锁定调度员

Question

Is it necessary to lock code snippet where multiple threads access same wpf component via dispatcher?

Example:

void ladder_OnIndexCompleted(object sender, EventArgs args)
{
    lock (locker)
    {
        pbLadder.Dispatcher.Invoke(new Action(() => { pbLadder.Value++; }));
    }
}

pbLadder is a progress bar and this event can be raised from multiple threads in the same time.

Answer 1

You should not acquire a lock if you're then going to marshal to another thread in a synchronous fashion - otherwise if you try to acquire the same lock in the other thread (the dispatcher thread in this case) you'll end up with a deadlock.

If pbLadder.Value is only used from the UI thread, then you don't need to worry about locking for thread safety - the fact that all the actions occur on the same thread isolates you from a lot of the normal multi-threading problems. The fact that the original action which caused the code using pbLadder.Value to execute occurred on a different thread is irrelevant.

Answer 2

All actions executed on the Dispatcher are queued up and executed in sequence on the UI thread. This means that data races like that increment cannot occur. The Invoke method itself is thread-safe, so also adding the action to the queue does not require any locking.

From MSDN :

Executes the specified delegate with the specified arguments synchronously on the thread the Dispatcher is associated with.

and:

The operation is added to the event queue of the Dispatcher at the specified DispatcherPriority.

Answer 3

Even though this one is pretty old it was at the top of my search results and I'm pretty new (4 months since I graduated) so after reading other peoples comments, I went and spoke with my senior coder. What the others are saying above is accurate but I felt the answers didn't provide a solution, just information. Here's the feedback from my senior coder:

"It's true that the Dispatcher is running on its own thread, but if another thread is accessing an object that the dispatcher wants to access then all UI processing stops while the dispatcher waits for the access. To solve this ideally, you want to make a copy of the object that the dispatcher needs to access and pass that to the dispatcher, then the dispatcher is free to edit the object and won't have to wait on the other thread to release their lock."

Cheers!