Develop Reference

No threads displayed in the Threads Debug Window

I am trying to track my BackgroundWorker threads as described in this article the Threads Debug Windows is empty:
And yes, I'm debugging.
What am I missing?

You need to pause your application first; the debugger will not display anything while your code is running.
You can either set a breakpoint and wait until it gets hit, or click the Pause button to break immediately.

Why doesn't visual studio exit the debugger on application exit?

I am trying to figure out what changed within a project that now causes the application to not exit the debugger when I close the application I am debugging. I have also noticed that unhandled exceptions no longer invoke the unhandled exception handler, I am not sure if it's related.

This is probably due to other threads, which are not set as background threads, keeping the application alive. Background threads will terminate when the application does, whereas foreground threads will keep the rest of the application alive until they complete.
One way to check for the thread(s) that are responsible is in the debugger, as follows:
Run your app in the debugger
"Exit" your app.
Wait until the app should have exited but hasn't
Break into the app using the "pause" button
Open the "Threads" debugging window
Look for any threads in this window. Chances are there will be one, or a few threads showing. See if you can identify what they are by the info in the window.
It may also help to open the "Stack" debugging window and then double-click on each thread in the "Threads" window in turn and look at the contents of the "Stack" window. You may be able to see what any stuck threads are trying to do.

The debugger exits when all the threads exit. You've probably created an extra thread and done nothing to terminate it.
And this would explain unhandled exceptions being uncaught: if they happen on a different thread you won't see them on the main thread.

Most certainly the application has not actually terminated. Do you have any other threads running? An application will not shut down until all foreground threads have terminated.

Process not terminated at the end of the program

After I close the main window of my program, the process is still running in the background.
How can I check what is the cause for this weird problem?
(I don't know which part of my program code is relevant)

First check that the value of your application's ShutdownMode property is equal to ShutdownMode.OnMainWindowClose -- if it is not, see if the actual value is preventing the app from closing.
If this does not solve the problem, then you have one or more non-background threads still running after the main window closes, preventing the process from shutting down. Break into the debugger and see how many threads are still alive and what they are doing; this will lead you to a solution.

To see the Thread that is currently running and stopping your application from termination just press "Break All" at the Visual Studio and see where the cursor stops.
Note: you can force application to terminate by calling Environment.Exit();

Another reason could be that someone set the Application.ShutdownMode to OnExplicitShutdown.

One possibility is that you create threads which are not set to background or you're calling code that does that; that may keep your process "alive" even after you shut down your main thread.
Make sure that you terminate all of your threads before you exit or you set their IsBackground property to true.

Reason why my application would not exit?

We have an application written in .net, c#, winforms. We noticed that sometimes when closing the application, the process remains.
I'm not sure how I can reproduce that behavior, so I'm looking for some clues as to why the application wouldn't exit.
The application uses a bit of background threads. Thread pools. Wondering if that could be the cause. Anything else could have this effect?

If you have thread's that have IsBackground property set to false that are alive after application is closed, they will remain

The application uses a bit of
background threads. Thread pools.
Wondering if that could be the cause.
Anything else could have this effect?
It most definitely could be the cause though I cannot be certain that it actually is. One way to test this hypothesis to make sure all threads that have been explicitly created are designated as background threads. This can be done by setting Thread.IsBackground = true which will allow the application to terminate if the main thread ends. If there is at least one thread for which IsBackground = false then the CLR keeps the host process running.

Its likely to be a thread left running. If you have a look at the process in task manager you can see when a thread starts, and how many are left running when it exits by adding the 'Threads' column from the view menu.
I would start by making sure you start and end on the same thread count.

Get Process Explorer and possibly Process Monitor and see what thread is left suspended or running.

If you are able to run in debug and you close the form, VS should not return to the normal code edit mode (it will still have the pause and stop buttons from debug active). You can then press Pause and check the Threads window to see where the call stacks are stuck for threads that are still active.

Application.Exit() vs Application.ExitThread() vs Environment.Exit()

I am trying to figure out which I should be using. On closing my WinForm app fires of a Form in Dialog mode. That form runs a Background worker that Syncs the DB with the remote DB and displays it's progress on the "Splash Form."
I have a method like so:
private void CloseMyApp()
{
SaveUserSettings();
splashForm = new SplashForm();
splashForm.ShowDialog();
Application.ExitThread();
//Application.Exit();
}
which is what I call to close my app from Menu --> Exit and in the Form_FormClosing() event. Application.Exit() gives the following error -->
Collection was modified; enumeration operation may not execute.
Now I read that Environment.Exit() is brutal and means there is probably something wrong with your app (see here).
Application.ExitThread() works but I am concered that it may only be APPEARING to work and as I have never used it before I am not sure when it is normally appropriate to do so.

Unfortunately, the problem isn't caused by any of these, and really exists (even if you don't get the message) in all of these scenarios.
Your problem is this:
On closing my WinForm App fires of a Form in Dialog mode. That form runs a Background worker that Syncs the DB with the remote DB and displays it's progress on the "Splash Form."
Since you're not actually shutting down when you request a shutdown, all of the "Exit" functions are trying to tear down your background thread. Unfortunately, this is probably happening in the middle of your DB sync, and an enumeration working in the save logic is probably providing that error.
I would recommend not using any of these - just call myMainForm.Close() instead. That should close your main form, which will fire your closing logic appropriately. Once the main form in your application closes, it will shut down gracefully.

Environment.Exit() is used for console apps.
You want to use: System.Windows.Forms.Application.Exit()
By exiting thread, you are only exiting the current thread context, while leaving any started foreground threads running. I suspect the thread that is causing the error is still running, so you've essentially masked the problem, not worked around it. I would try and figure out why you are getting this error "Collection was modified; enumeration operation may not execute." on exit. It's being exposed by Application.Exit(), but it's not caused by it.