使用异步调用时异常处理的良好模式

Question

I want to consume an Web API and I see many people recommending System.Net.Http.HttpClient .

That's fine... but I have only VS-2010, so I cannot use async/await just yet. Instead, I guess I could use Task<TResult> in combination to ContinueWith . So I tried this piece of code:

var client = new HttpClient();
client.DefaultRequestHeaders.Accept.Add(
    new MediaTypeWithQualityHeaderValue("application/json"));

client.GetStringAsync(STR_URL_SERVER_API_USERS).ContinueWith(task =>
{                 
   var usersResultString = task.Result;
   lbUsers.DataSource = JsonConvert.DeserializeObject<List<string>>(usersResultString);
});

My first observation was to realize that it doesn't generate any error if URL is not available, but maybe there will be more errors like this...

So I am trying to find a way to handle exceptions for such async calls (particularly for HttpClient). I noticed that "Task" has IsFaulted property and an AggregateException which maybe could be used, but I am not sure yet how.

Another observation was that GetStringAsync returns Task<string> , but GetAsync returns Task<HttpResponseMessage> . The latter could be maybe more useful, since it presents a StatusCode .

Could you share a pattern on how to use the async calls correctly and handle exceptions in a good way? Basic explanation would be appreciated as well.

Answer 1

I would not use a separate ContinueWith continuation for successful and faulted scenarios. I'd rather handle both cases in a single place, using try/catch :

task.ContinueWith(t =>
   {
      try
      {
          // this would re-throw an exception from task, if any
          var result = t.Result;
          // process result
          lbUsers.DataSource = JsonConvert.DeserializeObject<List<string>>(result);
      }
      catch (Exception ex)
      {
          MessageBox.Show(ex.Message);
          lbUsers.Clear();
          lbUsers.Items.Add("Error loading users!");
      }
   }, 
   CancellationToken.None, 
   TaskContinuationOptions.None, 
   TaskScheduler.FromCurrentSynchronizationContext()
);

If t is a non-generic Task (rather than a Task<TResult> ), you can do t.GetAwaiter().GetResult() to re-throw the original exception inside the ContinueWith lambda; t.Wait() would work too. Be prepared to handle AggregatedException , you can get to the inner exception with something like this:

catch (Exception ex)
{
    while (ex is AggregatedException && ex.InnerException != null)
        ex = ex.InnerException;

    MessageBox.Show(ex.Message);
}

If you're dealing with a series of ContinueWith , usually you don't have to handle exceptions inside each ContinueWith . Do it once for the outermost resulting task, eg:

void GetThreePagesV1()
{
    var httpClient = new HttpClient();
    var finalTask = httpClient.GetStringAsync("http://example.com")
        .ContinueWith((task1) =>
            {
                var page1 = task1.Result;
                return httpClient.GetStringAsync("http://example.net")
                    .ContinueWith((task2) =>
                        {
                            var page2 = task2.Result;
                            return httpClient.GetStringAsync("http://example.org")
                                .ContinueWith((task3) =>
                                    {
                                        var page3 = task3.Result;
                                        return page1 + page2 + page3;
                                    }, TaskContinuationOptions.ExecuteSynchronously);
                        }, TaskContinuationOptions.ExecuteSynchronously).Unwrap();
            }, TaskContinuationOptions.ExecuteSynchronously).Unwrap()
        .ContinueWith((resultTask) =>
            {
                httpClient.Dispose();
                string result = resultTask.Result;
                try
                {
                    MessageBox.Show(result);
                }
                catch (Exception ex)
                {
                    MessageBox.Show(ex.Message);
                }
            },
            CancellationToken.None,
            TaskContinuationOptions.None,
            TaskScheduler.FromCurrentSynchronizationContext());
}

Any exceptions thrown inside inner tasks will propagate to the outermost ContinueWith lambda as you're accessing the results of the inner tasks ( taskN.Result ).

This code is functional, but it's also ugly and non-readable. JavaScript developers call it The Callback Pyramid of Doom . They have Promises to deal with it. C# developers have async/await , which you're unfortunately not able to use because of the VS2010 restrain.

IMO, the closest thing to the JavaScript Promises in TPL is Stephen Toub's Then pattern . And the closest thing to async/await in C# 4.0 is his Iterate pattern from the same blog post, which uses the C# yield feature.

Using the Iterate pattern, the above code could be rewritten in a more readable way. Note that inside GetThreePagesHelper you can use all the familiar synchronous code statements like using , for , while , try/catch etc. It is however important to understand the asynchronous code flow of this pattern:

void GetThreePagesV2()
{
    Iterate(GetThreePagesHelper()).ContinueWith((iteratorTask) =>
        {
            try
            {
                var lastTask = (Task<string>)iteratorTask.Result;

                var result = lastTask.Result;
                MessageBox.Show(result);
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
                throw;
            }
        },
        CancellationToken.None,
        TaskContinuationOptions.None,
        TaskScheduler.FromCurrentSynchronizationContext());
}

IEnumerable<Task> GetThreePagesHelper()
{
    // now you can use "foreach", "using" etc
    using (var httpClient = new HttpClient())
    {
        var task1 = httpClient.GetStringAsync("http://example.com");
        yield return task1;
        var page1 = task1.Result;

        var task2 = httpClient.GetStringAsync("http://example.net");
        yield return task2;
        var page2 = task2.Result;

        var task3 = httpClient.GetStringAsync("http://example.org");
        yield return task3;
        var page3 = task3.Result;

        yield return Task.Delay(1000);

        var resultTcs = new TaskCompletionSource<string>();
        resultTcs.SetResult(page1 + page1 + page3);
        yield return resultTcs.Task;
    }
}

/// <summary>
/// A slightly modified version of Iterate from 
/// http://blogs.msdn.com/b/pfxteam/archive/2010/11/21/10094564.aspx
/// </summary>
public static Task<Task> Iterate(IEnumerable<Task> asyncIterator)
{
    if (asyncIterator == null)
        throw new ArgumentNullException("asyncIterator");

    var enumerator = asyncIterator.GetEnumerator();
    if (enumerator == null)
        throw new InvalidOperationException("asyncIterator.GetEnumerator");

    var tcs = new TaskCompletionSource<Task>();

    Action<Task> nextStep = null;
    nextStep = (previousTask) =>
    {
        if (previousTask != null && previousTask.Exception != null)
            tcs.SetException(previousTask.Exception);

        if (enumerator.MoveNext())
        {
            enumerator.Current.ContinueWith(nextStep,
                TaskContinuationOptions.ExecuteSynchronously);
        }
        else
        {
            tcs.SetResult(previousTask);
        }
    };

    nextStep(null);
    return tcs.Task;
}