Child tasks are not awaited by parent task

Question

This is my code

static void Main(string[] args)
{
    List<Thing> collection = new List<Thing>
    { 
        new Thing { IntProp = 1, BoolProp = true }, 
        new Thing { IntProp = 1, BoolProp = true }, 
        new Thing { IntProp = 2, BoolProp = true }, 
        new Thing { IntProp = 1, BoolProp = false } 
    };

    int number = 0;

    var task = Task.Factory.StartNew<bool>(() =>
    {
        TaskFactory ts = new TaskFactory(TaskCreationOptions.AttachedToParent, TaskContinuationOptions.ExecuteSynchronously);

        foreach (var item in collection)
        {
            if (item.BoolProp)
            {
                ts.StartNew(() =>
                    number += GetNum1(item.IntProp));
            }
            else
            {
                ts.StartNew(() =>
                    number += GetNum2(item.IntProp));
            }
        }

        return true;
    });

    task.Wait();

    Console.WriteLine(number);
}

here is are definitions of GetNum1 and GetNum2

static int GetNum1(int num)
{
    for (int i = 0; i < 1000000000; i++) { } // simulate some job

    return 10;
}

static int GetNum2(int num)
{
    for (int i = 0; i < 500000000; i++) { } // simulate some job

    return 3;
}

and here is the Thing class

class Thing
{
    public bool BoolProp { get; set; }
    public int IntProp { get; set; }
}

basically what I am doing is just creating the collection of Thing objects. then I create a single parent task which will have several child task (which it should await, I guess).

there is a number variable which is incremented by child task by the amount returned from GetNum1 and GetNum2 method (10 or 3). the code above should output 33 (10 + 10 + 10 + 3) as I guess, but 10 is outputted instead, Because just the first child task is awaited. If I put the breakpoint in the code and go step by step than the output is correct. Why does this happen. does it have to do something with the foreach loop inside the parent task ? Please do not start asking question like "why you need this" and "there is no need for that", this is just an example code.

Answer 1

The parent task is in fact waiting for (not "awaiting") the child tasks. Your problem is that the code is accessing the number variable from multiple threads without synchronization:

var mutex = new object();
int number = 0;

var task = Task.Factory.StartNew<bool>(() =>
{
    TaskFactory ts = new TaskFactory(TaskCreationOptions.AttachedToParent, TaskContinuationOptions.ExecuteSynchronously);

    foreach (var item in collection)
    {
        if (item.BoolProp)
        {
            ts.StartNew(() =>
            {
                var value = GetNum1(item.IntProp);
                lock (mutex) number += value;
            });
        }
        else
        {
            ts.StartNew(() =>
            {
                var value = GetNum2(item.IntProp);
                lock (mutex) number += value;
            });
        }
    }

    return true;
});

task.Wait();

lock (mutex)
    Console.WriteLine(number);

Recommended reading: Parallel Tasks and Dynamic Task Parallelism.