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

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I want to send a request to a server and process the returned value:

private static string Send(int id)
{
    Task<HttpResponseMessage> responseTask = client.GetAsync("aaaaa");
    string result = string.Empty;
    responseTask.ContinueWith(x => result = Print(x));
    responseTask.Wait(); // it doesn't wait for the completion of the response task
    return result;
}

private static string Print(Task<HttpResponseMessage> httpTask)
{
    Task<string> task = httpTask.Result.Content.ReadAsStringAsync();
    string result = string.Empty;
    task.ContinueWith(t =>
    {
        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?

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<HttpResponseMessage> responseTask = client.GetAsync("aaaaa");
    string result = string.Empty;
    Task continuation = responseTask.ContinueWith(x => result = Print(x));
    continuation.Wait();
    return result;
}

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

How to wait until all the tasks finish before returning from method in C, When all can be used to create a new task which completes when all tasks in it completes, you can then proceed fetching results out of that task using the  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.

It waits for client.GetAsync("aaaaa");, but doesn't wait for result = Print(x)

Try responseTask.ContinueWith(x => result = Print(x)).Wait()

--EDIT--

Task responseTask = Task.Run(() => { 
    Thread.Sleep(1000); 
    Console.WriteLine("In task"); 
});
responseTask.ContinueWith(t=>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(() => { 
    Thread.Sleep(1000); 
    Console.WriteLine("In task"); 
});
Task newTask = responseTask.ContinueWith(t=>Console.WriteLine("In ContinueWith"));
newTask.Wait();
Console.WriteLine("End");

Task.Wait Method (System.Threading.Tasks), You can use a CancellationToken and Task.WaitAll() to do this public class A { private readonly CancellationTokenSource tokenSource = new  Waiting for a single Task to finish is as easy as calling the Wait() method or one of its overloaded versions: In short from top to bottom: Wait until the Task completes, is cancelled or throws an exception Wait until the cancellation token is cancelled or the task completes, is cancelled or throws an exception…

A clean example that answers the Title

string output = "Error";
Task task = Task.Factory.StartNew(() =>
{
    System.Threading.Thread.Sleep(2000);
    output = "Complete";
});

task.Wait();
Console.WriteLine(output);

Using Task.Wait() for waiting while task is finished (even if it already , from the scheduler and execute it inline on the current thread. Wait(TimeSpan) is a synchronization method that causes the calling thread to wait for the current task instance to complete until one of the following occurs: The task completes successfully. The task itself is canceled or throws an exception. In this case, you handle an AggregateException exception.

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<string>:

private static string Send(int id)
{
    Task<HttpResponseMessage> responseTask = client.GetAsync("aaaaa");
    Task<string> result;
    responseTask.ContinueWith(x => 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<string> Print(Task<HttpResponseMessage> httpTask)
{
    Task<string> task = httpTask.Result.Content.ReadAsStringAsync();
    string result = string.Empty;
    task.ContinueWith(t =>
    {
        Console.WriteLine("Result: " + t.Result);
        result = t.Result;
    });
    return task;
}

await vs Task.Wait - Deadlock?, when you know it is done. You don't really need to do anything manually, await keyword pauses the function execution until blah() returns. We do NOT want to (or have to) wait for one database call to be completed before we make the next. They can all run at the same time. But, before making all of the calls, we need to perform a "starting" task. And when all of the calls are complete, we need to perform a "finished" task. Here's where I'm at now:

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);
            });
        }
    }
}

A Tour of Task, Part 6: Results, Waiting for a single Task to finish is as easy as calling the Wait() method or one of its overloaded versions: In short from top to bottom: Wait until  How do I wait until Task is finished in C#? (4) 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.

Wait for a void async method, The task starts, and (later in Main) we call Wait() for it to finish. async Task<int> HandleFileAsync() { string file = @"C:\programs\enable1.txt"; // Part 2: status  WaitAll(Task[], CancellationToken) Waits for all of the provided Task objects to complete execution unless the wait is cancelled. WaitAll(Task[], TimeSpan) Waits for all of the provided cancellable Task objects to complete execution within a specified time interval. WaitAll(Task[], Int32, CancellationToken)

Wait for a single Task in .NET C#: the Wait() method, In this article I will discuss the various ways by which we can wait for task or multiple tasks completion in C#. There are many ways in which we  How do I wait until myVar == true. C# / C Sharp Forums on Bytes.

C# async, await Examples, Whenever you call a method that returns a Task or Task<T> you should not ignore its return value. will complete in one second, but nothing is waiting for that task to finish. ForEach(async c => await SendEmailAsync(c));. I am new to Task logic, I want to wait Task until a flag is true, however I fail, I sent a message to a port and before it returns, my code does serialize which end up with wrong value. So "serialize" Task should wait until I get the port answer and make the flag true. Then perform the serialization.

Comments
  • As an aside, the pattern where you call async and then immediately wait on it, is pretty much the same as just calling synchronously.
  • This is the best solution when u are stuck with .net Framework 4.0 and you have problems adding microsoft.bcl.async nuget pkg inorder to use Await.
  • But I call task.Wait() within Print() method.
  • When you call task.Wait() you wait the original Task, not the one you created with ContinueWith
  • Why don't you call responseTask.Wait() before newTask.Task()?
  • @OskarK., there is no need to wait for previous task. ContinueWith will guarantee previous task is completed.
  • not testing ? fails for me. Task<string> result is null
  • for not console application ? not Readline available, any solution?