WebClient.CancelAsync() and TargetInvocationException

[deerchao]

I found a thread (http://groups.google.com/group/
microsoft.publi c.dotnet.langua ges.csharp/browse_thread/thread/
b154f44522b5e08 3/11974fe21507faa e?
lnk=gst&q=webcl ient.CancelAsyn c&rnum=1#11974f e21507faae) about this
issue, but it can't solve my problem.
I simply put a progressbar (to show the progress of the downloading),
a lable (to show what's downloading), and a button (to cancel the
download) on a form. If I click cancel button while downloading, there
comes a TargetInvocatio nException. How should I avoid it, or at least
catch it?

And here is the code:

DownloadProgres sForm f=new DownloadProgres sForm(url, message)
if(f.ShowDialgo ()==DialogResul t.OK)
{
...
}


public partial class DownloadProgres sForm : Form
{
private readonly string url;
private byte[] data;
private WebClient client;

public DownloadProgres sForm(string url, string message)
{
InitializeCompo nent();
lblStatus.Text = message;
this.url = url;
}

public byte[] Data
{
get { return data; }
}

private void DownloadProgres sForm_Load(obje ct sender,
EventArgs e)
{
client = new WebClient();
client.Download ProgressChanged += DownloadProgres sChanged;
client.Download DataCompleted += DownloadDataCom pleted;
client.Download DataAsync(new Uri(url));
}

void DownloadDataCom pleted(object sender,
DownloadDataCom pletedEventArgs e)
{
if (e.Cancelled)
{
DialogResult =
System.Windows. Forms.DialogRes ult.Cancel;
}
else
{
DialogResult = System.Windows. Forms.DialogRes ult.OK;
}
data = e.Result;
Close();
}

void DownloadProgres sChanged(object sender,
DownloadProgres sChangedEventAr gs e)
{
progressBar1.Va lue = (int)(e.BytesRe ceived * 100 /
e.TotalBytesToR eceive);
}

private void btnCancel_Click (object sender, EventArgs e)
{
client.CancelAs ync();
}
}

[deerchao]

Re: WebClient.Cance lAsync() and TargetInvocatio nException

I don't know what's better than Application.Thr eadException event to
catch this exception. But it works strangely: if in the event I pop a
MessageBox, the application continues to run, If I don't, it will quit
immediately! I believe it should be my decision to make whether or not
exit the application, but not Application or whatever.
What's wrong with my code?

static void Main()
{
Application.Ena bleVisualStyles ();
Application.Set CompatibleTextR enderingDefault (false);

Application.Thr eadException +=
Application_Thr eadException;
Application.Run (new MainForm());
}

static void Application_Thr eadException(ob ject sender,
ThreadException EventArgs e)
{
//logger.Error(e. Exception.Messa ge);
//if the statement below is not present, application will
exit at once!
//but if it's here, application continues to run.
MessageBox.Show (e.Exception.Me ssage);
}