Run Task in loop throw ArgumentOutOfRangeException

Question

I want to wrap Task class, but when I run my code it throw ArgumentOutOfRangeException. It’s strange because when I’m using debugger no exception has been thrown. Here is ‘working’ example of my code.

public class TaskService
{
private Task<int>[] _tasks;
private List<DateTime> _arguments = new List<DateTime>();

public void AddArgument(DateTime argument)
{
    _arguments.Add(argument);
}

public void RunTask(Func<DateTime, int> doWork)
{
    _tasks = new Task<int>[_arguments.Count];

    for (int i = 0; i < _arguments.Count; i++)
    {
        _tasks[i] = Task.Factory.StartNew<int>(() => doWork(_arguments[i])); // Exception is throwing here: _arguments[i]
        // by some reason i = 2, but _arguments.Count = 2 so it should never happed
    }
}

public void Wait()
{
    Task.WaitAll(_tasks);
}
}

class Program
{
static void Main(string[] args)
{
    TaskService taskService = new TaskService();

    taskService.AddArgument(new DateTime(2014, 1, 1));
    taskService.AddArgument(new DateTime(2014, 2, 1));
    taskService.RunTask(DoWork);
    taskService.Wait();
}

public static int DoWork(DateTime day)
{
    Console.WriteLine(day);
    return 0;
}
}

use i<_arguments.Count - 1

Poomrokc The 3years
– Poomrokc The 3years

2014-04-16 15:06:26 +00:00
Commented Apr 16, 2014 at 15:06 — Poomrokc The 3years
– Poomrokc The 3years, Commented Apr 16, 2014 at 15:06

BartoszKP · Accepted Answer · 2014-04-16 15:07:59Z

4

Your problem occurs because of the closure (further reading). The i variable is captured within the lambda. After you finish creating tasks (after the loop) value of i is 2. And when the tasks start, they are still referring to this particular int instance. Try this:

public void RunTask(Func<DateTime, int> doWork)
{
    _tasks = new Task<int>[_arguments.Count];

    for (int i = 0; i < _arguments.Count; i++)
    {
        var index = i;
        _tasks[i] = Task.Factory.StartNew<int>(() => doWork(_arguments[index]));
    }
}

answered Apr 16, 2014 at 15:07

BartoszKP

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