How to run a set of functions in parallel and wait for the results upon completion?

Question

I have a requirement to run a set of heavy functions asynchronously at sametime and populate the results in a list. Here is the pseudo code for this :

List<TResult> results = new List<TResults>();
List<Func<T, TResult>> tasks = PopulateTasks();

foreach(var task in tasks)
{
    // Run Logic in question
    1. Run each task asynchronously/parallely
    2. Put the results in the results list upon each task completion
}

Console.WriteLine("All tasks completed and results populated");

I need the logic inside the foreach bock. Can you guys plz help me?

I have some constraint : The solution must be .net 3.5 compliant (not .net 4, but a .net 4 alternative solution would be appreciated for my knowledge purpose)

Thanks in advance.

Answer 1

A simple 3.5 implementation could look like this

List<TResult> results = new List<TResults>();
List<Func<T, TResult>> tasks = PopulateTasks();

ManualResetEvent waitHandle = new ManualResetEvent(false);
void RunTasks()
{
    int i = 0;
    foreach(var task in tasks)
    {
        int captured = i++;
        ThreadPool.QueueUserWorkItem(state => RunTask(task, captured))
    }

    waitHandle.WaitOne();

    Console.WriteLine("All tasks completed and results populated");
}

private int counter;
private readonly object listLock = new object();
void RunTask(Func<T, TResult> task, int index)
{
    var res = task(...); //You haven't specified where the parameter comes from
    lock (listLock )
    {
       results[index] = res;
    }
    if (InterLocked.Increment(ref counter) == tasks.Count)
        waitHandle.Set();
}