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?

Answer 1

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];                        
    });
}