Sample code to illustrate a deadlock by using lock(this)

Question

I\'ve read several articles and posts that say that lock(this), lock(typeof(MyType)), lock(\"a string\") are all bad practice because

Answer 1

The problem is that lock("a string") is locking on a singleton. This means that other objects that use the same lock could be an infinite wait.

for example:

using System;
using System.Threading;

namespace ThreadLock
{
    class Program
    {
        static void Main(string[] args)
        {
            lock ("my lock")
            {
                ManualResetEvent evt = new ManualResetEvent(false);
                WorkerObject worker = new WorkerObject(evt);
                Thread t = new Thread(new ThreadStart(worker.Work));
                t.Start();
                evt.WaitOne();
            }
        }
    }

    class WorkerObject
    {
        private ManualResetEvent _evt;
        public WorkerObject(ManualResetEvent evt)
        {
            _evt = evt;
        }
        public void Work()
        {
            lock ("my lock")
            {
                Console.WriteLine("worked.");
                _evt.Set();
            }
        }
    }
}

In this case, the calling code creates a lock on a string then makes a worker object. The worker object in Work() locks on the same string, which is a singleton in C#. It ends up in deadlock because the caller owns the lock and is waiting for a signal which will never come.