Why does this async code sometimes fail, and only when not observed?

Question

This is the original code that had been running fine for a few weeks. In a test I just did, it failed 0 out of 100 attempts.

using (var httpClient = new Http         


        

Answer 1

Edit: unaccepting my own answer, but leaving it for reference. The code works, with a catch: ContinueWith loses the SynchronizationContext

---

Thanks to @jbl and @MattSmith for putting me on the right track.

The problem was indeed that Task.WhenAll does not wait on the continuations. The solution is to set TaskContinuationOptions.AttachedToParent.

So this

private static Task GetAsync(HttpClient httpClient, Uri URI, Action> continuationAction)
{
    return httpClient.GetAsync(URI)
        .ContinueWith(request =>
        {
            request.Result.EnsureSuccessStatusCode();

            request.Result.Content.ReadAsAsync()
                .ContinueWith(continuationAction);
        });
}

becomes this

private static Task GetAsync(HttpClient httpClient, Uri URI, Action> continuationAction)
{
    return httpClient.GetAsync(URI)
        .ContinueWith(request =>
        {
            request.Result.EnsureSuccessStatusCode();

            request.Result.Content.ReadAsAsync()
                .ContinueWith(continuationAction, TaskContinuationOptions.AttachedToParent);
        }, TaskContinuationOptions.AttachedToParent);
}

More info available on MSDN: Nested Tasks and Child Tasks