Await in catch block

Question

I have the following code:

WebClient wc = new WebClient();
string result;
try
{
  result = await wc.DownloadStringTaskAsync( new Uri( "http://badurl" ) );
}
catch
{
  result = await wc.DownloadStringTaskAsync( new Uri( "http://fallbackurl" ) );
}

Basically I want to download from a URL and when it fails with an exception I want to download from another URL. Both time async of course. However the code does not compile, because of

error CS1985: Cannot await in the body of a catch clause

OK, it's forbidden for whatever reason but what's the correct code pattern here?

EDIT:

The good news is that C# 6.0 will likely allow await calls both in catch and finally blocks.

Solution 1

Update: C# 6.0 supports await in catch

---

Old Answer: You can rewrite that code to move the await from the catch block using a flag:

WebClient wc = new WebClient();
string result = null;
bool downloadSucceeded;
try
{
  result = await wc.DownloadStringTaskAsync( new Uri( "http://badurl" ) );
  downloadSucceeded = true;
}
catch
{
  downloadSucceeded = false;
}

if (!downloadSucceeded)
  result = await wc.DownloadStringTaskAsync( new Uri( "http://fallbackurl" ) );

Solution 2

Awaiting in a catch block is now possible as of the End User Preview of Roslyn as shown here (Listed under Await in catch/finally) and will be included in C# 6.

The example listed is

try … catch { await … } finally { await … }

Update: Added newer link, and that it will be in C# 6

Solution 3

This seems to work.

        WebClient wc = new WebClient();
        string result;
        Task<string> downloadTask = wc.DownloadStringTaskAsync(new Uri("http://badurl"));
        downloadTask = downloadTask.ContinueWith(
            t => {
                return wc.DownloadStringTaskAsync(new Uri("http://google.com/")).Result;
            }, TaskContinuationOptions.OnlyOnFaulted);
        result = await downloadTask;

Solution 4

Give this a try:

         try
        {
            await AsyncFunction(...);
        }

        catch(Exception ex)
        { 
            Utilities.LogExceptionToFile(ex).Wait();
            //instead of "await Utilities.LogExceptionToFile(ex);"
        }

(See the Wait() ending)

Solution 5

Use C# 6.0. see this Link

public async Task SubmitDataToServer()
{
  try
  {
    // Submit Data
  }
  catch
  {
    await LogExceptionAsync();
  }
  finally
  {
    await CloseConnectionAsync();
  }
}

View more solutions

Comments

[György Balássy]

Thanks svick, that's quite obvious, anything better, more connected to async?

[svick]

I don't think anything like that exists.

[Stephen Cleary]

In your case, you could also use task continuations. But the code in svick's answer is cleaner than code using continuations.

[Etienne Maheu]

If you even need to rethrow the exception without loosing the callstack, you can also use the System.Runtime.ExceptionServices.ExceptionDispatchInfo static class. Simply call ExceptionDispatchInfo.Capture(ex) in your catch block and store the return value, the captured exception, in a local variable. Once you're done with your async code, you can use capturedException.Throw() which will properly rethrow the original exception.

[Zia Ur Rahman]

awesome technique

[Tim Lovell-Smith]

And what VS version is that in? Answer: C# 6 is in the box in Visual Studio 2015 (and its preview).

[svick]

This makes the lambda async void, which should not be used, unless you have to.