(); Just close the current window. If it is not the main form, it cannot exit the program. In addition, if there is a managed thread (not the main thread), it cannot exit cleanly;
(); Force all messages to abort and exit all forms, but if there is a managed thread (non-main thread), it cannot exit cleanly;
(); Forced abort all messages on the calling thread, and the problem that other threads cannot exit correctly;
(0); This is the most thorough exit method. No matter what thread is forced to exit, the program ends cleanly.
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