.NET 4.5 - C# - variable with ThreadStatic attribute in for loop

Question

4.00/5 (1 vote)

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I'm testing the ThreadStatic attribute in a simple class.
Here's the code:

C#

public class TestClass
    {
        [ThreadStatic ( )] 
        static int reps = 0;

        [ThreadStatic ( )]
        static int count = 20;

        public void Run ( )
        {
            var t1 = new Task ( ( ) =>
                {
                    for ( int i = 0; i < count; i++ )
                    {
                        Console.WriteLine ( "Thread 1:" + reps++ );
                        Thread.Sleep ( 0 );
                    }
                } );

            var t2 = new Task ( ( ) =>
            {
                for ( int i = 0; i < 20; i++ )
                {
                    Console.WriteLine ( "Thread 2:" + reps++ );
                    Thread.Sleep ( 0 );
                }
            } );



            t1.Start ( );
            t2.Start ( );

            t1.ContinueWith ( ( e ) => 
            { 
                Console.WriteLine ( e.Status ); 
            }, TaskContinuationOptions.OnlyOnRanToCompletion );
            
            t2.ContinueWith ( ( e ) => 
            {
                Console.WriteLine ( e.Status );
            }, TaskContinuationOptions.OnlyOnRanToCompletion );
        }
    }

Here's the output:

Thread 2:0
Thread 2:1
Thread 2:2
Thread 2:3
Thread 2:4
Thread 2:5
Thread 2:6
Thread 2:7
Thread 2:8
Thread 2:9
Thread 2:10
Thread 2:11
Thread 2:12
Thread 2:13
Thread 2:14
Thread 2:15
Thread 2:16
Thread 2:17
Thread 2:18
Thread 2:19
RanToCompletion
RanToCompletion

As you can see only the 2nd Task outputs something. The only difference between the two Tasks is that in the first one I use a static int variable marked with the ThreadStatic attribute to calculate the exit condition of the for loop.

Why the first Task does not output the values?

Thanks

Posted 7-Aug-15 2:45am

Mattia Locatelli

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Comments

Sergey Alexandrovich Kryukov 7-Aug-15 10:40am

This is a nice educative experiment, but you would easily add more output and dig out what's going on by yourself. Nevertheless, I up-voted your question by 4.
—SA

2 solutions

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Sergey Alexandrovich Kryukov · Answer 1 · 2015-08-07T04:38:00

This is because count is thread-static, one task is using this value, and another one uses hard-coded immediate constant 20 (bad by itself). A thread-static field becomes not static, the attribute ThreadStaticAttribute makes it behave as two or more different static fields per each thread, which are, importantly, are initialized separately in each thread. Its initial value becomes 0, so the loop using it will undergo zero number of iterations. To see it, if would be enough to output count before each loop.

Please see: https://msdn.microsoft.com/en-us/library/system.threadstaticattribute%28v=vs.110%29.aspx[^].

Note that having such field would be pointless, because this is constant, so its value can be used in two thread. To make both tasks' loops work, you would need to declare

C#

const count = 20; // without the attribute

and remove hard-coded 20 from other places.

Note that your experiment is not 100% correct, because tasks are not exactly threads.

—SA

Mattia Locatelli · Answer 2 · 2015-08-07T02:58:00

Solution 1

Find out by myself looking on MSDN.

https://msdn.microsoft.com/en-us/library/system.threadstaticattribute.aspx[^]

Remainder for me: RTFM!

Posted 7-Aug-15 2:58am