inconsistent variable values in C# multithreading -

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• captured variable in loop in c# 6 answers

i trying learn how use multithreading in c#. working this tutorial, explains using lambda expressions pass arguments. built toy program test this, confused output.

here code:

using system; using system.threading;  namespace multithreadingapplication {    class threadcreationprogram    {       public static void calltochildthread(int id)       {          console.writeline(string.format("child thread {0} starts", id));       }        static void main(string[] args)       {         (int i=0; i<10; i++)         {             console.writeline(string.format("i: {0}", i));             thread childthread = new thread( () => calltochildthread (i) );              childthread.start();         }       }    } } 

and here output:

i: 0 i: 1 child thread 1 starts i: 2 child thread 2 starts i: 3 child thread 3 starts i: 4 child thread 4 starts child thread 5 starts i: 5 i: 6 child thread 6 starts i: 7 i: 8 child thread 7 starts i: 9 child thread 9 starts child thread 8 starts child thread 10 starts 

the child function prints out value of id passed. expected these range 0 9, calltochildthread instead showing 1 10. can please explain why?

i ran program , got following result:

i: 0 i: 1 i: 2 child thread 2 starts i: 3 child thread 2 starts child thread 3 starts i: 4 child thread 4 starts child thread 4 starts i: 5 i: 6 i: 7 child thread 7 starts child thread 7 starts i: 8 i: 9 child thread 9 starts child thread 9 starts child thread 10 starts 

this nicely demonstrates 1 of problems of multi-threading: shared variables. in line:

thread childthread = new thread( () => calltochildthread (i) ); 

you assume create lambda current value of i. don't. create lambda reference i, loop variable. when child thread reaches beginning of calltochildthread (which might happen @ later time), value of i evaluated , copied local variable id.

the fix easy:

int _i = i; thread childthread = new thread(() => calltochildthread(_i)); 

this yield child threads 0 9.