Solved

Threadpool arguments have strange values

Posted on 2012-04-04
3
322 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

Online Training Solution

Drastically shorten your training time with WalkMe's advanced online training solution that Guides your trainees to action. Forget about retraining and skyrocket knowledge retention rates.

Question has a verified solution.

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

Summary Displaying images in RichTextBox is a common requirement with limited solutions available. Pasting through clipboard or embedding into RTF content only support static images.  This article describes how to insert Windows control objects int…
In my previous two articles we discussed Binary Serialization (http://www.experts-exchange.com/A_4362.html) and XML Serialization (http://www.experts-exchange.com/A_4425.html). In this article we will try to know more about SOAP (Simple Object Acces…
In this video, viewers are given an introduction to using the Windows 10 Snipping Tool, how to quickly locate it when it's needed and also how make it always available with a single click of a mouse button, by pinning it to the Desktop Task Bar. Int…
Michael from AdRem Software outlines event notifications and Automatic Corrective Actions in network monitoring. Automatic Corrective Actions are scripts, which can automatically run upon discovery of a certain undesirable condition in your network.…

690 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