Question

Simple copy pasta here:

static void Main(string[] args)
{
    List<Task> Tasks = new List<Task>();

    Random r = new Random();

    for (int o = 0; o < 5; o++)
        Tasks.Add(Task.Factory.StartNew(() => { int i = r.Next(0, 3000); Thread.Sleep(i); Console.WriteLine("{0}: {1}", o, i); }));

    Task.WaitAll(Tasks.ToArray());

    Console.Read();
}

When you run that, you will get something like this:

5: 98
5: 198
5: 658
5: 1149
5: 1300

What am I not understanding about this? Writing each iteration of o is showing as 5 for all threads when I’d expect to see numbers 0 through 4 in random order.

I tried using an actual method instead of anonymous and it does the same thing. What am I missing?

Edit: I just found the problem with my very first post and edited the question, so sorry if you answered about the improper order problem. However, I am curious as to why o is not writing properly.

Answer 1

() => 
 { 
   int i = r.Next(0, 3000); 
   Thread.Sleep(i); 
   Console.WriteLine("{0}: {1}", o, i); 
 })

Your are closing over your loop variable o with the delegate you use for your task – by the time it is executed your loop has finished and you only get the end value 5 for o. Remember you are creating a closure over the loop variable, not it’s current value – the value is only evaluated when the delegate is executed once the task is started.

You have to create a local copy of the loop variable instead, which you can then use safely:

for (int o = 0; o < 5; o++)
{
   int localO = o;
   Tasks.Add(Task.Factory.StartNew(() => { int i = r.Next(0, 3000); Thread.Sleep(i); Console.WriteLine("{0}: {1}", localO, i); }));
}