C#: Different results when using sync, async or async with shared HttpClient

Question

Introduction

I've been working in ASP.NET Web API where I bumped into some weird interactions when trying to call another API. I've tried 3 different ways to use HttpClient, all with different results.
Have tested everything using Postman, hence some results.

1. Sync HttpClient call

private static string GetAPI(string url)
{
    using (HttpClient client = new HttpClient())
    {
        client.BaseAddress = new Uri(url);
        client.DefaultRequestHeaders.Accept.Clear();
        client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
        client.DefaultRequestHeaders.Add("x-api-key", "SomeSecretApiKey");

        HttpResponseMessage response = client.GetAsync(url).Result;
        string contents = response.Content.ReadAsStringAsync().Result;

        if (response.IsSuccessStatusCode)
        {
            return contents;
        }
    }

    return null;
}

Result

Does work, but I want to use async

2. Async httpClient call

private static async Task<string> GetAPI(string url)
{
    using (HttpClient client = new HttpClient())
    {
        client.BaseAddress = new Uri(url);
        client.DefaultRequestHeaders.Accept.Clear();
        client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
        client.DefaultRequestHeaders.Add("x-api-key", "SomeSecretApiKey");

        HttpResponseMessage response = await client.GetAsync(url);
        string contents = await response.Content.ReadAsStringAsync();

        if (response.IsSuccessStatusCode)
        {
            return contents;
        }
    }

    return null;
}

Result

Does not work. Will not go beyond the line HttpResponseMessage response = await client.GetAsync(url);, since there's never a response?

3. Async httpClient call with shared HttpClient

private static readonly HttpClient client = new HttpClient();

private static async Task<string> GetAPI(string url)
{
    client.BaseAddress = new Uri(url);
    client.DefaultRequestHeaders.Accept.Clear();
    client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
    client.DefaultRequestHeaders.Add("x-api-key", "SomeSecretApiKey");

    HttpResponseMessage response = await client.GetAsync(url);
    string contents = await response.Content.ReadAsStringAsync();

    if (response.IsSuccessStatusCode)
    {
        return contents;
    }

    return null;
}

Result

Does work once. Will then throw the error: System.AggregateException: One or more errors occurred. ---> System.InvalidOperationException: This instance has already started one or more requests. Properties can only be modified before sending the first request. (as suggested by this SO answer).

Question

I would appreciate it if someone could explain why these results are so different and/or give an alternative way to make a basic HttpClient call (still would like to use async).

FYI: Your third example is _not_ a singleton. Its just a shared HttpClient (which is the way you should be using it). Also your third example is not even Async (nor would it compile). You need to return a Task and mark the method as `async`. — maccettura, May 23 '18 at 14:36
@maccettura Ah thank you for pointing out my mistake, I had made a copy-paste mistake in my third example, I have fixed it now. About the Singleton: that's what the linked answer said and I took it for granted, but will change it right now. Any more on the actual question? — Wouter Vanherck, May 23 '18 at 14:47
Example 2 is most likely a common deadlock which occurs when you mix async with synchronous code (ie. not using async all the way). This could be caused by a call to `GetAPI` but using `.Result` instead of `await`. See also releated [An async/await example that causes a deadlock](https://stackoverflow.com/q/15021304/1260204). — Igor, May 23 '18 at 14:48
@WouterVanherck Well once you make your third example compile, that looks like every bit of HttpClient code I have written. Nothing stands out as "wrong". You have to worry about DNS issues with a shared HttpClient (it wouldnt respect TTL). — maccettura, May 23 '18 at 14:49
@Igor I have indeed used `.Result` somewhere around my `GetAPI`-call. Can you point me in the direction of a proper way to fully use async? — Wouter Vanherck, May 23 '18 at 14:56
Here's a good article about using the HttpClient: https://blogs.msdn.microsoft.com/henrikn/2012/08/07/httpclient-httpclienthandler-and-webrequesthandler-explained/ — Stevo, May 23 '18 at 14:58
Here is a popular article that I found easy to digest when I started working with async / await. https://blog.stephencleary.com/2012/07/dont-block-on-async-code.html — Igor, May 23 '18 at 14:59
I am not sure what your calling context is though, is it a console application or wpf or asp.net or something else? That will determine how you can start an async call chain. — Igor, May 23 '18 at 15:00
Steve J, Thank you, I'll check that out | @Igor, I'll check that link out too, since you're probably right about the deadlock. I'm calling the funcion from my API endpoint in ASP.NET Web API — Wouter Vanherck, May 23 '18 at 15:08
Then make sure your web api method is also `async` and use `await` all the way down into your `GetAPI` method. — Igor, May 24 '18 at 10:12

Wouter Vanherck · Accepted Answer · 2018-07-06T09:07:19.707

@Igor has informed me about the deadlock issue in my second example.
So I currently used the HttpClient as such (async):

private static async Task<string> GetAPI(string url)
    {
    // TODO: Use a shared instance of HttpClient
    using (HttpClient client = new HttpClient())
    {
        client.BaseAddress = new Uri(url);
        client.DefaultRequestHeaders.Accept.Clear();
        client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
        client.DefaultRequestHeaders.Add("x-api-key", "SomeSecretApiKey");

        var jsonString = await client.GetStringAsync(url).ConfigureAwait(false);
        return jsonString;
    }
}

While some things have been made clear, the entire question hasn't been answered yet. I ~~therefore won't accept this as an answer, yet~~ like to thank those for providing me good information on the use of HttpClient.

C#: Different results when using sync, async or async with shared HttpClient

Introduction

1. Sync HttpClient call

Result

2. Async httpClient call

Result

3. Async httpClient call with shared HttpClient

Result

Question

1 Answers1