UI is freezed for 5 seconds when async button is clicked at the first time

Question

is it okay when UI is freezed for 5 seconds, when a button is clicked at first time? I mean when I click the button to load content from website, then I cannot move the window for 5 seconds.

My code looks like this:

private async void Button_Click(object sender, RoutedEventArgs e)
{            
    var html = await GetHtmlAsync("http://msdn.microsoft.com"); 
    MessageBox.Show(html.Substring(0, 10));
}


public async Task<string> GetHtmlAsync(string url)
{
    var webClient = new WebClient();
    return await webClient.DownloadStringTaskAsync(url);
}

However, if I click the above button the second time or click other async buttons, then there is no UI freezing.

So is it okay when UI is freezed for 5 seconds, when a button is clicked at first time?

Be more precise. It is ok for the results to be delayed, not for the UI to freeze. How do you establish that it is frozen? — H H, Jun 10 '18 at 14:43
Try to change the `async` call to `var html = await Task.Run(AsyncFoo);` ? — Fabjan, Jun 10 '18 at 14:43
@HenkHolterman I mean when I click the button to load content from website, then I cannot move the window for 5 seconds. — StepUp, Jun 10 '18 at 14:48
@Fabjan yeah, if I use `var html = await Task.Run(AsyncFoo);` then there is no UI freezing. Is it possible to achieve the same result without using `Task.Run()`? — StepUp, Jun 10 '18 at 14:52
It shouldn't happen - make sure this is an mcve. That means, create a scrap project with _only_ this code and see if it demonstrates the problem. We should not be looking at a fragment of a much larger program. — H H, Jun 10 '18 at 14:57
@Fabjan - that is a horrible brute force approach. Maybe it will be needed but the OP should look for other options first. — H H, Jun 10 '18 at 14:59
@HenkHolterman By all means I'm not suggesting it as a solution. It's merely a small test... It's weird though that `DownloadStringTaskAsync` is blocking the thread. — Fabjan, Jun 10 '18 at 15:00
It shouldn't. And therefore I'm not ruling out `new WebClient`, that is a strange class. — H H, Jun 10 '18 at 15:01
@StepUp As an aside: you could remove `async` keyword from method signature and return a `Task` object directly - `return webClient.DownloadStringTaskAsync(url);` — Fabjan, Jun 10 '18 at 15:03
@HenkHolterman I see that it should work okay. However, I've created a scrap project with only this code and problem is always occurred, but only when I click the button at the first time. — StepUp, Jun 10 '18 at 15:04
@StepUp Maybe you've some stuff not related to this in the UI that is initializing and blocking the thread ? — Fabjan, Jun 10 '18 at 15:05
OK, try creating a single WebCLient in the constructor or OnLoad. Just a hunch. — H H, Jun 10 '18 at 15:05
@HenkHolterman yeah, I've created a single WebCLient in the constructor, but there is still some UI freezing for 5 seconds. — StepUp, Jun 10 '18 at 15:08
@Fabjan `As an aside: you could remove async keyword from method signature and return a Task object directly - return webClient.DownloadStringTaskAsync(url);` I've tried it, but there is still some UI freezing for 5 seconds. — StepUp, Jun 10 '18 at 15:11
Yes, 'eliding async' is a small optimization but it couldn't have helped here. — H H, Jun 10 '18 at 15:13
It is not the async mechanism that is blocking you here, it is WebClient and/or your network layer. — H H, Jun 10 '18 at 15:14
See for instance [this question](https://stackoverflow.com/q/10568321/60761), I think you have the same problem Also worth a try: HttpClient. Use it as a singleton. — H H, Jun 10 '18 at 15:16
When it freezes, press pause in the debugger.... what's it doing? — MineR, Jun 10 '18 at 16:52
@HenkHolterman please, write your reply to use `HttpClient`, and I'll mark as answer. You are right. Now UI is not freezed when I click at the button at the first time. — StepUp, Jun 10 '18 at 18:50
You can post a self-answer... I was just guessing. Maybe somenody comes along to tell us why HttpClient does better. — H H, Jun 10 '18 at 20:02

paparazzo · Answer 1 · 2018-06-10T17:38:32.727

1

There could be some caching. Try another URL each call. But with await it should not be blocking.

I tested and I have no lag. I would start a fresh client. Something strange is going on.

Put in some Stopwatch Debug to try and figure where the lag is.

private async void btnURLclick(object sender, RoutedEventArgs e)
{
    Stopwatch sw = new Stopwatch();
    sw.Start();
    var html = await GetHtmlAsync("http://msdn.microsoft.com");
    Debug.WriteLine($"btn {sw.ElapsedMilliseconds}");
    MessageBox.Show(html.Substring(0, 10));

}
public async Task<string> GetHtmlAsync(string url)
{
    Stopwatch sw = new Stopwatch();
    sw.Start();
    var webClient = new WebClient();
    Debug.WriteLine($"taskA {sw.ElapsedMilliseconds}");
    return await webClient.DownloadStringTaskAsync(url);
}

edited Jun 10 '18 at 17:38

answered Jun 10 '18 at 16:14

paparazzo

44,497
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This will not tell you what part is freezing. – H H Jun 10 '18 at 16:44
The OP has already established that it is the `DownloadStringTaskAsync()` call... We have eliminated `new WebClient()` , that wasn't to likely anyway. – H H Jun 10 '18 at 16:53
@HenkHolterman I am not seeing where that is established. It is not going to hurt to get some numbers. Moving on. – paparazzo Jun 10 '18 at 17:02

score 1 · Accepted Answer · answered Jun 10 '18 at 22:47

The HttpClient is the way to go but let's look at code.

Pulling the code for the corefx source. If you're actually getting a HttpWebRespone this snippet may point to the issue your seeing.

public override Stream GetResponseStream()
{
    CheckDisposed();
    return _httpResponseMessage.Content.ReadAsStreamAsync().GetAwaiter().GetResult();
}

score -1 · Answer 3 · edited Jun 10 '18 at 20:00

-1

That's probably due to some communications initialization. Try this:

public async Task<string> GetHtmlAsync(string url)
{
    await Task.Delay(1).ConfigureAwait(false);
    var webClient = new WebClient();
    return await webClient.DownloadStringTaskAsync(url).ConfigureAwait(false);
}

edited Jun 10 '18 at 20:00

H H

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answered Jun 10 '18 at 18:51

Paulo Morgado

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Then better use `Task.Yield`, or directly `Task.Run` – Kevin Gosse Jun 10 '18 at 19:02
@KevinGosse, can you use `ConfigureAwait` with `Task.Yield`? – Paulo Morgado Jun 10 '18 at 23:18

UI is freezed for 5 seconds when async button is clicked at the first time

3 Answers3