Hi! Since the data processing procedure is running in UI thread, so no message processing operation will be performed, and that is the reason of blocking of the user interface. A simple solution is when user click the "Start" button, set the state of the UI elements (e.g. enable or disable) correctly to avoid invalid user operation and run the data operation from another thread. You can place a process bar on the form to indicat that the application is in "Busy" state while data processing is running.