.net Lock - Two Questions

Question

Two questions about the Lock() construct in .net

First, I am aware that if an object is locked within one class and another class attempts to lock the same object this produces a deadlock. But why? I have read about it on MSDN but MSDN is rarely all that clear.

----Edit Question One---- Still confused. I have a main thread (UI thread) that spawns many Threadpool threads. Each child thread locks the data before it works with it. This works fine every time.

If I then attempt to lock the same data from the UI thread to check if I should even bother creating a new thread for an edge case I create deadlock nearly every time.

----Edit Question Two---- Secondly, If I have a compound object that I lock are all child objects within it locked as well? Short code Demo:

internal sealed class Update
{
    //Three objects instantiated via other external assemblies
    public DataObject One { get; set; }
    public DataObject Two { get; set; }
    public ReplayStatus Status { get; set; }
}

If I call lock(UpdateObject) are each of the three internal objects and all of there child objects locked as well?

So I should do somthing like this to prevent threads from playing with my data objects:

Lock(UpdateObject.One)
{
    Lock(UpdateObject.Two)
    {
        Lock(UpdateObject.Status)
        {
            //Do Stuff
        }
    }
} 

Answer 1

First, I am aware that if an object is locked within one class and another class attempts to lock the same object this produces a deadlock.

No. If one thread locks an object and a second thread attempts to lock that object, that second thread must wait for the first thread to exit the lock.

Deadlock is something else:

1. thread1 locks instanceA
2. thread2 locks instanceB
3. thread1 attempts to lock instanceB and now must wait on thread2
4. thread2 attempts to lock instanceA and now must wait on thread1

These two threads can no longer execute, and so never release their locks. What a mess.

If I call lock(UpdateObject) are each of the three internal objects and all of there child objects locked as well?

No, the "lock" is only on the locked instance. Note: the lock doesn't prevent anything other than a second thread from acquiring a lock on that instance at the same time.