Are threads waiting on a lock FIFO?

Question

Let\'s say I have the following code

static class ...
{
    static object myobj = new object();

    static void mymethod()
    {
        lock(myobj)
                


        

Answer 1

A simple example tell us that order is not guaranteed to be FIFO

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


namespace ConsoleApplication
{
    class Program
    {
        private static Info info = new Info();

        static void Main(string[] args)
        {
            Thread[] t1 = new Thread[5];
            for (int i = 0; i < 5; i++)
            {
                t1[i] = new Thread(info.DoWork);
            }

            Thread[] t2 = new Thread[5];
            for (int i = 0; i < 5; i++)
            {
                t2[i] = new Thread(info.Process);
            }

            for (int i = 0; i < 5; i++)
            {
                t1[i].Start();
                t2[i].Start();
            }

            Console.ReadKey();
        }
    }

    class Info
    {
        public object SynObject = new object();

        public void DoWork()
        {
            Debug.Print("DoWork Lock Reached: {0}", Thread.CurrentThread.ManagedThreadId);
            lock (this.SynObject)
            {
                Debug.Print("Thread Lock Enter: {0}", Thread.CurrentThread.ManagedThreadId);
                Thread.Sleep(5000);
                Debug.Print("Thread Lock Exit: {0}", Thread.CurrentThread.ManagedThreadId);
            }
        }

        public void Process()
        {
            Debug.Print("Process Lock Reached: {0}", Thread.CurrentThread.ManagedThreadId);
            lock (this.SynObject)
            {
                Debug.Print("Process Lock Enter: {0}", Thread.CurrentThread.ManagedThreadId);
                Thread.Sleep(5000);
                Debug.Print("Process Lock Exit: {0}", Thread.CurrentThread.ManagedThreadId);
            }
        }
    }
}
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Diagnostics;


namespace ConsoleApplication
{
    class Program
    {
        private static Info info = new Info();

        static void Main(string[] args)
        {
            Thread[] t1 = new Thread[5];
            for (int i = 0; i < 5; i++)
            {
                t1[i] = new Thread(info.DoWork);
            }

            Thread[] t2 = new Thread[5];
            for (int i = 0; i < 5; i++)
            {
                t2[i] = new Thread(info.Process);
            }

            for (int i = 0; i < 5; i++)
            {
                t1[i].Start();
                t2[i].Start();
            }

            Console.ReadKey();
        }
    }

    class Info
    {
        public object SynObject = new object();

        public void DoWork()
        {
            Debug.Print("DoWork Lock Reached: {0}", Thread.CurrentThread.ManagedThreadId);
            lock (this.SynObject)
            {
                Debug.Print("Thread Lock Enter: {0}", Thread.CurrentThread.ManagedThreadId);
                Thread.Sleep(5000);
                Debug.Print("Thread Lock Exit: {0}", Thread.CurrentThread.ManagedThreadId);
            }
        }

        public void Process()
        {
            Debug.Print("Process Lock Reached: {0}", Thread.CurrentThread.ManagedThreadId);
            lock (this.SynObject)
            {
                Debug.Print("Process Lock Enter: {0}", Thread.CurrentThread.ManagedThreadId);
                Thread.Sleep(5000);
                Debug.Print("Process Lock Exit: {0}", Thread.CurrentThread.ManagedThreadId);
            }
        }
    }
}

Execution will procede something like this

Process Lock Reached: 15
Process Lock Enter: 15
DoWork Lock Reached: 12
Process Lock Reached: 17
DoWork Lock Reached: 11
DoWork Lock Reached: 10
DoWork Lock Reached: 13
DoWork Lock Reached: 9
Process Lock Reached: 18
Process Lock Reached: 14
Process Lock Reached: 16
Process Lock Exit: 15
Thread Lock Enter: 9
Thread Lock Exit: 9
Process Lock Enter: 14
Process Lock Exit: 14
Thread Lock Enter: 10
Thread Lock Exit: 10
Thread Lock Enter: 11
Thread Lock Exit: 11
Process Lock Enter: 16
Process Lock Exit: 16
Thread Lock Enter: 12
Thread Lock Exit: 12
Process Lock Enter: 17
Process Lock Exit: 17
Thread Lock Enter: 13
Thread Lock Exit: 13
Process Lock Enter: 18
Process Lock Exit: 18

As you ca see the process of reach lock is different that lock enter.