multithreading - Threading in C#: How to keep loop-index thread-safe?

Question

In the following example, it seems that the index i of the for-loop is modified independently by each thread leading to strange values of i (multiples, even values larger than System.Environment.ProcessorCount-1) in DoWork_Threaded()

How are multithreaded loops properly done in C# ?

// Prepare all threads
Thread[] threads = new Thread[System.Environment.ProcessorCount];

// Start all threads
for (int i = 0; i < System.Environment.ProcessorCount; i++)
{
   threads[i] = new Thread(() => DoWork_Threaded(i));
   threads[i].Start();
}

// Wait for completion of all threads
for (int i = 0; i < System.Environment.ProcessorCount; i++)
{
   threads[i].Join();
}

Answer 1

First of all, rather than rolling your own thread scheduling service, why not use the task parallel library? It already has logic that determines the number of processors to schedule threads onto.

To answer your actual question, you are closing over a loop variable. See my article on why that is wrong:

https://ericlippert.com/2009/11/12/closing-over-the-loop-variable-considered-harmful-part-one/

In short: i is a variable and variables change. When your lambda executes, it executes with the current value of i, not the value it used to have when the delegate was created.