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Excel Object Model: range.locked

By default, the cells of a (new) worksheet are locked - but they still can be edited. In order to protect locked cells from being edited by a user, the worksheet needs to be protected using Home -> Cells -> Format -> Protect Sheet or Review -> Protect -> Protect Sheet.

See also

The worksheet function cell(protect, reference) allows to determine if a cell is locked.
Setting application.dataEntryMode to xlOn or xlStrict prevents a user from entering data into locked cells of the currently selected Range.
The option contents of worksheet.protect

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