inconsistent variable values in C# multithreading

Question

I am trying to learn how to use multithreading in C#. I am working from this tutorial, which explains using lambda expressions to pass arguments. I built a toy program to test this, but am confused by the output.

Here is my 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)
      {
        for (int i=0; i<10; i++)
        {
            Console.WriteLine(string.Format("i: {0}", i));
            Thread childThread = new Thread( () => CallToChildThread (i) );
             childThread.Start();
        }
      }
   }
}

And here is the 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 only prints out the value of the id it is passed. I expected these to range from 0 to 9, but CallToChildThread is instead showing 1 to 10. Can someone please explain why?

This has been asked several times. I've marked your question as duplicate. Search with the term "Closing over the loop variable" — Sriram Sakthivel, Apr 22 '15 at 14:13
@SriramSakthivel - thanks for the information. The main problem was knowing what to search for. Even now that you mention the right terms, "Closing over the loop variable" is not obvious to me. — Roberto, Apr 22 '15 at 14:18

score 2 · Accepted Answer · answered Apr 22 '15 at 14:12

I just ran the program and got the 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 one of the problems of multi-threading: Shared variables. In this line:

Thread childThread = new Thread( () => CallToChildThread (i) );

you would assume that you create a lambda with the current value of i. You don't. You create a lambda with a reference to i, the loop variable. Only when your child thread reaches the beginning of CallToChildThread (which might happen at some later time), the value of i is evaluated and copied to the local variable id.

The fix is easy:

int _i = i;
Thread childThread = new Thread(() => CallToChildThread(_i));

This will yield child threads 0 to 9.

inconsistent variable values in C# multithreading

1 Answers1