Carlos,
I don't know why it doesn't have one, but with inheritance it hardly matters because a simple change to your custom baseclass can fix the problem everywhere.
> I'm asking me why checkbox doesn't has a readonly property.
> Will do in VFP 6?
This is a list of classes with a ReadOnly property from the VFP6 help file: CheckBox, Column, ComboBox, Cursor, EditBox, File Object, Grid, Spinner, TextBox