Thread parameters being changed

Question

When starting multiple threads, the id parameter I'm parsing is sometimes wrong. Here is my startup:

for (int i = 0; i < _threadCount; i++)
{
    Thread thread = new Thread(() => WorkerThread(i));
    thread.Start();
    _threads.Add(thread);
}

And my thread function:

private void WorkerThread(int id)
{
    Console.WriteLine("[{0}] Thread started {1}", DateTime.Now.ToLongTimeString(), id);
}

The output of this code is:

[19:10:54] Thread start 3
[19:10:54] Thread start 9
[19:10:54] Thread start 4
[19:10:54] Thread start 12
[19:10:54] Thread start 11
[19:10:54] Thread start 3
[19:10:54] Thread start 12
[19:10:54] Thread start 6
[19:10:54] Thread start 9
[19:10:54] Thread start 6
[19:10:54] Thread start 13
[19:10:54] Thread start 2
[19:10:54] Thread start 15
[19:10:54] Thread start 9
[19:10:54] Thread start 15

Where in my mind, this code should create every thread with a unique id instead of duplicates as seen above.

Compiler info:

Platform target: x64

Target Framework: .NET Framework 4.5

Answer 1

You should be careful about accidentally modifying captured variables like i after starting the thread, because the i is shared. The i variable refers to the same memory location throughout the loop’s lifetime. The solution is to use a temporary variable like this:

for (int i = 0; i < _threadCount; i++)
{
      var i1 = i;
      Thread thread = new Thread(() => WorkerThread(i1));
      thread.Start();
      _threads.Add(thread);
}

Read more about Closures here : The Beauty of Closures from (Jon Skeet) and Lambda expressions and captured variables from (Joseph Albahari).