How do I wait until Task is finished in C

Question

I want to send a request to a server and process the returned value   private static string Send int id        Task lt HttpResponseMessage gt  responseTask   client GetAsync  aaaaa        string result   string Empty      responseTask ContinueWith x   gt  result   Print x        responseTask Wait       it doesn t wait for the completion of the response task     return result     private static string Print Task lt HttpResponseMessage gt  httpTask        Task lt string gt  task   httpTask Result Content ReadAsStringAsync        string result   string Empty      task ContinueWith t   gt                Console WriteLine  Result      t Result           result   t Result              task Wait        it does wait     return result      Am I using Task correctly  I don t think so because the Send   method returns string Empty every time  while Print returns the correct value   What am I doing wrong  How do I get the correct result from a server

User · Answer

When working with continuations I find it useful to think of the place where I write .ContinueWith as the place from which execution immediately continues to the statements following it, not the statements 'inside' it. In that case it becomes clear that you would get an empty string returned in Send. If your only processing of the response is writing it to the console, you don't need any Wait in Ito's solution - the console printout will happen without waits but both Send and Print should return void in that case. Run this in console app and you will get printout of the page.

IMO, waits and Task.Result calls (which block) are necessary sometimes, depending on your desired flow of control, but more often they are a sign that you don't really use asynchronous functionality correctly.

namespace TaskTest
{
    class Program
    {
        static void Main(string[] args)
        {
            Send();
            Console.WriteLine("Press Enter to exit");
            Console.ReadLine();
        }

        private static void Send()
        {
            HttpClient client = new HttpClient();
            Task<HttpResponseMessage> responseTask = client.GetAsync("http://google.com");
            responseTask.ContinueWith(x => Print(x));
        }

        private static void Print(Task<HttpResponseMessage> httpTask)
        {
            Task<string> task = httpTask.Result.Content.ReadAsStringAsync();
            Task continuation = task.ContinueWith(t =>
            {
                Console.WriteLine("Result: " + t.Result);
            });
        }
    }
}

User · Answer

async Task lt int gt  AccessTheWebAsync                 You need to add a reference to System Net Http to declare client        HttpClient client   new HttpClient              GetStringAsync returns a Task lt string gt   That means that when you await the          task you ll get a string  urlContents         Task lt string gt  getStringTask         client GetStringAsync  http   msdn microsoft com              You can do work here that doesn t rely on the string from GetStringAsync        DoIndependentWork              The await operator suspends AccessTheWebAsync            - AccessTheWebAsync can t continue until getStringTask is complete            - Meanwhile  control returns to the caller of AccessTheWebAsync            - Control resumes here when getStringTask is complete             - The await operator then retrieves the string result from      getStringTask        string urlContents   await getStringTask            The return statement specifies an integer result           Any methods that are awaiting AccessTheWebenter code hereAsync retrieve the length      value        return urlContents Length

User · Answer

Your Print method likely needs to wait for the continuation to finish  ContinueWith returns a task which you can wait on    Otherwise the second ReadAsStringAsync finishes  the method returns  before result is assigned in the continuation    Same problem exists in your send method   Both need to wait on the continuation to consistently get the results you want  Similar to below  private static string Send int id        Task lt HttpResponseMessage gt  responseTask   client GetAsync  aaaaa        string result   string Empty      Task continuation   responseTask ContinueWith x   gt  result   Print x        continuation Wait        return result     private static string Print Task lt HttpResponseMessage gt  httpTask        Task lt string gt  task   httpTask Result Content ReadAsStringAsync        string result   string Empty      Task continuation   task ContinueWith t   gt                Console WriteLine  Result      t Result           result   t Result              continuation Wait          return result

User · Answer

It waits for client GetAsync  aaaaa     but doesn t wait for result   Print x   Try responseTask ContinueWith x   gt  result   Print x   Wait    --EDIT--  Task responseTask   Task Run      gt         Thread Sleep 1000        Console WriteLine  In task         responseTask ContinueWith t  gt Console WriteLine  In ContinueWith     responseTask Wait    Console WriteLine  End      Above code doesn t guarantee the output   In task In ContinueWith End   But this does  see the newTask   Task responseTask   Task Run      gt         Thread Sleep 1000        Console WriteLine  In task         Task newTask   responseTask ContinueWith t  gt Console WriteLine  In ContinueWith     newTask Wait    Console WriteLine  End

User · Answer

A clean example that answers the Title  string output    Error   Task task   Task Factory StartNew      gt        System Threading Thread Sleep 2000       output    Complete        task Wait    Console WriteLine output

User · Answer

I m an async novice  so I can t tell you definitively what is happening here  I suspect that there s a mismatch in the method execution expectations  even though you are using tasks internally in the methods  I think you d get the results you are expecting if you changed Print to return a Task lt string gt    private static string Send int id        Task lt HttpResponseMessage gt  responseTask   client GetAsync  aaaaa        Task lt string gt  result      responseTask ContinueWith x   gt  result   Print x        result Wait        responseTask Wait       There s likely a better way to wait for both tasks without doing it in this awkward  consecutive way      return result Result     private static Task lt string gt  Print Task lt HttpResponseMessage gt  httpTask        Task lt string gt  task   httpTask Result Content ReadAsStringAsync        string result   string Empty      task ContinueWith t   gt                Console WriteLine  Result      t Result           result   t Result              return task

[c#] How do I wait until Task is finished in C#?

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