Datamismatch when called method using multithreading

Question

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Please find below code snippet and help to resolve it.

Program.cs:

C#

class Program
   {
       static void Main(string[] args)
       {
           for (int i = 0; i < 5; i++)
           {
               Class1 obj = new Class1();
               new Thread(() => obj.Show(i)).Start();
           }

           Console.ReadLine();
       }
   }

Class1:

C#

class Class1
    {
        public void Show(int i)
        {
            Console.WriteLine("Show: " + i);
            Thread.Sleep(i * 100);
        }
    }

Excepted Output:
Show: 0
Show: 1
Show: 2
Show: 3
Show: 4

But due to threading i am not getting result in above way
the result is unpredictable like
Show: 3
Show: 3
Show: 3
Show: 5
Show: 4

Can any one let me know, how to resolve this kind of issue?

Posted 21-May-15 0:37am

indyarock

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3 solutions

Solution 1

Try to change the method Show to this.

C#

public void Show(int i)
{
    Console.WriteLine("Show: " + i);
    Thread.Sleep(i * 100);
    Console.WriteLine("Show: " + i);
}

It makes more sense to put the sleep before the print out.

Posted 21-May-15 1:10am

George Jonsson

Updated 21-May-15 1:13am

v2

Comments

indyarock 21-May-15 9:36am

No, i didn't solve :(

Solution 2

I have tried this and working as per your expecteation.

for (int i = 1; i <= 10; i++)
{
    Class1 obj = new Class1();
    obj.Show(i);
    //new Thread(() => obj.Show(i)).Start();
}
Console.ReadLine();

Posted 21-May-15 2:36am

Pankit Patel

Comments

indyarock 21-May-15 9:37am

i need it with threading!!

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Richard Deeming · Accepted Answer · 2015-05-21T03:34:00

Solution 3

You have captured the loop variable in a closure. That doesn't work as you'd expect:
Closing over the loop variable considered harmful : Eric Lippert’s Blog[^]

In .NET 4.5, this was fixed for foreach loops; however, for loops retain the old behaviour.

To fix it, you need to create a copy of the loop variable, and use that copy in your closure:

C#

for (int i = 0; i < 5; i++)
{
    int iCopy = i;
    Class1 obj = new Class1();
    new Thread(() => obj.Show(iCopy)).Start();
}

Posted 21-May-15 3:34am

Richard Deeming

Comments

indyarock 21-May-15 9:40am

it fixed :)
Thank you

Pankit Patel 22-May-15 0:15am

this will print 0 to 5 but not wait for output...as you mention Thread.Sleep()

Richard Deeming 22-May-15 8:10am

If you look at the question, you'll see that the Main method has a Console.ReadLine(); after this loop. The program will keep running until the user presses the "Enter" key, giving the threads time to complete.

Pankit Patel 22-May-15 8:27am

but when u run this console application it will print 0 to 4 immediate .It will not wait for a second as per statement ( i * 100).
and Console.ReadLine() will always wait for User input then after it will exit.
But main matter is about Threading.

Richard Deeming 22-May-15 8:30am

If you read the original question, you'll see that this is what the OP wanted.

The question is not "how do I get the values to print after the call to Thread.Sleep". The question is "how do I get the correct values to print".

Pankit Patel 22-May-15 9:00am

ok then why he wrote Thread.Sleep(i * 100);

Richard Deeming 22-May-15 9:04am

You'd have to ask him - my psychic powers aren't working at the moment.

Pankit Patel 22-May-15 9:11am

let it go for here.He got solution then it doesn't matter for more discussion.Enjoy ..Thanks...
-PP