How do you evaluate the result of multiple tasks

Question

Using TPL, i have multiple tasks set up (created dynamically, could be a large number), each returns a bool, i want to check if all task return values are true. how would i do this? is it possible? if not, is it possible to pass in a shared object to each task and have the tasks update this object?

EDIT: This would be an example of my task. ProcessEntity returns a bool. Now I have multiple of these tasks created and executed. I need to check that all the results are true.

 private Task<bool> CreateTask(MyEntity entity, Action onStart, Action onComplete)
    {
        return (new Task<bool>(
            () =>
            {
                onStart.Invoke();
                var result = false;
                try
                {
                    result = ProcessEntity(myEntity);
                }
                catch (Exception ex)
                {
                }

                onComplete.Invoke();
                return result;
            })
               );
    }

 for (int i = 0; i < counter; i++)
        {
            CreateTask(entities[i], () => _taskCounter++, () => _taskCounter--).Start();
        }

So at this point I need to continue execution of other code and that must only happen if all tasks returned true.

All of these are possible. If you were to show us your code and what you've tried that didn't work, it would be easier to help. — Matt Smith, Apr 15 '13 at 13:34
possible duplicate of [Limit number of tasks with TPL](http://stackoverflow.com/questions/16012096/limit-number-of-tasks-with-tpl) This appears to be about the same problem. Rather than create new questions revise the current. You have had several requests for specifics. — paparazzo, Apr 15 '13 at 14:30
Do you want to synchronously wait for the `Task`s to complete or do this asynchronously? Also, what should happen if some of the `Task`s fail? — svick, Apr 15 '13 at 14:40
@Blam I don't think so. This seems to be a separate question, though both contain very little specifics. — svick, Apr 15 '13 at 15:03
@Blam...its a different question, it has nothing to do with controlling the number of threads, it's about accessing the results — newbie_86, Apr 15 '13 at 16:18

score 2 · Accepted Answer · answered Apr 15 '13 at 14:41

Simply query the Task.Result property of each, this will both wait for the task to complete and return the result:

void Main()
{
    var tasks = new List<Task<bool>>();

    // spawn all the tasks
    for (int index = 0; index < 10; index++)  
        tasks.Add(Task.Factory.StartNew(new Func<bool>(GetValue)));

    // now wait for them to return
    bool didAllReturnTrue = tasks.All(t => t.Result);
    // note that if one task returns false, the rest of the tasks will not be
    // waited upon, and will finish in their own time.

    // show the results (LINQPad syntax)
    didAllReturnTrue.Dump();
}

public bool GetValue()
{
    Thread.Sleep(500);
    return true;
}

Note that this will not handle exceptions in any of the tasks well, you will have to build that in. Also, with the new async/await support in C# 5.0 / .NET 4.5, you can write the above code to be a little bit more async-friendly as well, but I'll leave that alone for this answer.

thanks! i need all the tasks to complete first and then to access the results and continue with further processing — newbie_86, Apr 15 '13 at 16:17

How do you evaluate the result of multiple tasks

1 Answers1