Why doesn't the async method return immediately?
I thought this is a sane pattern that can be used to call WebClient DownloadData asynchronously in an event handler:
private async void button1_Click(object sender, EventArgs e)
{
WebClient wc = new WebClient();
//wc.Proxy = null;
byte[] bytes = await wc.DownloadDataTaskAsync("http://www.google.com");
label1.Text = Encoding.ASCII.GetString(bytes);
}
But I can find DownloadDataTaskAsync blocks for about 5 seconds before returning (unless the expression wc.Proxy = null is met). What's the point of the expected method if it can, on a whim, do non-trivial work before even returning the task?
Presumably this means I have to be safe. I should never call the xAsync methods like above, but instead should always complete them in Task.Run () to be sure. Or not?
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This is a known issue with WebClient
/ HttpWebRequest
: proxies and DNS queries are always done synchronously. This is a bug, but Microsoft has chosen not to fix it for backward compatibility reasons.
First of all, I recommend using HttpClient
. If that doesn't work and you want async, you can end the call in Task.Run
.
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It turns out WebClient.DownloadDataTaskAsync is calling HttpWebRequest.BeginGetResponse
MSDN points out:
The BeginGetResponse method requires some synchronous configuration tasks to complete (DNS resolution, proxy discovery, and TCP socket connections, for example) before this method becomes asynchronous. As a result, this method should never be called on the user interface (UI) thread because it can take a significant amount of time (up to several minutes depending on network settings) to complete the initial synchronous configuration task before an error exception is thrown or the method succeeds.
http://msdn.microsoft.com/en-us/library/system.net.httpwebrequest.begingetresponse(v=vs.110).aspx
Unfortunately, the MSDN documentation for WebClient.DownloadDataTaskAsync says:
This operation will not be blocked.
which doesn't seem to be strict.
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