6.2 Parallelism: locks, C#

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//6.2 Parallelism: locks

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;
using System.Threading;

namespace Rextester
{
    public class Program
    {
        static object _list_lock = new object();
        public static void Main(string[] args)
        {
            List<string> list = new List<string>() {"one", "two"};
                
            //Documentation on List<T> says that it's not thread-safe meaning you can't read/write to it from multiple threads (or undefined bahaviour may occur)
            //Therefore we must use locks. Locks are special mechanism so that only one thread can aquire lock at a time. Locks use variables to lock on 
            //(and it's best to use something like private static object variable named clearly what data structure it's guarding, like shown here).
            ThreadPool.QueueUserWorkItem(f =>
                                         {
                                             lock(_list_lock)   //this statement will block if some other thread has aquired the lock and haven't yet released
                                             {
                                                 list.Add("four"); //now we have aquired a lock and shall release it only when execution will get out of this scope                                           
                                             }
                                             
                                         });
            ThreadPool.QueueUserWorkItem(f =>
                                         {
                                             lock(_list_lock)
                                             {
                                                 list.Add("five");
                                             }
                                         });
            
            lock(_list_lock)
            {
                list.ForEach(Console.WriteLine);  //even iteration is not thread-safe while some other thread is writing to list, so we use lock here too
            }                 
            
            Thread.Sleep(3000); //let's wait for other threads to end, 
                                //although it's not garanteed that they actually will terminate after 3 seconds and not even garanteed that they would start by then.
                                //We have queued them for execution and they will be executed as soon as possible, but on an idle system 3 sec is more than enough
            
            lock(_list_lock)
            {
                list.ForEach(Console.WriteLine);
            }  
        }
    }
}

Show compiler warnings [ + ] Show input
Compilation time: 0.12 sec, absolute running time: 3.1 sec, cpu time: 0.08 sec, average memory usage: 22 Mb, average nr of threads: 8