Need to understand the relation between ManualResetEvent and thread

Question

please help me to understand the below code
what is resetEvent.Set() and WaitOne() ? what it does ?
var resetEvent = new ManualResetEvent(false);
ThreadPool.QueueUserWorkItem(
arg =>
DoWork();
resetEvent.Set();
resetEvent.WaitOne();
again help me to understand the below code
var events = new List<ManualResetEvent>();
foreach(var job in jobs)
var resetEvent = new ManualResetEvent(false);
ThreadPool.QueueUserWorkItem(
arg =>
DoWork(job);
resetEvent.Set();
events.Add(resetEvent);
WaitHandle.WaitAll(events.ToArray());
why this line WaitHandle.WaitAll(events.ToArray()); is required ?
why adding ManualResetEvent instance to list ?
looking for discussion. thanks

Answer 1

WaitOne() will cause the current (main) thread to wait until the thread pool thread has called the Set() method. If you don't call the WaitOne() method, the code line below ThreadPool.QueueUserWorkItem(...) will get executed before the delegate that you
pass do the ThreadPool.QueueUserWorkItem method has actually finished its execution because the threads are being executed simultaneously:
var resetEvent = new ManualResetEvent(false);
ThreadPool.QueueUserWorkItem(
arg =>
//this will be executed simultaneously on thread B
Thread.Sleep(10000);
//tell thread A to continue
resetEvent.Set();
//thread A waits here until thread B calls the Set() method...
resetEvent.WaitOne();
string s = "abc"; //this line won't get executed until thread B has called the Set() method provided that the WaitOne() method is called by thread A above.
>>why this line WaitHandle.WaitAll(events.ToArray()); is required ?
It causes the main thread to wait until the Set method of all ManualResetEvent objects in the events list has been called. Othwerwise the execution of the main thread would continue immediately after the foreach loop has been executed. By this time the DoWork
method of all jobs has not yet been completed because they are being executed on another thread.
WaitHandle.WaitAll(events.ToArray());
string s = "abc"; //this line won't get executed until all the Set() method of all ManualResetEvent objects in events has been called
>>why adding ManualResetEvent instance to list ?
Because you (appearantly) want to wait for all objects in the list, i.e. you want to wait for the DoWork method to complete for all jobs before you continue the execution of the main thread (and go on to assign the string s to the value "abc"
in the above example).
Hope that helps.
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