Question

I see locking using static readonly objects alot, what is the significance of this statement in the below given code?I know lock is applied on an object , is this just an object for making locks work, is it just a hack , because i don’t need any object here but just creating an object in order to make locking work?

Also instead of just saying object obj1= new object() i use readonly and static, i guess due to increasing performance but how readonly and static helps increasing performance?

static readonly object locker = new object();

Code is as follows:

class ThreadSafe 
{
  static bool done;
  static readonly object locker = new object();

  static void Main()
  {
    new Thread (Go).Start();
    Go();
  }

  static void Go()
  {
    lock (locker)
    {
      if (!done) { Console.WriteLine ("Done"); done = true; }
    }
  }
}

Answer 1

Having an object whose sole purpose is to provide a lock simplifies things a bit. In order to keep outside code from messing with your locks and causing you all kinds of grief, you really really want a lock that’s solely visible to you. Easiest way to accomplish that, would be to create an object. (If you have another object that no one else will have access to, you could conceivably use that…but having the lock separate from everything else tends to keep stuff conceptually simpler.)

As for why it’s static…that’s not so much a performance thing as a correctness one. In order to lock in a static method or synchronize access to a static resource, you need an object that belongs to the class, not to an instance. Otherwise, every instance would be locking its own lock rather than sharing one — which would be pretty useless unless there was only ever one instance, and incorrect even then. For a non-static method, you wouldn’t use static unless for some reason you were accessing static properties or something like that. If you did use it, and didn’t need to synchronize access between instances, you’d end up locking unnecessarily and slowing stuff down.