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

Sure, here you go.

Note that the common example for a deadlock is when you acquire multiple locks, and two or more threads end up waiting for each other.

For instance, two threads that locks like this:

Thread 1               Thread 2
 Lock "A"               Lock "B"
 Lock "B"               Lock "A" <-- both threads will stop dead here
                                     waiting for the lock to be come
                                     available.

However, in this example I didn't bother with that, I just let one thread lock indefinitely. You really don't want to loose control over your locks, so while this is a contrived example, the fact that the background thread can completely block the main thread like this, is bad.

using System;
using System.Threading;

namespace ConsoleApplication7
{
    public class Program
    {
        public static void Main(string[] args)
        {
            LockableClass lockable = new LockableClass();
            new Thread(new ParameterizedThreadStart(BackgroundMethod)).Start(lockable);
            Thread.Sleep(500);
            Console.Out.WriteLine("calling Reset");
            lockable.Reset();
        }

        private static void BackgroundMethod(Object lockable)
        {
            lock (lockable)
            {
                Console.Out.WriteLine("background thread got lock now");
                Thread.Sleep(Timeout.Infinite);
            }
        }
    }

    public class LockableClass
    {
        public Int32 Value1 { get; set; }
        public Int32 Value2 { get; set; }

        public void Reset()
        {
            Console.Out.WriteLine("attempting to lock on object");
            lock (this)
            {
                Console.Out.WriteLine("main thread got lock now");
                Value1 = 0;
                Value2 = 0;
            }
        }
    }

}