Pass non reference value to Task

Question

I often get strange resulst when passing int variables to tasks such as in this example:

List<List<object>> ListToProcess = new List<List<object>>();

// place some lists in list to process
foreach (var temp in Foo)
    ListToProcess.Add(temp);

foreach (var tempArray in ListToProcess)
{
    // initialize each list in ListToProcess                    
}

int numberOfChunks = ListToProcess.Count; 
Task[] tasks = new Task[numberOfChunks];
for (int counter = 0; counter < numberOfChunks; counter++)
{
    tasks[counter] = Task.Factory.StartNew(() =>
        {
            // counter is always = 5  why?   <---------------------------
            var t = ListToProcess[counter];                        
        });
}

How can I solve this problem?

Closures capture variables, not values. See http://stackoverflow.com/questions/271440/c-sharp-captured-variable-in-loop — Kenneth Ito, Apr 11 '12 at 23:00

score 3 · Accepted Answer · answered Apr 11 '12 at 23:01

This is known as a closure. You are not using the value of the variable, you are using the variable itself. When the code executes, it uses the value at the time of execution, not the value when the Task was defined.

To fix this issue, I believe you would do something like this:

for (int counter = 0; counter < numberOfChunks; counter++)
{
    int cur = counter;
    tasks[counter] = Task.Factory.StartNew(() =>
    {
        // counter is always = 5  why?   <---------------------------
        var t = ListToProcess[cur];                        
    });
}

score 1 · Answer 2 · answered Apr 11 '12 at 23:01

There is no guarantee as to when the 'counter' variable in the Action block of StartNew will be accessed. What is likely to happen is that all 5 values are looped through, and the tasks are created, then the tasks are scheduled to run.

When they do run, the following is executed:

var t = ListToProcess[counter];

But at this stage count is already equal to 5.

Perhaps you should look at parallel collections?

ListToProcess.AsParallel().ForAll(list => dosomething(list));

There are many other options around this area.

score 0 · Answer 3 · answered Apr 11 '12 at 23:02

    for (int counter = 0; counter < numberOfChunks; counter++)
    {
        var referenceVariable = new{val=counter};
        tasks[counter] = Task.Factory.StartNew(() =>
            {
                var t = ListToProcess[referenceVariable.val];                        
            });
    }

score 0 · Answer 4 · answered Apr 11 '12 at 23:02

Since variables are captured, you can solve this by redeclaring a new variable in each loop.

for (int counter = 0; counter < numberOfChunks; counter++)
{
    int localCounter = counter;
    tasks[localCounter] = Task.Factory.StartNew(() =>
        {
            // counter is always = 5  why?   <---------------------------
            var t = ListToProcess[localCounter];                        
        });
}

Pass non reference value to Task

4 Answers4