如何同步多个线程

Question

I wrote this console program:

static void Main(string[] args)
{
    object sync = new object();
    Thread[] t = new Thread[10];
    int count = 0;

    for (var i = 0; i < t.Length; i++)
    {
        t[i] = new Thread(() =>
        {
            lock (sync)
            {
                int inc = count;
                Console.WriteLine("Count: {0}", count);
                count = inc + 1;
            }
        });
    }

    foreach (var t1 in t)
    {
        t1.Start();
    }

    foreach (var t1 in t)
    {
        t1.Join();
        Console.WriteLine("\nFinal Count= {0}", count);
        Console.ReadKey();
    }
}

I get this result in the output :

Count: 0
Count: 1
Count: 2
Count: 3
Count: 4
Count: 5
Count: 6
Count: 7

Final Count= 7
Count: 8
Count: 9

and when I run the app multiple time I'm getting more different results but I want to see this result :

Count: 0
Count: 1
Count: 2
Count: 3
Count: 4
Count: 5
Count: 6
Count: 7
Count: 8
Count: 9

Final Count= 10

why does it return different results and how do I fix this?

Answer 1

Try do don't manually create/terminate threads like this, it's a time consuming operation and it doesn't scale well.

Use the ThreadPool instead.

Or better: use tasks , and synchronize them by using await Task.WhenAll(list of your tasks)

Answer 2

Well, instead of this :

foreach (var t1 in t)
{
    t1.Join();
    Console.WriteLine("\nFinal Count= {0}", count);
    Console.ReadKey();
}

You should write:

foreach (var t1 in t)
{
    t1.Join();

}
Console.WriteLine("\nFinal Count= {0}", count);
Console.ReadKey();

Otherwise you will have a racing and your code will be undeterministic.