Question

I was trying to create an example for deadlock. I tried the following code. But instead of creating deadlock, it worked like charm. Help me in understanding why it didn’t create a deadlock. What change in this code would create a deadlock?

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

namespace ReferenceTypes
{
    class DeadLockExample
    {
        static int a;
        static int b;

        public static void Main(string[] args)
        {
            DeadLockExample.a = 20;
            DeadLockExample.b = 30;

            DeadLockExample d = new DeadLockExample();

            Thread tA = new Thread(new ThreadStart(d.MethodA));
            Thread tB = new Thread(new ThreadStart(d.MethodB));

            tA.Start();
            tB.Start();

            Console.ReadLine();
        }

        private void MethodA()
        {
            lock (this)
            {
                Console.WriteLine(a);
                Thread.Sleep(1000);
                Console.WriteLine(b);
            }
        }

        private void MethodB()
        {
            lock (this)
            {
                Console.WriteLine(b);
                Thread.Sleep(1000);
                Console.WriteLine(a);
            }
        }
     }
}

Answer 1

As everyone else has said, two locks acquired in different orders, so that each is waiting on the other. I also changed one of the Sleep lengths to ensure a high probability of the deadlock occurring.

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

namespace ReferenceTypes
{
    class DeadLockExample
    {
        static int a;
        static int b;

        static object lockedObjA = new object();
        static object lockedObjB = new object();

        public static void Main(string[] args)
        {
            DeadLockExample.a = 20;
            DeadLockExample.b = 30;

            DeadLockExample d = new DeadLockExample();

            Thread tA = new Thread(new ThreadStart(d.MethodA));
            Thread tB = new Thread(new ThreadStart(d.MethodB));

            tA.Start();
            tB.Start();

            Console.ReadLine();
        }

        private void MethodA()
        {
            lock (DeadLockExample.lockedObjA)
            {
                Console.WriteLine(a);
                Thread.Sleep(1200);

                lock (DeadLockExample.lockedObjB) {
                    Console.WriteLine(b);
                }
            }
        }

        private void MethodB()
        {
            lock (DeadLockExample.lockedObjB)
            {
                Console.WriteLine(b);
                Thread.Sleep(1000);

                lock (DeadLockExample.lockedObjA) {
                    Console.WriteLine(a);
                }
            }
        }
     }
}