Unobserved Task exceptions in .NET 4.5 still crash app

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You do have a Task with an exception (the one returned by GetStringAsync). However, the await is observing the Task exception, which then propagates out of the DownloadAsync method (which is async void).

Exceptions propagating out of async void methods behave differently; they are raised on the SynchronizationContext that was active when the async void method started (in this case, a thread pool SynchronizationContext). This is not considered an unobserved exception.

If you change DownloadAsync to return Task, then you will have an actual unobserved Task exception, which will be ignored (correctly):

static void Main(string[] args)
{
  DownloadAsync("http://an.invalid.url.com");
  Console.ReadKey();
}

async static Task DownloadAsync(string url)
{
  using (var client = new System.Net.Http.HttpClient())
  {
    string text = await client.GetStringAsync(url);
    Console.WriteLine("Downloaded {0} chars", text.Length);
  }
}

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