HttpClient 在 GetAsync 上使用 await 时不会抛出异常

Question

I'm using the following code to get an endpoint and write it to a cache:

public async Task UpdateCacheFromHttp(string Uri)
{
    if (string.IsNullOrEmpty(Uri))
        return;

    var httpClient = new HttpClient();
    var response = await httpClient.GetAsync(Uri);

    if ((response != null) && (response.IsSuccessStatusCode))
    {
        var responseStream = await response.Content.ReadAsStreamAsync();
        WriteToCache(responseStream);
    }
}

The code is running on IIS.

If the endpoint can't be reached I'd expect GetAsync to throw an exception. Even with a Try-Catch, it never seems to fail. GetAsync never returns (I tried a 5 second timeout on the HttpClient, still didn't return).

This does throw an exception:

public Task UpdateCacheFromHttp(string Uri)
{
    var updateCacheTask = Task.Factory.StartNew(new Action(() =>
    {
        if (string.IsNullOrEmpty(Uri))
            return;

        var httpClient = new HttpClient();
        var response = httpClient.GetAsync(Uri).Result;

        if (response.IsSuccessStatusCode)
        {
            var responseStream = response.Content.ReadAsStreamAsync().Result;
            WriteToCache(responseStream);
        }
    }));

    return updateCacheTask;
}

I get the expected "Unable to connect to the remote server".

I suspect it has something to do with the code running in IIS, but why? How do I get it to properly throw the exception without the need to start a new task?

Answer 1

My intuition tells me that you're calling Wait or Result further up your call stack.

If that is correct, then you're causing a deadlock, as I explain on my blog .

Answer 2

As I encountered the same behaviour with no Exception being thrown, I created a sample to demonstrate the problem with a possible solution:

using System;
using System.Linq;
using System.Net.Http;
using System.Threading.Tasks;

namespace Exam
{
    public static class Program
    {
        private static async Task<string> GetWebPage(string uri)
        {
            var httpClient = new HttpClient();
            var response = await httpClient.GetAsync(new Uri(uri, UriKind.Absolute), HttpCompletionOption.ResponseContentRead);
            return await response.Content.ReadAsStringAsync();
        }

        public static void Main(string[] args)
        {
            try
            {
                // These two lines do not work - i.e. it terminates the application without any exception being thrown...
                //string s = await GetWebPage(@"https://www.dwd.de/DE/leistungen/klimadatendeutschland/klimadatendeutschland.html");
                //Console.WriteLine(s);

                // This works:
                Task<string> getPageTask = GetWebPage(@"https://www.dwd.de/DE/leistungen/klimadatendeutschland/klimadatendeutschland.html");
                getPageTask.Wait();
                if (getPageTask.IsCompleted)
                    Console.WriteLine(getPageTask.Result);
            }
            catch (AggregateException aex)
            {
                aex.InnerExceptions.AsParallel().ForAll(ex => Console.WriteLine(ex));
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex);
            }
            Console.ReadKey();
        }
    }
}

When you additionally change the URI to something like @"invalid https://...." you will retrieve the AggregateException. Hope, it helps anyone :-)