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

I've read several articles and posts that say that lock(this), lock(typeof(MyType)), lock("a string") are all bad practice because another thread could lock on the same key and cause a deadlock. In order to understand this problem, I was trying to create some sample code to illustrate the deadlock but have been unable to wrap my head around this.

Can someone write a concise bit of code that illustrates this classic problem? Please keep it short, I can digest code in smaller chunks only.

Edit: I think lassevk sums it up well; that the real problem is that you have lost control over your locks. Once that happens, you cannot control the order the locks are called, and you are allowing a potential deadlock situation.

lock(this), lock(typeof(MyType)), etc all are situations where you have chosen a lock that is impossible to control.

c# .net multithreading deadlock

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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.

answered 2009-05-21T17:22:51.767

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

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