HttpClient PostAsync caught in a deadlock

Question

User-1641476077 posted

Hi, 

I am doing a web page to pull some JSON data from a site and to display this data out to end user. 

Below is my code:

protected void Page_Load(object sender, EventArgs e)
    {

        startApi().Wait();

        lblCookie.Text = message;
    }

    protected static async Task<int> startApi()
    {
        var responseString = await GetLogin();
       
        return 0;

    }
    protected static async Task<string> GetLogin()
    {
        var responseString = string.Empty;

        try
        {
            string username = ConfigurationManager.AppSettings["username"];
            string password = ConfigurationManager.AppSettings["password"];
            string loginUrl = ConfigurationManager.AppSettings["loginUrl"];
            using (HttpClient client = new HttpClient())
            {
                client.DefaultRequestHeaders.Accept.Clear();
                client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));

                List<KeyValuePair<string, string>> postData = new List<KeyValuePair<string, string>>();

                postData.Add(new KeyValuePair<string, string>("username", username));
                postData.Add(new KeyValuePair<string, string>("password", password));

                FormUrlEncodedContent content = new FormUrlEncodedContent(postData);
                
                //code is in deadlock after the line below
                HttpResponseMessage response = await client.PostAsync(loginUrl, content);

                if (response.IsSuccessStatusCode)
                {
                    message += "response ok <br />";                    
                }
                else
                {
                    message += "response failed <br />";

                }

            }
            
        }
        catch (Exception exe)
        {
           message += "exception: " + exe.Message + "<br />";
        }
        return responseString;

    }

Apparently the code is stuck at the line HttpResponseMessage response = await client.PostAsync(loginUrl, content);

Answer 1

User-1641476077 posted

i tried adding ConfigureAwait(false) to the 2 lines of code with the await. i got the following exception

Exception: An error occurred while sending the request.
Stack Trace: at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) 
             at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) 
             at System.Runtime.CompilerServices.ConfiguredTaskAwaitable`1.ConfiguredTaskAwaiter.GetResult() 
             at _Default.d__4.MoveNext() in d:\projects\IOT Monitoring System\Default.aspx.cs:line 62

Default.aspx.cs:line 62 points to 

HttpResponseMessage response = await client.PostAsync(loginUrl, content).ConfigureAwait(false);

Answer 2

User36583972 posted

Hi bczm8703,

   //code is in deadlock after the line below
                HttpResponseMessage response = await client.PostAsync(loginUrl, content);

You have use asynchronous call and it will go async all the way unless you you use .Result.

That is what is causing deadlock.

           using (var client = new HttpClient())
            {
                var baseUrl = Request.Url.GetLeftPart(UriPartial.Authority);
                var uri = new Uri(baseUrl);            
                client.BaseAddress = uri;             
                var values = new List<KeyValuePair<string, string>>();
                values.Add(new KeyValuePair<string, string>("customerID", "CUS01"));
                using (var content = new FormUrlEncodedContent(values))
                {
                    using (var response = client.PostAsync("/api/FormUrlEncoded", content).Result)
                    {
                        Console.WriteLine(response.StatusCode);
                    }
                }
            }

Best Regards,

Yong Lu

Answer 3

User-1641476077 posted

Hi bczm8703,

bczm8703

//code is in deadlock after the line below
                HttpResponseMessage response = await client.PostAsync(loginUrl, content);

You have use asynchronous call and it will go async all the way unless you you use .Result.

That is what is causing deadlock.

           using (var client = new HttpClient())
            {
                var baseUrl = Request.Url.GetLeftPart(UriPartial.Authority);
                var uri = new Uri(baseUrl);            
                client.BaseAddress = uri;             
                var values = new List<KeyValuePair<string, string>>();
                values.Add(new KeyValuePair<string, string>("customerID", "CUS01"));
                using (var content = new FormUrlEncodedContent(values))
                {
                    using (var response = client.PostAsync("/api/FormUrlEncoded", content).Result)
                    {
                        Console.WriteLine(response.StatusCode);
                    }
                }
            }

Best Regards,

Yong Lu

But if i use .Result, i will not be able to use the await keyword. i am under the assumption that for HTTPClient, i should be using it with await and async

Answer 4

User36583972 posted

Hi bczm8703,

But if i use .Result, i will not be able to use the await keyword. i am under the assumption that for HTTPClient, i should be using it with await and async

You can use the response.Content.ReadAsStringAsync() to serialize the HTTP content

                   using (var response = client.PostAsync("/api/FormUrlEncoded", content).Result)
                    {
                         Console.WriteLine(response.StatusCode);

                         string s = await response.Content.ReadAsStringAsync();
                    }

Best Regards,

Yong Lu