未设置断点时在单元测试期间引发异常

Question

I could not figure out why the exception is thrown when I don't have a breakpoint during debugging for my unit test. However, when I debug with breakpoint set at the following line, it runs okay.

Here is my code.

MainPageViewModel.cs

public class MainPageViewModel
{
    public async Task<IEnumerable<Event>> GetAllEvents()
    {
        WebClient client = new WebClient();
        string xmlDoc = await FXNewsAlert.Misc.Extensions.DownloadStringTask(client, new Uri("http://www.forexfactory.com/ffcal_week_this.xml")); //breakpoint here

        var events = XDocument.Parse(xmlDoc)
            .Descendants("event")
            .Select(n => new Event
            {
                Title = n.Element("title").Value,
                Country = n.Element("country").Value,
                DateTime = this.ToLocalTime(String.Format("{0} {1}", n.Element("date").Value, n.Element("time").Value)),
                Impact = n.Element("forecast").Value,
                Forecast = n.Element("forecast").Value,
                Previous = n.Element("previous").Value
            }).ToList();

        return events;
    }
}

EventsTest.cs

[TestClass]
public class EventsTest : WorkItemTest
{
    [TestMethod]
    [Asynchronous]
    public async void GetAllEventsTest()
    {
        MainPageViewModel vm = new MainPageViewModel();
        IEnumerable<Event> events = await vm.GetAllEvents(); //WebException is thrown here.

        Assert.IsTrue(events.Count() > 0);
        EnqueueTestComplete();
    }
}

My guess is that somehow it takes time to load the url, so by having a breakpoint it gives that extra time to so the xml I'm downloading could be completed on time.

But it is actually an async method, so it doesn't matter right? I'm pretty much lost now. Any help would be greatly appreciated.

Edit: Implementation for DownloadStringTask

    public static Task<string> DownloadStringTask(this WebClient webClient, Uri uri)
    {
        var tcs = new TaskCompletionSource<string>();

        webClient.DownloadStringCompleted += (s, e) =>
        {
            if (e.Error != null)
            {
                tcs.SetException(e.Error);
            }
            else
            {
                tcs.SetResult(e.Result);
            }
        };

        webClient.DownloadStringAsync(uri);

        return tcs.Task;
    }

StackTrace for the System.Net.WebException

at System.Net.Browser.AsyncHelper.BeginOnUI(SendOrPostCallback beginMethod, Object state) at System.Net.Browser.ClientHttpWebRequest.EndGetResponse(IAsyncResult asyncResult) at System.Net.WebClient.GetWebResponse(WebRequest request, IAsyncResult result) at System.Net.WebClient.DownloadBitsResponseCallback(IAsyncResult result) --- End of stack trace from previous location where exception was thrown --- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at System.Runtime.CompilerServices.TaskAwaiter 1.GetResult() at FXNewsAlert.Resources.MainPageViewModel.<GetAllEvents>d__2.MoveNext() --- End of stack trace from previous location where exception was thrown --- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at System.Runtime.CompilerServices.TaskAwaiter 1.GetResult() at FXNewsAlert.Resources.MainPageViewModel.<GetAllEvents>d__2.MoveNext() --- End of stack trace from previous location where exception was thrown --- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at System.Runtime.CompilerServices.TaskAwaiter 1.GetResult() at FXNewsAlertTest.UnitTests.EventsTest.d__0.MoveNext()

Answer 1

I have a feeling that something is going wrong in FXNewsAlert, the implementation of which isn't presented above. I recommend digging into (or replacing) FXNewsAlert.Misc.Extensions.DownloadStringTask() . My guess is that it's yielding before the its completed its download.

In order to test this code, I first created an Event class to contain the data and minimize changes to your code:

public class Event
{
    public string Title { get; set; }
    public string Country { get; set; }
    public DateTime DateTime { get; set; }
    public string Impact { get; set; }
    public string Forecast { get; set; }
    public string Previous { get; set; }
}

I then slightly modified your code to usr the WebClient's DownloadStringTaskAsync() method:

public class MainPageViewModel
{
    public async Task<IEnumerable<Event>> GetAllEvents()
    {
        WebClient client = new WebClient();

        // HACK: Don't have access to the implementation to FXNewsAlert...
        //string xmlDoc = await FXNewsAlert.Misc.Extensions.DownloadStringTask(
        //    client, new Uri("http://www.forexfactory.com/ffcal_week_this.xml")); //breakpoint here

        string xmlDoc = await client.DownloadStringTaskAsync(
            new Uri("http://www.forexfactory.com/ffcal_week_this.xml")); //breakpoint here

        var events = XDocument.Parse(xmlDoc)
            .Descendants("event")
            .Select(n => new Event
            {
                Title = n.Element("title").Value,
                Country = n.Element("country").Value,
                DateTime = DateTime.Parse(String.Format("{0} {1}", n.Element("date").Value, n.Element("time").Value)),
                Impact = n.Element("forecast").Value,
                Forecast = n.Element("forecast").Value,
                Previous = n.Element("previous").Value
            }).ToList();

        return events;
    }
}

Rather than create a test project, I whipped up the following console app:

class Program
{
    static void Main(string[] args)
    {
        var vm = new MainPageViewModel();
        var task = vm.GetAllEvents();
        task.Wait();
        foreach (Event e in task.Result)
        {
            Console.WriteLine("{0} [{1}]", e.Title, e.DateTime);
            Console.WriteLine("Forecast: {0} (Previous: {1})", e.Forecast, e.Previous);
            Console.WriteLine();
        }
    }
}

This code executes fine with and without a breakpoint.

Answer 2

Change your test method from async void to async Task .

This won't clear up the WebException , but it should make it consistent so you can debug it.