// Because SyncContext is set, all the iterator code runs on the GUI thread
private IEnumerator<Int32> GetWebData(AsyncEnumerator ae, String[] uris)
{
ToggleStartAndCancelButtonState(false);
m_lbResults.Items.Clear();
// Auto-cancel after 5 seconds if the user desires
if (m_chkAutoCancel.Checked)
ae.SetCancelTimeout(5000, ae);
// Issue several web requests (all in discard group 0) simultaneously
foreach (String uri in uris) {
WebRequest webRequest = WebRequest.Create(uri);
// If the AsyncEnumerator is canceled, DiscardWebRequest cleans-up
// any outstanding operations as they complete in the future
webRequest.BeginGetResponse(ae.EndVoid(0, DiscardWebRequest), webRequest);
}
yield return uris.Length; // Process the completed web requests after all complete
String resultStatus; // Ultimate result of processing shown to user
// Check if iterator was canceled
Object cancelValue;
if (ae.IsCanceled(out cancelValue)) {
// Tell the AE to auto-cleanup any operations issued as part of discard group 0
ae.DiscardGroup(0);
// Note: In this example calling DiscardGroup above is not mandatory
// because the whole iterator is stopping execution; causing all
// discard groups to be discarded automatically.
resultStatus = (cancelValue == ae) ? "Timeout" : "User canceled";
goto Complete;
}
// Iterator wasn't canceled, process all the completed operations
for (Int32 n = 0; n < uris.Length; n++) {
// Grab the result of a completed web request
IAsyncResult result = ae.DequeueAsyncResult();
// Get the WebRequest object used to initate the request
// (see BeginGetResponse's last argument above)
WebRequest webRequest = (WebRequest)result.AsyncState;
// Build the String showing the result of this completed web request
String s = "URI=" + webRequest.RequestUri + ", ";
try {
using (WebResponse webResponse = webRequest.EndGetResponse(result)) {
s += "ContentLength=" + webResponse.ContentLength;
}
}
catch (WebException e) {
s += "Error=" + e.Message;
}
m_lbResults.Items.Add(s); // Add result of operation to listbox
}
resultStatus = "All operations completed.";
Complete:
// All operations have completed or cancelation occurred, tell user
MessageBox.Show(this, resultStatus);
// Reset everything so that the user can start over if they desire
m_ae = null; // Reset since we're done
ToggleStartAndCancelButtonState(true);
}