Over a million developers have joined DZone.

ConcurrentDIctionary.GetOrAdd may call the valueFactory method more than once

·

When you assume, you are making an ass out of yourself, you stupid moronic idiot with no sense whatsoever. The ass in question, if anyone cares to think about it, is yours truly.

Let us take a look at the following code snippet:

var concurentDictionary = new ConcurrentDictionary<int, int>();
var w = new ManualResetEvent(false);
int timedCalled = 0;
var threads = new List<Thread>();
for (int i = 0; i < Environment.ProcessorCount; i++)
{
    threads.Add(new Thread(() =>
    {
        w.WaitOne();
        concurentDictionary.GetOrAdd(1, i1 =>
        {
            Interlocked.Increment(ref timedCalled);
            return 1;
        });
    }));
    threads.Last().Start();
}

w.Set();//release all threads to start at the same time
foreach (var thread in threads)
{
    thread.Join();
}

Console.WriteLine(timedCalled);

What would you say would be the output of this code?

Well, I assumes that it would behave in an atomic fashion, that the implementation is something like:

if(TryGetValue(key, out value))
   return value;

lock(this)
{
   if(TryGetValue(key, out value))
      return value;

   AddValue( key, valueFactory());
}

Of course, the whole point of the ConcurentDictionary is that there are no locks. Well, that is nice, except that because I assumed that the call is only made once, I called that with a function that had side effects when called twice.

That was a pure hell to figure out, because in my mind, of course that there was no error with this function.

Topics:

Published at DZone with permission of Ayende Rahien, DZone MVB. See the original article here.

Opinions expressed by DZone contributors are their own.

The best of DZone straight to your inbox.

SEE AN EXAMPLE
Please provide a valid email address.

Thanks for subscribing!

Awesome! Check your inbox to verify your email so you can start receiving the latest in tech news and resources.
Subscribe

{{ parent.title || parent.header.title}}

{{ parent.tldr }}

{{ parent.urlSource.name }}