Confused about multi-threading in a loop for C# [duplicate]

Question

Possible Duplicate:
C# Captured Variable In Loop

I am pretty new to multi-threading programming. When I ran the code below and only the last child got executed. Can some one tell me what happened? Thank you very much.

private void Process()
{
    Dictionary<int, int> dataDict = new Dictionary<int, int>();
    dataDict.Add(1, 2000);
    dataDict.Add(2, 1000);
    dataDict.Add(3, 4000);
    dataDict.Add(4, 3000);

    foreach (KeyValuePair<int, int> kvp in dataDict)
    {
        Console.WriteLine("Ready for [" + kvp.Key.ToString() + "]");
        Task.Factory.StartNew(() => DoSomething(kvp.Value, kvp.Key));
    }

private static void DoSomething(int waitTime, int childID)
{
    {               
        Console.WriteLine("Start task [" + childID.ToString() + "]");
        Thread.Sleep(waitTime);
        Console.WriteLine("End task [" + childID.ToString() + "]");
    }
}

Output

Ready for [1]
Ready for [2]
Ready for [3]
Ready for [4]
Start task [4]
Start task [4]
Start task [4]
Start task [4]
End task [4]
End task [4]
End task [4]
End task [4]

Botz3000 · Accepted Answer · 2012-03-08 07:39:43Z

12

By using the loop variable in your lambda, all of the effectively refer to the same variable, which is the last item of your dictionary at the time they run.

You need to assign the loop variable to another variable local to the loop before passing it to a lambda. Do this:

foreach (KeyValuePair<int, int> kvp in dataDict)
{
    var pair = kvp;
    Console.WriteLine("Ready for [" + pair.Key.ToString() + "]");
    Task.Factory.StartNew(() => DoSomething(pair.Value, pair.Key));
}

EDIT: It seems this little pitfall is fixed in C#5. That's why it might work for others ;) See comment by labroo

edited Mar 8, 2012 at 7:39

answered Mar 8, 2012 at 7:14

Botz3000

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9 Comments

Nadir Muzaffar Over a year ago

people should really explain why they downvoted, seems like a reasonable explanation to me

labroo Over a year ago

Why the downvote, I dont know if this is the solution, but it is a valid change....... blogs.msdn.com/b/ericlippert/archive/2009/11/12/…

Botz3000 Over a year ago

Was wondering, too. Can't see anything wrong with my answer.

Nadir Muzaffar Over a year ago

btw, what Botz3000's answer is implying is that all of the threads run...however, by the time they get the chance to run, kvp == 4

Dreteh Over a year ago

I figured that too, but I just don't understand why.

|

saintedlama · Accepted Answer · 2012-03-08 07:18:31Z

1

You can prevent that behavior by assigning kvp to a local variable in the for loop and pass the variables fields Key and Value to the DoSomething method.

answered Mar 8, 2012 at 7:18

saintedlama

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1 Comment

Dreteh Over a year ago

Thanks. I figured that too, but I just don't understand why.

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