Creating an Array of Threads in C#

Question

I have a very strange problem with my code. It will fully run the 1st for loop, then complete the foreach, but then it will skip back to the "ThreadStart IMAPDelegate" (line 1 of the for loop) and then crash because of an ArgumentOutOfRangeException. Can someone explain why the program is doing this? I debugged it line by line and it literally just skips back up into the a line in the for loop. If it had normally run the for loop again, it would have set x back to 0 and it would not have crashed. Any suggestions?

for (int x = 0; x < UserInfo.Count; x++)
{
    ThreadStart IMAPDelegate = delegate{SendParams(UserInfo[x], IMAPServers[x]); };
    MyThreads.Add(new Thread(IMAPDelegate));
}

foreach (Thread thread in MyThreads)
{
    thread.Start();
}

Hans Passant · Accepted Answer · 2010-12-30 16:17:09Z

5

This is by design when you use an anonymous method like that. As soon as the thread starts running, it executes the SendParams() method call. Which then bombs because the "x" variable is already incremented beyond UserInfo.Count. Fix:

for (int x = 0; x < UserInfo.Count; x++)
{
    int user = x;
    ThreadStart IMAPDelegate = delegate{SendParams(UserInfo[user], IMAPServers[user]); };
    MyThreads.Add(new Thread(IMAPDelegate));
}

answered Dec 30, 2010 at 16:17

Hans Passant

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10 Comments

Tim Lloyd Over a year ago

+1 Otherwise know as "Closing Over the Loop Variable":(blogs.msdn.com/b/ericlippert/archive/2009/11/12/….

Franz Payer Over a year ago

When when UserInfo and MyTheads both have more than 1 element, it gives me the error. Count = Function evaluation disabled because a previous function evaluation timed out. You must continue execution to reenable function evaluation.

Hans Passant Over a year ago

That's not an error, just the debugger complaining that it temporarily lost its marbles. That happens. There's a hotfix for debugger problems with threads when you use VS2008. Or press F5. support.microsoft.com/kb/957912

Franz Payer Over a year ago

Now only the 1st thread in the array will start, all others will not.

Hans Passant Over a year ago

Hmya, threads don't just refuse to start when you call their Start() method. I'm going to guess that anonymous methods really confuse the stuffing out of you. There's no compelling reason to use them here, this will work just as well when you use a little private helper method as the ThreadStart delegate target.

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