ASP.NET MVC AsyncController Exception Handling - asp.net-mvc

Question

On ASP.NET MVC, I try to write an async Controller action with the old asynchronous programming model (actually, it is the current one, new one is still a CTP).
Here, I am trying to run 4 operations in parallel and it worked great. Here is the complete code:
public class SampleController : AsyncController {
public void IndexAsync() {
AsyncManager.OutstandingOperations.Increment(4);
var task1 = Task<string>.Factory.StartNew(() => {
return GetReponse1();
});
var task2 = Task<string>.Factory.StartNew(() => {
return GetResponse2();
});
var task3 = Task<string>.Factory.StartNew(() => {
return GetResponse3();
});
var task4 = Task<string>.Factory.StartNew(() => {
return GetResponse4();
});
task1.ContinueWith(t => {
AsyncManager.Parameters["headers1"] = t.Result;
AsyncManager.OutstandingOperations.Decrement();
});
task2.ContinueWith(t => {
AsyncManager.Parameters["headers2"] = t.Result;
AsyncManager.OutstandingOperations.Decrement();
});
task3.ContinueWith(t => {
AsyncManager.Parameters["headers3"] = t.Result;
AsyncManager.OutstandingOperations.Decrement();
});
task4.ContinueWith(t => {
AsyncManager.Parameters["headers4"] = t.Result;
AsyncManager.OutstandingOperations.Decrement();
});
task3.ContinueWith(t => {
AsyncManager.OutstandingOperations.Decrement();
}, TaskContinuationOptions.OnlyOnFaulted);
}
public ActionResult IndexCompleted(string headers1, string headers2, string headers3, string headers4) {
ViewBag.Headers = string.Join("<br/><br/>", headers1, headers2, headers3, headers4);
return View();
}
public ActionResult Index2() {
ViewBag.Headers = string.Join("<br/><br/>", GetReponse1(), GetResponse2(), GetResponse3(), GetResponse4());
return View();
}
}
And these below ones are the methods that the async operations are running:
string GetReponse1() {
var req = (HttpWebRequest)WebRequest.Create("http://www.twitter.com");
req.Method = "HEAD";
var resp = (HttpWebResponse)req.GetResponse();
return FormatHeaders(resp.Headers);
}
string GetResponse2() {
var req2 = (HttpWebRequest)WebRequest.Create("http://stackoverflow.com");
req2.Method = "HEAD";
var resp2 = (HttpWebResponse)req2.GetResponse();
return FormatHeaders(resp2.Headers);
}
string GetResponse3() {
var req = (HttpWebRequest)WebRequest.Create("http://google.com");
req.Method = "HEAD";
var resp = (HttpWebResponse)req.GetResponse();
return FormatHeaders(resp.Headers);
}
string GetResponse4() {
var req = (HttpWebRequest)WebRequest.Create("http://github.com");
req.Method = "HEAD";
var resp = (HttpWebResponse)req.GetResponse();
return FormatHeaders(resp.Headers);
}
private static string FormatHeaders(WebHeaderCollection headers) {
var headerStrings = from header in headers.Keys.Cast<string>()
select string.Format("{0}: {1}", header, headers[header]);
return string.Join("<br />", headerStrings.ToArray());
}
I have also Index2 method here which is synchronous and does the same thing.
I compare two operation execution times and there is major difference (approx. 2 seconds)
But I think I am missing lots of things here (exception handling, timeouts, etc). I only implement the exception handling on task3 but I don't think it is the right way of doing that.
What is the healthiest way of handling exceptions for this kind of operations?

Answer 1

Before accessing the result of a task you should test whether it completed successfully. If there was an error you should not try to access the results but instead do something with the exception:
task1.ContinueWith(t =>
{
if (!t.IsFaulted)
{
AsyncManager.Parameters["headers1"] = t.Result;
}
else if (t.IsFaulted && t.Exception != null)
{
AsyncManager.Parameters["error"] = t.Exception;
}
AsyncManager.OutstandingOperations.Decrement();
});