Thread start passed integer changes itself beyond what it should be

Question

I have no idea what is going on in this. I'm trying to test thread safety of a class by spawning 100 threads to access it constantly, but it seems my anonymous method parameters are changing themselves to values they should never be and I'm confused as to why. As I have no idea what's going on, I'll just post all the functions involved in testing. Somehow I'm ending up with "Thread 98" getting the parameter "num = 100"... That shouldn't be possible and I have no idea what kind of hokey pokey is going on that is changing the integer. (in method "ThreadWriting(int num)" you'll see the point where I check for "num" to equal 100, where I put a break point to catch the offending thread in the act, and it breaks every time. Otherwise it throws an "IndexOutofRangeException" on the array "counts". I'm just trying to see if my threads are generally getting equal access to the class they're all trying to use at once.

    public delegate void TempDel();
    public TempDel InvokeTest;

    public void TRTest3(Form1 sender)
    {
        InvokeTest = new TempDel(UpdateInvoke);

        Thread t = new Thread(() => ConsoleUpdateTest(sender));
        t.IsBackground = true;
        t.Start();

        POConsole.Instance.MaxLines = 20;

        for(int i = 0; i < 100; i++)
        {

            Thread t2 = new Thread(() => ThreadWriting(i));
            t2.IsBackground = true;
            t2.Name = String.Format("Thread {0}", i);
            t2.Start();
        }

    }

    public ulong[] counts = new ulong[100];

    public void ThreadWriting(int num)
    {
        if(num == 100)
        {
            bool stop = true;
        }
        while (true)
        {
            POConsole.Instance.WriteLine("Hello from Thread " + num);
            counts[num]++;
        }
    }

    public void ConsoleUpdateTest(Form1 sender)
    {
        while(true)
        {
            sender.Invoke(InvokeTest);
            Thread.Sleep(5);
        }
    }



    public void UpdateInvoke()
    {
        QuickTestBox.Text = POConsole.Instance.FullFeed;
    }

All my threads are named, as you can see, and none of them receives the name "Thread 100" so I have no idea how one of the other threads could get passed a parameter of 100 or the parameter could be corrupted in some way.

Apparently my thread-safety checking isn't thread safe in some way?

Answer 1

This is a simple closure issue, you should not be using the for loop counter as a threading parameter issue, issue happens out here, for loop and thread execution do not run at same speed, so value of i can change for multiple threads:

for(int i = 0; i < 100; i++)
        {

            Thread t2 = new Thread(() => ThreadWriting(i));
            t2.IsBackground = true;
            t2.Name = String.Format("Thread {0}", i);
            t2.Start();
        }

Use following modification, create a local variable from loop counter

for(int i = 0; i < 100; i++)
        {
            int j = i;
            Thread t2 = new Thread(() => ThreadWriting(j));
            t2.IsBackground = true;
            t2.Name = String.Format("Thread {0}", j);
            t2.Start();
        }