what's difference between Environment.Exit() and Application.Shutdown()?

Environment.Exit() is a more brutal way of closing down your application, yes. But in general, if you need to kill your application to make it close then I think you're looking at the problem in the wrong way. You should rather look into why the other threads aren't closing gracefully.

You could look into the FormClosing event on the main form and close down any resources that are hanging up the application, preventing it from closing.

This is how I have found resources hanging up the app.

1. In debug mode, enable showing of threads. (This will allow you to see all the threads your application is running.)
2. Close the application in the way that it does not close correctly.
3. Press pause in Visual studio.
4. Look at the threads list, and click on them to see where is the code they are hanging. Now that you can see what resources are blocking your application from closing, go to your FormClosing event and close/dispose them there.
5. Repeat until the app closes correctly :)

Be aware that the threads list in debug mode will show some threads that are run but not under your control. These threads rarely have a name and when you click on them you get a message saying you have no symbols. These can be safely ignored.

One of the reasons for making sure your application is closing gracefully is that some resources (let's say a FileStream) are not done working, so using some API to force it to quit can make all sorts of "random" problems come in, like settings/data files not being written and so on.

1. You should NEVER call TerminateThread
2. Make sure that all the threads that you spawn are mark as background, this way when you close the application it won't wait for them to complete.

As Shay said, NEVER call TerminateThread, TerminateThread kills just one thread without letting it clean up after itself this can lead to deadlocks and corruptions in other threads in the process.

TerminateProcess on the other had will kill the entire process and let the OS clean up, it's the fastest way to close a process - you just have to make sure you are not holding any resources the OS can't clean up (it also helps to close windows before calling TerminateProcess).

I think, but I haven't checked, that Environemnt.Exit calls TerminateProcess.

Application.Shutdown is very different, it doesn't immediately kill the process - it sends all the closing and shutdown notifications and waits for all the application's windows and threads close themselves.