Cancelling a DownloadStringTaskAsync and restart it

Question

I'm downloading a string from a webpage using System.Net.WebClient.DownloadStringTaskAsync asynchronously.

About 4 out of 5 times, this completes successfully, but occasionally, it hangs, and I will have to restart the program to "reset" it.

Thus I'm thinking of implementing a time-out that automatically cancels the Task after 5 seconds and restarts it:

async void DoTasks()
{
    string output;

    int timeout = 5000;
    WebClient client = new WebClient() { Encoding = Encoding.UTF8 };
    Task<string> task = client.DownloadStringTaskAsync(url);
    if (await Task.WhenAny(task, Task.Delay(timeout)) == task)
    {
        output = task.Result;
    }
    else
    {
        client.CancelAsync();
        DoTasks();
    }
}

This, however, returns a NullReferenceException.

There is no way that this code could cause `NullReferenceException`. Probably it is inside `GetTasks` method. Also, you're not disposing the `WebClient`. — Sriram Sakthivel, Dec 25 '14 at 14:17
I'm sorry, GetTasks was supposed to be DoTasks - restarting the method — jacobz, Dec 25 '14 at 14:22
Nothing obvious that could cause `NullReferenceException` here. Which line throws the exception? — Sriram Sakthivel, Dec 25 '14 at 14:25
You should also post the original code that causes the hang. You're actually asking two unrelated questions. — Yuval Itzchakov, Dec 25 '14 at 14:29
The NullReferenceException seems to be caused by code executed after, code which uses "output", so it seems that when DoTasks is being restarted, it doesn't execute properly. I'm certain that DownloadStringTaskAsync ocasionally causes it to hang and it seems to it sometime fails to connect. — jacobz, Dec 25 '14 at 14:41
Currently just using `DoTasks()`. Should I use `Task.Run(() => DoTasks())`? — jacobz, Dec 25 '14 at 14:48
`DoTasks` should be `async Task` and not `async void`. And then you need to `await DoTasks()`. — Yuval Itzchakov, Dec 25 '14 at 14:49

Fredou · Answer 1 · 2014-12-25T16:20:25.460

I'm not an expert on await/async, i'm still learning myself but maybe you should do something like this?

using System;
using System.Net;
using System.Text;
using System.Threading;
using System.Threading.Tasks;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Inline code to test");
            Console.WriteLine(StartTask(true));

            Console.WriteLine("Get actual website to test, write the length of the text only");
            Console.WriteLine(StartTask(false).Length);

            Console.Write("Press any key");
            Console.ReadKey();
        }

        static string StartTask(bool DoTestInsteadOfGetUrl)
        {
            var task = DoTasks(DoTestInsteadOfGetUrl);
            Task.WaitAny(task);

            return task.Result;
        }

        async static Task<string> DoTasks(bool DoTestInsteadOfGetUrl)
        {
            int timeout = 20;
            int count = 1;
            CancellationTokenSource cts;
            string output;                
            do
            {
                cts = new CancellationTokenSource(timeout);
                if (DoTestInsteadOfGetUrl)
                {
                    output = await Task<string>.Run(() => test(count, cts.Token), cts.Token);
                }
                else
                {
                    WebClient client = new WebClient() { Encoding = Encoding.UTF8 };
                    output = await Task<string>.Run(() => client.DownloadStringTaskAsync("http://www.google.com"), cts.Token);
                }

                if (cts.IsCancellationRequested)
                {
                    Console.WriteLine("try #" + count++);
                    timeout += DoTestInsteadOfGetUrl ? 100 : 50;
                }
            } while (cts.IsCancellationRequested);

            return output;
        }

        async static Task<string> test(int count, CancellationToken ct)
        {
            int sleep = 400 - (count * 5);
            await Task.Run(() => Task.Delay(sleep), ct);

            if (!ct.IsCancellationRequested)
            {
                Console.WriteLine("succeed #" + count);
            }
            return sleep.ToString();
        }
    }
}

There are many things wrong with this code. For starters, it doesn't include the OP's `WebClient` call, which you'll need to make inside `DoTasks`. But your `DoTasks` is `void`, and doesn't contain the `async` modifier. This currently won't solve the OP's problem. — Yuval Itzchakov, Dec 25 '14 at 15:03

Cancelling a DownloadStringTaskAsync and restart it

1 Answers1