c# Thread Synchronization

Introduce a question

Consider the follow code snippet:

class Program
    {
        static int _hours = 0;

        static void Main(string[] args)
        {
            var tasks = new Task[2];
            for (int i = 0; i < tasks.Length; i++)
            {
                tasks[i] = new Task(Work);
                tasks[i].Start();
            }

            for (int i = 0; i < tasks.Length; i++)
            {
                tasks[i].Wait();
            }

            Console.WriteLine(_hours);
        }

        static void Work()
        {
            for (int i = 0; i < 8; i++)
            {
                Thread.Sleep(10);
                _hours += 1;
                Console.WriteLine("task:{0},nth:{1},hours:{2}", Thread.CurrentThread.ManagedThreadId, i, _hours);
            }
        }
    }

The run resut of above code is:

task:4,nth:0,hours:1
task:3,nth:0,hours:2
task:4,nth:1,hours:3
task:3,nth:1,hours:4
task:4,nth:2,hours:6
task:3,nth:2,hours:6
task:4,nth:3,hours:7
task:3,nth:3,hours:7
task:4,nth:4,hours:9
task:3,nth:4,hours:9
task:3,nth:5,hours:10
task:4,nth:5,hours:11
task:3,nth:6,hours:13
task:4,nth:6,hours:13
task:3,nth:7,hours:15
task:4,nth:7,hours:15
15
Press any key to continue . . .

Please observe the values of hours, and it's final value is 15 instead of 16.

From bussiness perspective, each task works 8 hours, so two tasks should work 16 hours. But the result is 15, why?

Solve the question

　　The problem is caused by Thread Synchronization.

　　We can resolve it using Synchronization technique.

　　1. Using Lock

class Program
    {
        static int _hours = 0;
        static object _synRoot = new object();

        static void Main(string[] args)
        {
            var tasks = new Task[2];
            for (int i = 0; i < tasks.Length; i++)
            {
                tasks[i] = new Task(Work);
                tasks[i].Start();
            }

            for (int i = 0; i < tasks.Length; i++)
            {
                tasks[i].Wait();
            }

            Console.WriteLine(_hours);
        }

        static void Work()
        {
            for (int i = 0; i < 8; i++)
            {
                Thread.Sleep(10);
                lock (_synRoot)
                {
                    _hours += 1;
                }
                Console.WriteLine("task:{0},nth:{1},hours:{2}", Thread.CurrentThread.ManagedThreadId, i, _hours);
            }
        }
    }

The run resut of above code is:

task:3,nth:0,hours:2
task:4,nth:0,hours:2
task:4,nth:1,hours:3
task:3,nth:1,hours:4
task:3,nth:2,hours:5
task:4,nth:2,hours:6
task:3,nth:3,hours:7
task:4,nth:3,hours:8
task:4,nth:4,hours:9
task:3,nth:4,hours:10
task:4,nth:5,hours:11
task:3,nth:5,hours:12
task:3,nth:6,hours:14
task:4,nth:6,hours:14
task:4,nth:7,hours:15
task:3,nth:7,hours:16
16
Press any key to continue . . .

　　As the result shown, we get the correct value 16 now.

2. Using Interlocked

class Program
    {
        static int _hours = 0;
        static object _synRoot = new object();

        static void Main(string[] args)
        {
            var tasks = new Task[2];
            for (int i = 0; i < tasks.Length; i++)
            {
                tasks[i] = new Task(Work);
                tasks[i].Start();
            }

            for (int i = 0; i < tasks.Length; i++)
            {
                tasks[i].Wait();
            }

            Console.WriteLine(_hours);
        }

        static void Work()
        {
            for (int i = 0; i < 8; i++)
            {
                Thread.Sleep(10);
                Interlocked.Increment(ref _hours);
                //_hours++; Using this statement may result in an unexpected value.
                Console.WriteLine("task:{0},nth:{1},hours:{2}", Thread.CurrentThread.ManagedThreadId, i, _hours);
            }
        }
    }

The run resut of above code is:

task:5,nth:0,hours:1
task:3,nth:0,hours:2
task:3,nth:1,hours:4
task:5,nth:1,hours:4
task:5,nth:2,hours:5
task:3,nth:2,hours:6
task:5,nth:3,hours:7
task:3,nth:3,hours:8
task:3,nth:4,hours:9
task:5,nth:4,hours:10
task:5,nth:5,hours:12
task:3,nth:5,hours:12
task:5,nth:6,hours:13
task:3,nth:6,hours:14
task:3,nth:7,hours:15
task:5,nth:7,hours:16
16
Press any key to continue . . .