So I am trying to learn the basics of using 'async' and 'await' in C#, but I am not sure what I am doing wrong here. I am expecting the following output:
Calling DoDownload DoDownload done [...output here...]
But I don't get the output of the download, and I also expect "done" but that takes a while. Shouldn't that be output immediately? Also, I can't seem to get the string result either. Here is my code:
namespace AsyncTest { class Program { static void Main(string[] args) { Debug.WriteLine("Calling DoDownload"); DoDownloadAsync(); Debug.WriteLine("DoDownload done"); } private static async void DoDownloadAsync() { WebClient w = new WebClient(); string txt = await w.DownloadStringTaskAsync("http://www.google.com/"); Debug.WriteLine(txt); } } }
This question is related to
c#
async-await
c#-5.0
To get the behavior you want you need to wait for the process to finish before you exit Main()
. To be able to tell when your process is done you need to return a Task
instead of a void
from your function, you should never return void
from a async
function unless you are working with events.
A re-written version of your program that works correctly would be
class Program { static void Main(string[] args) { Debug.WriteLine("Calling DoDownload"); var downloadTask = DoDownloadAsync(); Debug.WriteLine("DoDownload done"); downloadTask.Wait(); //Waits for the background task to complete before finishing. } private static async Task DoDownloadAsync() { WebClient w = new WebClient(); string txt = await w.DownloadStringTaskAsync("http://www.google.com/"); Debug.WriteLine(txt); } }
Because you can not await
in Main()
I had to do the Wait()
function instead. If this was a application that had a SynchronizationContext I would do await downloadTask;
instead and make the function this was being called from async
.
You are calling DoDownloadAsync() but you don't wait it. So your program going to the next line. But there is another problem, Async methods should return Task
or Task<T>
, if you return nothing and you want your method will be run asyncronously you should define your method like this:
private static async Task DoDownloadAsync() { WebClient w = new WebClient(); string txt = await w.DownloadStringTaskAsync("http://www.google.com/"); Debug.WriteLine(txt); }
And in Main method you can't await for DoDownloadAsync
, because you can't use await keyword in non-async function, and you can't make Main
async. So consider this:
var result = DoDownloadAsync(); Debug.WriteLine("DoDownload done"); result.Wait();
Source: Stackoverflow.com