C# httpclient with Task that return content

Question

I've the following function but can't debug it because response never come. Service on the other side is working. Any help will be preciated, I can't deduce how must be do it with other answers in SO

    public async Task<string> PostObjectToWebServiceAsJSON(object objectToPost, string validatorName, string method)
    {
        using (var client = new HttpClient())
        {
            client.BaseAddress = new Uri("myuri" + "/" + method);
            client.DefaultRequestHeaders.Accept.Clear();
            client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
            // HTTP POST
            var response = await client.PostAsJsonAsync("", objectToPost);
            if (response.IsSuccessStatusCode)
            {
                return await response.Content.ReadAsStringAsync();
            }
            else
            {
                string errplugin = "Error";

                return null;
            }
        }
    }

This is how I call it:

    public PaymentResponseInternal Post(some stuff here)
    {
        Task<string> retornoPluginAsync = PostObjectToWebServiceAsJSON(some stuff here);

        retornoPluginAsync.Wait();
        string result = retornoPluginAsync.Result;

        return JsonConvert.DeserializeObject<PaymentResponseInternal>(result);
    }

When you say "response never come", what do you mean? Is it hanging? How are you calling this function? It's likely a deadlock. — DavidG, Jan 08 '18 at 14:10
Yes, it goes to the service, the service response but doesn't do anymore after. Let me update the question with how I call it — Leandro Bardelli, Jan 08 '18 at 14:11
So yeah, it's a deadlock because you are calling `retornoPluginAsync.Result`. You need to `await` that call. See here: https://blog.stephencleary.com/2012/07/dont-block-on-async-code.html — DavidG, Jan 08 '18 at 14:13
It sounds like `public PaymentResponseInternal Post(some stuff here)` should be `public async Task Post(some stuff here)` and should internally make use of `await`. — David, Jan 08 '18 at 14:14
I miss an "await" in my question, sorry. I update it! but let me see what are you writing here. Thanks! — Leandro Bardelli, Jan 08 '18 at 14:15
Sorry but how I transform Task to PaymentResponseInternal then? — Leandro Bardelli, Jan 08 '18 at 14:17
You await that function too. async code is like a virus, once you start, it will slowly consume your whole project (which is a good thing) — DavidG, Jan 08 '18 at 14:18
lol!!! but I can't do that because I come from other call that can't wait :( — Leandro Bardelli, Jan 08 '18 at 14:20
Then look at [this](https://stackoverflow.com/questions/9343594/how-to-call-asynchronous-method-from-synchronous-method-in-c), unfortunately, only you will be able to tell which is the most suitable method to solve your issue. — DavidG, Jan 08 '18 at 14:21
@Sven that is the weird scenario that motivates this question, i get the data posted in the other service, and the service sends a response, but nothing happens :S — Leandro Bardelli, Jan 08 '18 at 14:35

score 3 · Accepted Answer · answered Jan 08 '18 at 16:56

As mentioned in the comments, you should make PaymentResponseInternal an async method that returns a Task<PaymentResponseInternal> and await it.

You shouldn’t mix synchronous and asynchronous code. One of the reasons for this is that you might deadlock as you have discovered here. Please refer to @Stephen Cleary's article on the subject for more information: https://msdn.microsoft.com/en-us/magazine/jj991977.aspx

If you can't make your Post method async for some reason, you could try not to capture the context in the PostObjectToWebServiceAsJSON method by calling ConfigureAwait(false) after each await:

public async Task<string> PostObjectToWebServiceAsJSON(object objectToPost, string validatorName, string method)
{
    using (var client = new HttpClient())
    {
        client.BaseAddress = new Uri("myuri" + "/" + method);
        client.DefaultRequestHeaders.Accept.Clear();
        client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));

        var response = await client.PostAsJsonAsync("", objectToPost).ConfigureAwait(false);
        if (response.IsSuccessStatusCode)
        {
            return await response.Content.ReadAsStringAsync().ConfigureAwait(false);
        }
        else
        {
            return null;
        }
    }
}

Please refer to @Stephen Cleary's blog post for information about this: https://blog.stephencleary.com/2012/07/dont-block-on-async-code.html

GeorgDangl · Answer 2 · 2018-01-08T14:23:46.707

1

With your update to the question, you're running into a deadlock situation when you call PostObjectToWebServiceAsJSON synchronously. You should add a call to retornoPluginAsync.ConfigureAwait(false); before you Wait() for it. ConfigureAwait(false) configures the Task to not require the original call context and should, in your case, solve the deadlock.

    public PaymentResponseInternal Post(some stuff here)
    {
        Task<string> retornoPluginAsync = PostObjectToWebServiceAsJSON(some stuff here);
        retornoPluginAsync.ConfigureAwait(false); // Add this
        retornoPluginAsync.Wait();
        string result = retornoPluginAsync.Result;

        return JsonConvert.DeserializeObject<PaymentResponseInternal>(result);
    }

edited Jan 08 '18 at 14:23

answered Jan 08 '18 at 14:12

GeorgDangl

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I do it that, and compiles but don't return anything. – Leandro Bardelli Jan 08 '18 at 14:13
Actually I miss the await in the question – Leandro Bardelli Jan 08 '18 at 14:13
3

Looks like the code in the question is a moving target which is invalidating responses being posted... – David Jan 08 '18 at 14:17
This answer was clearly never going to work anyway as OP definitely had code that compiled. – DavidG Jan 08 '18 at 14:17
yep, sorry David, my mistake, I remove await trying something but forget to write it again – Leandro Bardelli Jan 08 '18 at 14:18
@Georg Just remove the whole first part of your answer, it's not relevant or useful at all. – DavidG Jan 08 '18 at 14:23
@DavidG, I've removed the now obsolete part. – GeorgDangl Jan 08 '18 at 14:24
I tried, but same result :( I added that line and now at least I get the breakpoint on the retornoPluginAsync.Wait(); but never comes to if (response.IsSuccessStatusCode) and nothing happens. At the service side, the data is coming and working fine – Leandro Bardelli Jan 08 '18 at 14:45

C# httpclient with Task that return content

2 Answers2