FileStream WriteAsync and await confusion
.net, async-await, c#, filestream
Solution
I suspect the problem is with your understanding of what `await` does.
But if you do this, does it not wait for the call to finish? This makes it not very asynchronous...
When you `await` something, you "block asynchronously" - your asynchronous function will continue when the asynchronous operation has completed, but the immediate call will complete immediately.
So in your case, the UI thread becomes unblocked because `Button_Click` returns, but when the file write operation has completed, execution will return (still in the UI thread) to the end of the `await` expression... you'll then close the `FileStream` and complete the asynchronous function.
It would be clearer what was going on if you did something else after the `await` expression (e.g. updating the UI). But yes, it really is asynchronous - it just allows you to write code which looks synchronous.
Problem
I'm looking at the new `async` and `await` keywords in C# and just trying to get a feel for them. I'm looking at the MSDN `FileStream.WriteAsync()` example, and I wasn't sure I understood something. The example is as follows: ``` using System; using System.Text; using System.Threading.Tasks; using System.Windows; using System.Windows.Controls; using System.IO; namespace WpfApplication1 { public partial class MainWindow : Window { public MainWindow() { InitializeComponent(); } private async void Button_Click(object sender, RoutedEventArgs e) { UnicodeEncoding uniencoding = new UnicodeEncoding(); string filename = @"c:\Users\exampleuser\Documents\userinputlog.txt"; byte[] result = uniencoding.GetBytes(UserInput.Text); using (FileStream SourceStream = File.Open(filename, FileMode.OpenOrCreate)) { SourceStream.Seek(0, SeekOrigin.End); await SourceStream.WriteAsync(result, 0, result.Length); } } } } ``` What I don't get is the placement of the `await`. It seems that you can't take the `Task` item that is returned by `WriteAsync()` because this always gives me a syntax error. So the only way I could get this to compile is by leaving the `await` with the call. But if you do this, does it not wait for the call to finish? This makes it not very asynchronous...