>I am developing a Checkbox class. Among other things, I want to be able to control when the box gets checked, unchecked, or reset to null.
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>Under the debugger, I can see that the state of the checkbox has already been modified by the time it reaches the checkbox Click and InteractiveChange events. However, The state has not been modified by the time we reach the GotFocus event.
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>How can I override VFP's default handling of the checkbox control, and check, uncheck, or reset to .NULL. in my own code.

You can create your own checkbox out of a commandbutton - by changing the button's image according to the value. You'd need to create a .value property, and handle its changes of in .click() (as described elsewhere in this thread). Also, there would need to be some work about refreshing and saving, as commandbutton doesn't have a controlsource.