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copied!<p>It looks like you need <a href="http://msdn.microsoft.com/en-us/library/system.threading.monitor.wait" rel="noreferrer">Monitor.Wait</a> and <a href="http://msdn.microsoft.com/en-us/library/system.threading.monitor.pulse" rel="noreferrer">Monitor.Pulse</a>. There's a <a href="http://www.albahari.com/threading/part4.aspx#_How_to_Use_Wait_and_Pulse" rel="noreferrer">free eBook</a> out there on Threading (there are probably many but this one helped me).</p> <p>You can use a static object to lock on, and then have your threads call <code>Monitor.Pulse</code> to signal that they are "done with their turn" and <code>Monitor.Wait</code> to "wait for their next turn". Here's an example implementation using your basic code:</p> <pre><code>public class Class1 { // Use this to syncrhonize threads private static object SyncRoot = new object(); // First "turn" goes to thread 1 private static int threadInControl = 1; public static void thread1() { lock(SyncRoot) // Request exclusive access to SyncRoot { Console.WriteLine("1"); GiveTurnTo(2); // Let thread 2 have a turn WaitTurn(1); // Wait for turn to be given to thread 1 Console.WriteLine("t2 has printed 1, so we now print 2"); GiveTurnTo(2); // Let thread 2 have a turn WaitTurn(1); // Wait for turn to be given to thread 1 Console.WriteLine("t2 has printed 2, so we now print 3"); GiveTurnTo(2); // Let thread 2 have a turn } } public static void thread2() { lock(SyncRoot) // Request exclusive access to SyncRoot { WaitTurn(2); // Wait for turn to be given to thread 2 Console.WriteLine("t1 has printed 1, so we now print 1"); GiveTurnTo(1); // Let thread 1 have a turn WaitTurn(2); // Wait for turn to be given to thread 2 Console.WriteLine("t1 has printed 2, so we now print 2"); GiveTurnTo(1); // Let thread 1 have a turn WaitTurn(2); // Wait for turn to be given to thread 2 Console.WriteLine("t1 has printed 3, so we now print 3"); GiveTurnTo(1); // Let thread 1 have a turn } } // Wait for turn to use SyncRoot object public static void WaitTurn(int threadNum) { // While( not this threads turn ) while (threadInControl != threadNum) { // "Let go" of lock on SyncRoot and wait utill // someone finishes their turn with it Monitor.Wait(SyncRoot); } } // Pass turn over to other thread public static void GiveTurnTo(int nextThreadNum) { threadInControl = nextThreadNum; // Notify waiting threads that it's someone else's turn Monitor.Pulse(SyncRoot); } public static void void Main() { Thread t1 = new Thread(new ThreadStart(() => Class1.thread1())); Thread t2 = new Thread(new ThreadStart(() => Class1.thread2())); t1.Start(); t2.Start(); t2.Join(); t1.Join(); } } </code></pre> <p>As for using the <a href="http://msdn.microsoft.com/en-us/library/c5kehkcz%28v=VS.71%29.aspx" rel="noreferrer">lock keyword</a>, it is not limited to synchronization within the same function. Lock "guarantees" exclusive access to a resource (object) to a single thread (by exclusive, I mean that only one thread can obtain a lock on that resource at a time, locking does not prevent other threads from simply accessing the object itself).</p> <p>To simplify it, <code>lock(someObject)</code> is like a thread getting in line to use <code>someOject</code> and then waiting until all other threads in front of it have finished their turn before continuing. A thread ends its "turn" when it leaves the scope of the lock statement (unless you add things like <code>Monitor.Pulse</code> or <code>Monitor.Wait</code>).</p>

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