Solved

Threadpool arguments have strange values

Posted on 2012-04-04
3
319 Views
Last Modified: 2012-04-05
Hi,
I cannot figure out what I am doing wrong in my code below, I would expect a print of "1" then "2" but instead I get "3" and "3", please help.


        private void bTest_Click(object sender, EventArgs e)
        {

            for (int i = 1; i <= 2; i++)
            {
      XX xx = new XX();

                ThreadPool.QueueUserWorkItem(o =>
                    {
                        try
                        {
                            xx.SendMsg(i);
                        }
                        catch (Exception ex)
                        {
                            MessageBox.Show(ex.Message);
                        }
                    });                                
            }
        }

        public class XX
        {
            public void SendMsg(object n)
            {
                System.Diagnostics.Debug.Print( n.ToString());                
            }
        }
0
Comment
Question by:Alw1n
[X]
Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

  • Help others & share knowledge
  • Earn cash & points
  • Learn & ask questions
  • 2
3 Comments
 
LVL 75

Accepted Solution

by:
käµfm³d   👽 earned 500 total points
ID: 37809258
I just happened to read about this very scenario--though I cannot remember where, for the life of me  : \

Your problem is that the compiler will create a closure around the variable "i" since you passed it to your lambda. I don't know all the ins and outs of closures, but it basically means your lambda will see all of the updates to "i" that the for loop makes (depending on the timing of each thread). To rectify, introduce a new local variable and assign i's value to that. Then pass this new variable to the lambda.

e.g.

for (int i = 1; i <= 2; i++)
{
    XX xx = new XX();
    int x = i;

    ThreadPool.QueueUserWorkItem(o =>
    {
        try
        {
            xx.SendMsg(x);
        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.Message);
        }
    });
}

Open in new window

0
 

Author Closing Comment

by:Alw1n
ID: 37809673
Good answer!  I don't know about 'Closures' & will need to check that out. Out of interest, this situation only ocurrs when using lambda, the code below works fine:

ThreadPool.QueueUserWorkItem(new WaitCallback( xx.SendMsg), i);

The only benefit I see to using lambda is that you can use try/catch blocks etc. around the call, would there be any other reason to use lambda vs the original way?
0
 
LVL 75

Expert Comment

by:käµfm³d 👽
ID: 37810405
You can use try/catch within a regular function as well--you just don't see them until you actually navigate to that function (within the IDE). If there is a benefit to using one over the other, I don't know what it would be. To me it seems a matter of preference. Either way you are passing a function.
0

Featured Post

Space-Age Communications Transitions to DevOps

ViaSat, a global provider of satellite and wireless communications, securely connects businesses, governments, and organizations to the Internet. Learn how ViaSat’s Network Solutions Engineer, drove the transition from a traditional network support to a DevOps-centric model.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Suggested Solutions

It seems a simple enough task, yet I see repeated questions asking how to do it: how to pass data between two forms. In this article, I will show you the different mechanisms available for you to do just that. This article is directed towards the .N…
Many of us here at EE write code. Many of us write exceptional code; just as many of us write exception-prone code. As we all should know, exceptions are a mechanism for handling errors which are typically out of our control. From database errors, t…
Finds all prime numbers in a range requested and places them in a public primes() array. I've demostrated a template size of 30 (2 * 3 * 5) but larger templates can be built such 210  (2 * 3 * 5 * 7) or 2310  (2 * 3 * 5 * 7 * 11). The larger templa…
In a recent question (https://www.experts-exchange.com/questions/29004105/Run-AutoHotkey-script-directly-from-Notepad.html) here at Experts Exchange, a member asked how to run an AutoHotkey script (.AHK) directly from Notepad++ (aka NPP). This video…

730 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question