c# and Threadpool

Question

What is the difference between:

Task[] tasks = new Task[mylist.Count];
int i = 0;

foreach (DataTable dt in mylist)
{
            Task t = Task.Factory.StartNew(() => UpdateProductsData(dt, updateType));
            tasks[i] = t;
            i++;
}
Task.WaitAll(tasks);

and:

tasks[0] = Task.Factory.StartNew(() => UpdateProductsData(mylist[0], updateType));
tasks[1] = Task.Factory.StartNew(() => UpdateProductsData(mylist[1], updateType));
tasks[2] = Task.Factory.StartNew(() => UpdateProductsData(mylist[2], updateType));
tasks[3] = Task.Factory.StartNew(() => UpdateProductsData(mylist[3], updateType));
Task.WaitAll(tasks);

For some reason the first snippet terminates prematurely, although the second snippet behaves as expected. What needs to be changed in the first example in order to provide a dynamic implementation?

Impossible to tell without more context and better examples. — Simon Whitehead, Jul 10 '13 at 03:50
@SimonWhitehead but does the first snippet do exactly the same thing as the second one considering that `mylist` has 4 elements? — user2490355, Jul 10 '13 at 03:53

score 4 · Accepted Answer · edited May 23 '17 at 11:49

This is the classic Captured Variable In Loop issue. You need to copy the value of your loop iterator to a local variable within your loop body:

Task[] tasks = new Task[mylist.Count];
int i = 0;
foreach (DataTable dt in mylist)
{
    DataTable dtInner = dt;
    Task t = Task.Factory.StartNew(() => UpdateProductsData(dtInner, updateType));
    tasks[i] = t;
    i++;
}
Task.WaitAll(tasks);

Edit:

It may be simpler to express your code as a one-liner using Parallel.Foreach:

Parallel.ForEach(mylist, dt => UpdateProductsData(dt, updateType));

c# and Threadpool

1 Answers1