Question

I wrote a simple multi-threading snippet just to get myself used to this concept.

public void testThread(int arg1, ConsoleColor color)
    {
        for (int i = 0; i < 1000; i++)
        {
            Console.ForegroundColor = color;
            Console.WriteLine("Thread " + color.ToString() + " : " + i);
            Console.ResetColor();
            Thread.Sleep(arg1);
        }
    }

Thread t1 = new Thread(() => program.testThread(1000, ConsoleColor.Blue));
Thread t2 = new Thread(() => program.testThread(1000, ConsoleColor.Red));
t1.Start();
t2.Start();
t1.Join();
t2.Join();

What I saw in my output console window is

I just don’t understand why sometimes the thread decorated with Red color could change itself to white or light gray color (whatsoever). Could you please help enlighten this mind?

Thanks in advance.

Answer 1

Your code block is not atomic. This means that the two threads can intertwine. Example:

   Thread 1 (Red)               Thread 2 (Blue)
--------------------------------------------------------------
                                 Set Color Blue
   Set Color Red         
                                 Print Text (printed in Red!)
   Print Text (printed in Red!)

If you want the threads’ actions to be atomic, i.e. uninterruptable, you need to use locking:

private static readonly object myLock = new object();

public void testThread(int arg1, ConsoleColor color)
{
    for (int i = 0; i < 1000; i++)
    {
        lock (myLock) {
            Console.ForegroundColor = color;
            Console.WriteLine("Thread " + color.ToString() + " : " + i);
            Console.ResetColor();
        }
        Thread.Sleep(arg1);
    }
}

The lock statement ensures that only one of the threads in in the critical section at any given time:

   Thread 1 (Red)               Thread 2 (Blue)
--------------------------------------------------------------
                                 Enter critical section (lock)
   wait...                       Set Color Blue
                                 Print Text (printed in Red!)
                                 Reset Color
                                 Leave critical section
   Enter critical section (lock)
   ...