HttpClient not throwing exception when using await on GetAsync

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?

Stephen Cleary · Accepted Answer · 2013-04-08 17:49:14Z

19

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.

answered Apr 8, 2013 at 17:49

Stephen Cleary

460k78 gold badges721 silver badges858 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Cordt · Accepted Answer · 2021-01-01 18:11:33Z

0

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 :-)

answered Jan 1, 2021 at 18:11

Cordt

595 bronze badges

5 Comments

Theodor Zoulias Over a year ago

Uncommenting the line string s = await GetWebPage(... should cause a compile-time error, because the calling method is not async. I.e. I highly doubt that it causes the application to terminate without any exception being thrown.

Cordt Over a year ago

Hey Theodor, if you add "async" it works as described. I needed to remove "async" to not get the compile error, when the two marked lines are commented ;-)

Theodor Zoulias Over a year ago

Cordt if you add async to void Main you get an async void, which is something to avoid. The correct signature for an asynchronous Main() entry point is async Task Main().

Cordt Over a year ago

Theodor, thanks for the clarification. I know that async void is to be avoided as it does not forward exceptions. I came across this thread because when I was preparing myself for the C# exam I encountered the termination of my program without any exception. So i googled and found this thread. I don't know exactly why this had happen, but now the same piece of code is running without termination. I can only guess that the loading of the page took longer than today and that's why it works.

Theodor Zoulias Over a year ago

Yeap, this is a reasonable explanation. Btw the async void methods are not like fire-and-forget tasks that have their exceptions unobserved. An exception thrown by an async void is rethrown on the captured SychronizationContext, or on the ThreadPool if their isn't any, so you are surely going to observe it (the process will be crashed). For this reason the async voids are sometimes called fire-and-crash.

Collectives™ on Stack Overflow

HttpClient not throwing exception when using await on GetAsync

2 Answers 2

Comments

5 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

5 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related