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Inheritance tutorial
Hilmar Zonneveld, October 7, 2005
This is a step-by-step tutorial to show inheritance, specifically in Visual FoxPro forms, as a guidance for people who are not familiar with inheritance in general, or who don’t know how to implement it in Visual FoxPro. The basic idea of inheritance is that all your forms, or several of your for...
Summary
This is a step-by-step tutorial to show inheritance, specifically in Visual FoxPro forms, as a guidance for people who are not familiar with inheritance in general, or who don’t know how to implement it in Visual FoxPro. The basic idea of inheritance is that all your forms, or several of your forms, obtain their properties and methods from a "base form". It is possible to do changes only once, in your "base form"; the changes will be propagated to all your forms.
Description
Introduction The following should work in all versions of Visual FoxPro (i.e., version 3 and later). The examples are not useful by themselves, they only serve to show how to use inheritance. Create a new project 
modify project inheritance
Create the base class Select “Classes”, click on “New”. Name your class cMyForm, and base it on the class “Form”. Store it in class library MyClasses. Give your form some color, selecting the property “BackColor” in the Property Sheet. Save it. Create a derived form Create a form that is based on the class, with the command: 
create form MyForm as cMyForm from MyClasses
Note that the form has the same BackColor as the one you defined in the class. Save your form. Propagating changes Go back to the class (open class library MyClasses in the Project Manager, click on the “+” sign, and open class cMyForm). Change the background color (change property BackColor in the Property Sheet). Save the class, and open the form: 
modify form MyForm
Note that the form’s background color changed. Breaking inheritance Change the BackColor directly in the form. If you do this, inheritance is “broken”, that is, any change in the BackColor of the class will no longer be applied in the form. To restore inheritance, right-click on the BackColor in the Property Sheet, and select “Reset to Default”. Inheriting behavior Until now, we have only seen how to inherit properties. Now, go to the base class (cMyForm), add a button, and put the following command in the Button’s Click() Event: 
MessageBox("Hello")
Run your form: 
do form MyForm
then click on the button, and confirm that the message appears on screen. Adding to the inherited behavior If you write anything in the Click() Event of the button on the form, inheritance is broken – the code from the parent class is not executed. For example, write the following in the Click() Event of the form: 
MessageBox("This is the second command.")
The original code is no longer executed. Usually you will want to include the original commands, and just add something in a derived form. Change the button’s Click() Event to: 
DoDefault()
MessageBox("This is the second command.")
Run the form again, and verify that both MessageBoxes show, first one, then the other. Additional considerations
  • The practical value of all this is that any change done in the class is propagated to all forms that derive from the class. Of course, you can base several forms on the same form class.
  • A class can derive from another class; you can have a hierarchy of inheritance.
  • You can also use inheritance for Buttons, TextBoxes, etc. For example, you could define a common behavior for all TextBoxes that refer to money amounts.
  • In older versions of Visual FoxPro, a form class can not use a DataEnvironment. You have to open your tables with commands, in the Form’s Load() Event. Common commands include USE, SET ORDER, CursorSetProp() (for buffering), SELECT, SET RELATION.
  • For information about common problems, see also my other FAQ, on breaking inheritance – but first practice a while to get familiar with inheritance in general.
    Hilmar Zonneveld, Independent Consultant
    Hilmar Zonneveld works in programming since 1986, using dBASE, FoxPro and Visual FoxPro. He is available as an independent consultant. He currently works as a programmer at Bata Shoe Organization; also as an instructor at Cisco Networking Academy. You can contact him through the Universal Thread, or, via e-mail, at hilmarz@yahoo.com. Personal Web page (mainly in Spanish): www.geocities.com/hilmarz.
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