHosts.items.add(host.HostName);
     GetHostEntryFinished.Set();
    } 

Answer 1

If you wanted to execute an expensive activity on a separate thread and continue working while waiting for the result you can use this code snippet.

static void Main(string[] args)
{
    Console.WriteLine("About to call DNS");
    ResolveDnsAndPerformWorkWhileWaiting("74.125.237.112");
    Console.Read();
}

public static void ResolveDnsAndPerformWorkWhileWaiting(string ipaddress)
{
    var task = new Task<string>(() =>
        {
            var host = new IPHostEntry { AddressList = new[] { IPAddress.Parse(ipaddress) } };
            var result = Dns.GetHostEntry(host.AddressList[0]);
            return result.HostName;
        });

    // execute the work on another thread
    task.Start();

    // do other stuff

    // use the results (if the task has not finished the .Result property will hold until completed)
    var resolvedAddress = task.Result;
    Console.WriteLine("Resolved address:" + resolvedAddress);
}

If you are using a Windows Forms application then the easiest way would be to use a BackgroundWorker, here is a code sample

static void Main(string[] args)
{
    Console.WriteLine("About to call DNS");

    var worker = new BackgroundWorker();
    worker.DoWork += worker_DoWork;
    worker.RunWorkerCompleted += worker_RunWorkerCompleted;
    worker.RunWorkerAsync("74.125.237.112");


    Console.Read();
}

static void worker_DoWork(object sender, DoWorkEventArgs e)
{
    var host = new IPHostEntry { AddressList = new[] { IPAddress.Parse((string)e.Argument) } };
    var result = Dns.GetHostEntry(host.AddressList[0]);

    e.Result = result.HostName;
}

static void worker_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
{
    // You could set your UI control here like label1.text = (string)e.Result
    Console.WriteLine((string)e.Result);
}