Use Lock Keyword, C#

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//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5

//
//=======================================================
//          Code Examples from Exam Ref 70-483
//=======================================================
//
//AUTHOR: Wouter de Kort
//
//PURPOSE: Synchronizing Resources
//
//SECTION: Objective 1.2 - Chapter 1
//
//MODIFICATION HISTORY
//DATE          NAME                DESCRIPTION
//-----------   ----------------    --------------------
//10/19/2017    Cristian Canedo     Initial commit 
//10/19/2017    Cristian Canedo     Use lock keyword
//=======================================================

using System;
using System.Threading;
using System.Threading.Tasks;

namespace Rextester
{
    public class Program
    {
        ///<summary>
        /// EXPECTED: Ouput will always be 0 because there is no
        /// way one thread can change the value while the other
        /// thread is working with it
        ///</summary>
        public static void Main(string[] args)
        {
            
            int number = 0;
            
            object _lock = new object();
            
            var up = Task.Run(() =>
                              {
                                  for (int i = 0; i < 1000000; i++) {
                                      lock (_lock)
                                          number++;
                                  }
                              });
            
            for (int i = 0; i < 1000000; i++){
                lock (_lock)
                    number--;
            }
            
            
            up.Wait();
            Console.WriteLine(number);
            
        }
    }
}

Show compiler warnings [ + ] Show input
Compilation time: 0,12 sec, absolute running time: 0,13 sec, cpu time: 0,11 sec, average memory usage: 17 Mb, average nr of threads: 3