Doesn't ThreadPriority.Highest guarantee finishing before ThreadPriority.Lowest?

Question

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Threading;

namespace ConsoleApplication58
{
class MultiThreading
{
    static void Main(string[] args)
    {
        MultiThreading mul = new MultiThreading();

        Thread t1 = new Thread(new ThreadStart(mul.WriteX));
        Thread t2 = new Thread(new ThreadStart(mul.WriteO));

        t1.Priority = ThreadPriority.Lowest;
        t2.Priority = ThreadPriority.Highest;

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

    }

    private void WriteX()
    {
        for (int i = 0; i < 300; i++)
        {
            Console.Write("X");
        }
    }

    private void WriteO()
    {
        for (int i = 0; i < 300; i++)
        {
            Console.Write("O");
        }
    }
}
}

When I execute above code, I expect X's end of the printing job because I gave that method lowest priority but sometimes I get O's at the end. I mean doesn't giving high priority to 2nd thread guarantee it will finish sooner?

Answer 1

There is no practical guarantee about thread scheduling induced by the priority setting.

For example, the high priority thread could block on IO or a page fault. Then, another thread can execute.

This is not a good way to synchronize threads.