linq - Why decrementing one variable modifies incrementing of another one in C# Parallel.ForEach loop? -

running code parallel.foreach keeps spawning new threads few modifications

the output commented line:

//threadsremaining = interlocked.decrement(ref concurrentthreads); 

is "obvious", i.e. expected one:

[00:00] job 0 complete. 2 threads remaining. unsafecount=2 [00:00] job 1 complete. 1 threads remaining. unsafecount=1 [00:00] job 2 complete. 3 threads remaining. unsafecount=3 [00:00] job 3 complete. 4 threads remaining. unsafecount=4 [00:00] job 4 complete. 5 threads remaining. unsafecount=5 [00:00] job 5 complete. 6 threads remaining. unsafecount=6 [00:01] job 6 complete. 7 threads remaining. unsafecount=7 [00:01] job 8 complete. 8 threads remaining. unsafecount=8 [00:01] job 7 complete. 9 threads remaining. unsafecount=9 [00:01] job 9 complete. 10 threads remaining. unsafecount=10 

while output of same code upon uncommenting above line is:

[00:00] job 0 complete. 1 threads remaining. unsafecount=1 [00:00] job 1 complete. 0 threads remaining. unsafecount=0 [00:00] job 3 complete. 0 threads remaining. unsafecount=0 [00:00] job 2 complete. 1 threads remaining. unsafecount=1 [00:00] job 4 complete. 1 threads remaining. unsafecount=1 [00:00] job 5 complete. 1 threads remaining. unsafecount=1 [00:01] job 6 complete. 1 threads remaining. unsafecount=1 [00:01] job 8 complete. 1 threads remaining. unsafecount=1 [00:01] job 9 complete. 1 threads remaining. unsafecount=1 [00:01] job 7 complete. 0 threads remaining. unsafecount=0 

can explain me why decrementing 1 variable threadsremainin stops (or prevents) incrementing 1 unsafecount ?

the code of console app:

using system; using system.collections.generic; using system.linq; using system.threading; using system.threading.tasks;  namespace separallelforeachkeepsspawningnewthreads {   public class node   {     public node previous { get; private set; }     public node(node previous)     {       previous = previous;     }   }   public class program   {     public static void main(string[] args)     {       datetime startmoment = datetime.now;       int concurrentthreads = 0;       int unsafecount = 0;        var jobs = enumerable.range(0, 10);       paralleloptions po = new paralleloptions       {         maxdegreeofparallelism = environment.processorcount       };       parallel.foreach(jobs, po, delegate(int jobnr)       {         int threadsremaining = interlocked.increment(ref concurrentthreads);         unsafecount++;          int heavyness = jobnr % 9;          //give processor , garbage collector do...         list<node> nodes = new list<node>();         node current = null;         //for (int y = 0; y < 1024 * 1024 * heavyness; y++)         (int y = 0; y < 1024 * 4 * heavyness; y++)         {           current = new node(current);           nodes.add(current);         }          timespan elapsed = datetime.now - startmoment; //*****************          //threadsremaining = interlocked.decrement(ref concurrentthreads);         console.writeline("[{0:mm\\:ss}] job {1} complete. {2} threads remaining. unsafecount={2}",            elapsed, jobnr, threadsremaining, unsafecount);       });       console.writeline("finished");       console.readline();     }   } } 

this problem:

console.writeline(     "[{0:mm\\:ss}] job {1} complete. {2} threads remaining. unsafecount={2}",      elapsed, jobnr, threadsremaining, unsafecount); 

the final part should {3}, not {2}. you're printing out threadsremaining twice @ moment...