Lock thread using somthing other than an object

Question

when using a lock does the thing you are locking on have to be a object. For example is this legal

    static DateTime NextCleanup = DateTime.Now;
    const TimeSpan CleanupInterval = new TimeSpan(1, 0, 0);
    private static void DoCleanup()
    {
        lock ((object)NextCleanup)
        {
            if (NextCleanup < DateTime.Now)
            {
                NextCleanup = DateTime.Now.Add(CleanupInterval);
                System.Threading.ThreadPool.QueueUserWorkItem(new System.Threading.WaitCallback(cleanupThread));
            }
        }
        return;
    }

Answer 1

You can lock on any reference type.

Since DateTime is not a reference type, you cannot lock on it.
If you try, each call to Monitor.Enter will receive a different boxed DateTime value, and the lock will be effectively useless.

What are you trying to do?
As far as the runtime is concerned, it doesn't matter at all which object you lock on. (Locking on an object will not magically prevent other threads from using the object unless they explicitly lock on the same object)
When you lock on an object, you should choose an object that all of the necessary pieces of code will lock on.

See also CA2002.

Answer 2

It doesn't have to be of type System.Object, but is there a reason to not use an object?

Answer 3

Be aware that even if what you are trying to do here were possible (i.e., if DateTime were a reference type and therefore something you could lock on), it wouldn't be legal because you're setting NextCleanup to a new value within the lock block.

You cannot set a reference to a new object within a block of code which has locked on that object.

This is why it's advisable to have a dedicated "lock" object to use for all code that needs to be synchronized for a related purpose.

Answer 4

It looks like what you are trying to do is run a cleanup routine periodically. If that is the case, you can use System.Threading.Timer, which is specifically designed for running periodic tasks.