Why does WebClient.DownloadStringTaskAsync() block ? - new async API/syntax/CTP

问题

For some reason there is a pause after the program below starts. I believe that WebClient().DownloadStringTaskAsync() is the cause.

class Program
{
    static void Main(string[] args)
    {
        AsyncReturnTask();

        for (int i = 0; i < 15; i++)
        {
            Console.WriteLine(i);
            Thread.Sleep(100);
        }
    }

    public static async void AsyncReturnTask()
    {
        var result = await DownloadAndReturnTaskStringAsync();
        Console.WriteLine(result);
    }

    private static async Task<string> DownloadAndReturnTaskStringAsync()
    {
        return await new WebClient().DownloadStringTaskAsync(new Uri("http://www.weather.gov"));
    }
}

As far as I understand my program should start counting from 0 to 15 immediately. Am I doing something wrong?

I had the same problem with the original Netflix download sample (which you get with CTP) - after pressing the search button the UI first freezes - and after some time it is responsive while loadning the next movies. And I believe it didn't freeze in Anders Hejlsberg's presentation at PDC 2010.

One more thing. When instead of

return await new WebClient().DownloadStringTaskAsync(new Uri("http://www.weather.gov"));

I use my own method:

return await ReturnOrdinaryTask();

Which is:

public static Task<string> ReturnOrdinaryTask()
{
    var t = Task.Factory.StartNew(() =>
    {
        for (int i = 0; i < 10; i++)
        {
            Console.WriteLine("------------- " + i.ToString());
            Thread.Sleep(100);
        }
        return "some text";
    });
    return t;
}

It works as it should. I mean it doesn't load anything, but it starts immediately and doesn't block the main thread, while doing its work.

Edit

OK, what I believe right now is: the WebClient.DownloadStringTaskAsync function is screwed up. It should work without the initial blocking period, like this:

    static void Main(string[] args)
    {
        WebClient cli = new WebClient();
        Task.Factory.StartNew(() =>
            {
                cli.DownloadStringCompleted += (sender, e) => Console.WriteLine(e.Result);
                cli.DownloadStringAsync(new Uri("http://www.weather.gov"));
            });

        for (int i = 0; i < 100; i++)
        {
            Console.WriteLine(i);
            Thread.Sleep(100);
        }
    }

回答1:

While your program does block for a while, it does resume execution in the for loop, before the result is returned from the remote server.

Remember that the new async API is still single-threaded. So WebClient().DownloadStringTaskAsync() still needs to run on your thread until the request has been prepared and sent to the server, before it can await and yield execution back to your program flow in Main().

I think the results you are seeing are due to the fact that it takes some time to create and send the request out from your machine. First when that has finished, the implementation of DownloadStringTaskAsync can wait for network IO and the remote server to complete, and can return execution to you.

On the other hand, your RunOrdinaryTask method just initializes a task and gives it a workload, and tells it to start. Then it returns immediately. That is why you don't see a delay when using RunOrdinaryTask.

Here are some links on the subject: Eric Lippert's blog (one of the language designers), as well as Jon Skeet's initial blog post about it. Eric has a series of 5 posts about continuation-passing style, which really is what async and await is really about. If you want to understand the new feature in detail, you might want to read Eric's posts about CPS and Async. Anyways, both links above does a good job on explaining a very important fact:

• Asynchronous != parallel

In other words, async and await does not spin up new threads for you. They just lets you resume execution of your normal flow, when you are doing a blocking operation - times where your CPU would just sit and do nothing in a synchronous program, waiting for some external operation to complete.

Edit

Just to be clear about what is happening: DownloadStringTaskAsync sets up a continuation, then calls WebClient.DownloadStringAsync, on the same thread, and then yields execution back to your code. Therefore, the blocking time you are seeing before the loop starts counting, is the time it takes DownloadStringAsync to complete. Your program with async and await is very close to be the equivalent of the following program, which exhibits the same behaviour as your program: An initial block, then counting starts, and somewhere in the middle, the async op finishes and prints the content from the requested URL:

    static void Main(string[] args)
    {
        WebClient cli = new WebClient();
        cli.DownloadStringCompleted += (sender, e) => Console.WriteLine(e.Result);
        cli.DownloadStringAsync(new Uri("http://www.weather.gov")); // Blocks until request has been prepared

        for (int i = 0; i < 15; i++)
        {
            Console.WriteLine(i);
            Thread.Sleep(100);
        }
    }

Note: I am by no means an expert on this subject, so I might be wrong on some points. Feel free to correct my understanding of the subject, if you think this is wrong - I just looked at the PDC presentation and played with the CTP last night.

回答2:

Are you sure the issue isn't related to the proxy configuration settings being detected from IE/Registry/Somewhere Slow?

Try setting webClient.Proxy = null (or specifying settings in app.config) and your "blocking" period should be minimal.

回答3:

Are you pressing F5 or CTLR+F5 to run it? With F5 there's a delay for VS just to search for the symbols for AsyncCtpLibrary.dll...

来源：https://stackoverflow.com/questions/4057926/why-does-webclient-downloadstringtaskasync-block-new-async-api-syntax-ctp