Why is checkbox changing from checkmark to black square?

This is not checked, this is either .NULL. or 2, this is the third state of a checkbox, undecided.
Checkbox can take values 0,1 and 2, when initialised with any of these numbers of .T., .F. and .NULL., when intialised with any of these boolean values.
You actually don't need to do anything to avoid the third states 2 or .NULL., even if the checkbox is bound to a variable, which can be .null., the third state doesn't happen automatically, even if you set it to 2 or .null., interactive usage only toggles between checked and non checked state.

But most important: No, this isn't just a cosmetic thing, you really risc having a wrong value inside your table field, eg can fail to store .NULL. into a non nullable logical field or store .NULL. instead of the wanted .T. or .F.

.NULL. partly acts the same way as .F., but surely not as .T., eg an IF .NULL. executes the ELSE branch, like .F.

Bye, Olaf.

Thanks Olaf,

You were right, following your lead it was easy to track down...

Thanks,
Stanley

Just a notice: In previous versions of windows the look of the third state (2 or .NULL.) was more apperent to be that uncertain state in being displayed as a faint grey check icon instead of a box/dot. As you describe it I think most users will also see this as checked, not as half checked. So this is a bad redesign of the UI of Windows.

Bye, Olaf.