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, the whole point of a lock is that two sections of code can’t get ahold of the same lock at once. This is to coordinate multiple threads working with the same stuff without interfering with each other. If you have a lock on an object, then anyone else that tries to get the lock will block (wait) until the original lock is released (only one thread can have the lock at any given time). You only have a deadlock if the first thread never gives up the lock, or if both threads are waiting for something from each other and neither can proceed until each gets what it’s waiting for.

Second, if you lock an object in C#, you’re not really “locking” the object in any semantic sense. You’re acquiring a “lock” on the object (which you later release or relenquish). The object is purely a convenient token that is used to uniquely identify which lock you wish to obtain. So no, a lock on an object does not create a lock on any sub-parts of that object.