DZone
Thanks for visiting DZone today,
Edit Profile
  • Manage Email Subscriptions
  • How to Post to DZone
  • Article Submission Guidelines
Sign Out View Profile
  • Post an Article
  • Manage My Drafts
Over 2 million developers have joined DZone.
Log In / Join
  • Refcardz
  • Trend Reports
  • Webinars
  • Zones
  • |
    • Agile
    • AI
    • Big Data
    • Cloud
    • Database
    • DevOps
    • Integration
    • IoT
    • Java
    • Microservices
    • Open Source
    • Performance
    • Security
    • Web Dev
DZone >

ConcurrentDIctionary.GetOrAdd may call the valueFactory method more than once

Oren Eini user avatar by
Oren Eini
·
Apr. 06, 11 · · News
Like (0)
Save
Tweet
7.80K Views

Join the DZone community and get the full member experience.

Join For Free

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.

Snippet (programming) Lock (computer science) Implementation

Published at DZone with permission of Oren Eini, DZone MVB. See the original article here.

Opinions expressed by DZone contributors are their own.

Popular on DZone

  • Is Java Still Relevant?
  • Your Old Laptop Is Your New Database Server
  • How to Submit a Post to DZone
  • Password Authentication: How to Correctly Do It

Comments

Partner Resources

X

ABOUT US

  • About DZone
  • Send feedback
  • Careers
  • Sitemap

ADVERTISE

  • Advertise with DZone

CONTRIBUTE ON DZONE

  • Article Submission Guidelines
  • MVB Program
  • Become a Contributor
  • Visit the Writers' Zone

LEGAL

  • Terms of Service
  • Privacy Policy

CONTACT US

  • 600 Park Offices Drive
  • Suite 300
  • Durham, NC 27709
  • support@dzone.com
  • +1 (919) 678-0300

Let's be friends:

DZone.com is powered by 

AnswerHub logo