多线程中的循环界限

Question

I have a main thread that makes some other threads in two nested for.

private void mainthread()
        {
            List<Thread> ts= new List<Thread>();

            for (int w=0; w<7; w+=2)
                for (int h = 0; h < 5; h+=3)
                {
                    Thread t = new Thread(delegate() { otherthreads(w, h); });
                    ts.Add(t);
                    t.Start();
                }
            for (int i = 0; i < ts.Count; i++)
                ts[i].Join();
        }

        private void otherthreads(int w, int h)
        {                    
            listBox1.Invoke(new singleparam(addtolistbox), new object[] { "w:" + w.ToString() + ",h:" + h.ToString() });        
        }

Each thread adds it's input arguments to a Listbox. I am confused why the input arguments of some threads are not in the for bounds?

Answer 1

Your loop is running correctly, but what is happening is this: the delegate knows that it must pass w and h onto the otherthreads() function, but those values are not bound until it is actually invoked. In other words, prior to the delegate actually executing, it just knows it must use w and h . On your last iteration, you are asking for the delegate to execute, but before it can, w and h increment for the final time on the initiating thread, causing their values to be 8 and 6, respectively. The loops exit. Then, picomoments later, the delegate executes and NOW has the values of w and h ... but the values are now 8 and 6.

You can avoid this by "snapshotting" w and h with local variables to the tightest scope around the delegate and assigning their values appropriately:

for (int h = 0; h < 5; h+=3) 
{
    int h2=h;
    int w2=w;

    Thread t = new Thread(delegate() { otherthreads(w2, h2); });
    ts.Add(t);
    t.Start();
}