It seems that GetResponseAsync does not accept cancellationToken in Async/Await. So the question is how can I cancel the below procedure, provided I need to collect Cookies from response:
using (HttpWebResponse response = (HttpWebResponse) await request.GetResponseAsync())
{
cookies.Add(response.Cookies);
}
An alternative code to achieve the above is also welcome.
Something like this should work (untested):
public static class Extensions
{
public static async Task<HttpWebResponse> GetResponseAsync(this HttpWebRequest request, CancellationToken ct)
{
using (ct.Register(() => request.Abort(), useSynchronizationContext: false))
{
var response = await request.GetResponseAsync();
ct.ThrowIfCancellationRequested();
return (HttpWebResponse)response;
}
}
}
In theory, if cancellation is requested on ct
and request.Abort
is invoked, await request.GetResponseAsync()
should throw a WebException
. IMO though, it's always a good idea to check for cancellation explicitly when consuming the result, to mitigate race conditions, so I call ct.ThrowIfCancellationRequested()
.
Also, I assume that request.Abort
is thread-safe (can be called from any thread), so I use useSynchronizationContext: false
(I haven't verified that).
[UPDATED] to address the OP's comment on how to differentiate between WebException
caused by cancellation and any other error. This is how it can be done, so TaskCanceledException
(derived from OperationCanceledException
) will be correctly thrown upon cancellation:
public static class Extensions
{
public static async Task<HttpWebResponse> GetResponseAsync(this HttpWebRequest request, CancellationToken ct)
{
using (ct.Register(() => request.Abort(), useSynchronizationContext: false))
{
try
{
var response = await request.GetResponseAsync();
return (HttpWebResponse)response;
}
catch (WebException ex)
{
// WebException is thrown when request.Abort() is called,
// but there may be many other reasons,
// propagate the WebException to the caller correctly
if (ct.IsCancellationRequested)
{
// the WebException will be available as Exception.InnerException
throw new OperationCanceledException(ex.Message, ex, ct);
}
// cancellation hasn't been requested, rethrow the original WebException
throw;
}
}
}
}
public static async Task<T> WithCancellation<T>(this Task<T> task, CancellationToken cancellationToken, Action action, bool useSynchronizationContext = true)
{
using (cancellationToken.Register(action, useSynchronizationContext))
{
try
{
return await task;
}
catch (Exception ex)
{
if (cancellationToken.IsCancellationRequested)
{
// the Exception will be available as Exception.InnerException
throw new OperationCanceledException(ex.Message, ex, cancellationToken);
}
// cancellation hasn't been requested, rethrow the original Exception
throw;
}
}
}
Now you can use your cancellation token on any cancelable async method. For example WebRequest.GetResponseAsync:
var request = (HttpWebRequest)WebRequest.Create(url);
using (var response = await request.GetResponseAsync())
{
. . .
}
will become:
var request = (HttpWebRequest)WebRequest.Create(url);
using (WebResponse response = await request.GetResponseAsync().WithCancellation(CancellationToken.None, request.Abort, true))
{
. . .
}
See example http://pastebin.com/KauKE0rW
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