C# Threadpool Synchronization Inquiry

Question

I am running into a bit of difficulty using the threadpool class and an array of ManualResetEvents. Below is a simple example of what I am doing. The problem is that in the DoWork method I am getting null references to the resetEvent[param as int] object.

Can't seem to figure what I'm doing wrong.

(edit: got the code block working)

private static volatile ManualResetEvent[] resetEvents = new ManualResetEvent[NumThreads]; public void UpdateServerData() { for (int i = 0; i < NumThreads ; i++) { resetEvents[i] = new ManualResetEvent(false); ThreadPool.QueueUserWorkItem(new WaitCallback(DoWork), (object) i); } WaitHandle.WaitAll(resetEvents); } private void DoWork(object param) { //do some random work resetEvents[(int)param].Set(); }

EDIT: I have tried inserting a System.Threading.Thread.MemoryBarrier(); after each .Set() however i still get a null reference exception.

I had done multiple edits to get the code block working. If anyone saw the earlier iterations, excuse the mess that it was. — Setheron
– Setheron, Commented Aug 20, 2009 at 15:12
I've also tried to lock every .Set() call because that should cause a volatile read/write of the object however that doesn't seem to work either. Very frustrating. — Setheron
– Setheron, Commented Aug 20, 2009 at 15:59
Why on earth do you issue a memory barrier after the Set()? You need to see the updated array element before you call Set() on it! — Anton Tykhyy
– Anton Tykhyy, Commented Aug 21, 2009 at 15:03
As a side note, I hope you're using RegisteredWaitHandle.Unregister after using the thread, as advised by msdn, unlike in the code here provided. — cregox
– cregox, Commented Sep 27, 2012 at 13:13

Fredrik Mörk · Accepted Answer · 2009-08-20 15:24:28Z

You cannot use the as keyword to cast to int (since int is not a reference type). Use (int)param instead:

private void DoWork(object param) { //do some random work resetEvents[(int)param].Set(); }

Another approach that I feel is cleaner is to pass the wait handle to the method instead:

public void UpdateServerData() { for (int i = 0; i < NumThreads ; i++) { resetEvents[i] = new ManualResetEvent(false); ThreadPool.QueueUserWorkItem(new WaitCallback(DoWork), resetEvents[i]); } WaitHandle.WaitAll(resetEvents); } private void DoWork(object param) { //do some random work (param as ManualResetEvent).Set(); }

That way the worker method has no knowledge about how the wait handle is managed on the outside; and it also cannot reach wait handles for other threads by mistake.

I just quickly wrote a simple example. I didn't mean for it to be 100% syntatically correct but I will fix the example
dont think you meant to use the param as the indexer....should be able to just do (param as ManualResetEvent).Set();
@CSharpAtl: yes, I noticed that. My code died the copy/paste death, I guess ;o)

Anton Tykhyy · Accepted Answer · 2009-08-20 16:47:59Z

volatile ManualResetEvent[] doesn't mean that access to array elements follows the volatile semantics. Only access to the variable holding the reference to the array will be volatile. Try inserting a memory barrier after assigning the array element, or using Thread.VolatileWrite to set them, e.g.

Thread.VolatileWrite (ref resetEvents[i], new ManualResetEvent (false)) ;

Oh. I thought assigning volatile to the array causes the members to inhert the keyword. I'm not familiar with the other two ways you mentioned (memory barrier and Interlocked.Exchange)
Actually Interlocked.Xxx isn't a good idea here, since you don't need to read-and-update your data.

Setheron · Accepted Answer · 2009-08-20 18:03:21Z

Pretty much I found the issue was in

for (int i = 0; i < NumThreads ; i++) { resetEvents[i] = new ManualResetEvent(false); ThreadPool.QueueUserWorkItem(new WaitCallback(DoWork), resetEvents[i]); }

Instead of declaring a new ManualResetEvent I simply called Reset(). The issue seemed to have been that that although I would use MemoryBarrier or locks, the physical memory wouldn't be updated yet so it would point to null.

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