C# - Running a task in a for loop doesn't work

Question

I am trying to sum all the values of a byte array using 4 tasks. So I wrote a for loop in the main method with starting and running a new task each time it loops:

for (int i = 0; i < tasks.Length; i++)
{
    Task.Run(() => Sum(i));
}

The parameter of the Sum method is portionNumber. In the Sum method, the first thing it execute is:

Console.WriteLine("I is " + i);

When I run it, it shows me:

I is 4
I is 4
I is 4
I is 4

However, if I wrote it like this:

Task.Run(() => Sum(0));
Task.Run(() => Sum(1));
Task.Run(() => Sum(2));
Task.Run(() => Sum(3));

The result prints:

I is 0
I is 1
I is 2
I is 3

My question is why? How can I run the 4 task using for loops instead of duplicating code?

Thanks alot.

Change `Task.Run(() => Sum(i));` to `var bob = i; Task.Run(() => Sum(bob));` — mjwills, Dec 11 '18 at 23:53
https://www.bing.com/search?q=C%23+Running+task+in+a+for+loop+same will give you more questions to look at if duplicate is not enough. — Alexei Levenkov, Dec 12 '18 at 00:05

score 0 · Answer 1 · answered Dec 12 '18 at 00:01

you are running into a lambda capture common issue. you need

for (int i = 0; i < tasks.Length; i++)
{
   var capi = i;
   Task.Run(() => Sum(capi));
}

There is an excellent discusion from eric lippert somewhere on this

https://blogs.msdn.microsoft.com/ericlippert/2009/11/12/closing-over-the-loop-variable-considered-harmful/

C# - Running a task in a for loop doesn't work

1 Answers1