How to Create and Access Protected Properties (131133)

SUMMARY

In Visual FoxPro, programmers can make user-defined object properties visible only to the events and methods for the defined object by protecting them from access by outside objects. Protected properties can only be altered or referenced by using the object's methods and events.

By using the PROTECTED keyword for properties in a class definition, protected properties cannot be changed by other objects. For example, when designing a form class that has a command button, the command button is unable to modify the PROTECTED Methods or Properties of the form class.

This article shows by example how to create a new custom class with some protected properties and a custom method ProtProperty. The protected properties cannot be accessed by external commands unless they use the custom method (ProtProperty) of the object.

MORE INFORMATION

The Help menu says, "Properties, methods, and objects in a class definition that are designated protected with the PROTECTED keyword of the DEFINE CLASS command can be accessed only by other methods in the class definition."

Code Sample

*--------------------------------------------------------------*
* PROGRAM: PROTTEST.PRG                                        *
*                                                              *
* Copy this code into a PRG and then execute the PRG.          *
*--------------------------------------------------------------*
oMyObject = CREATEOBJECT("MyClass")

? oMyObject.One

* The command shown above fails because property ONE is not   *
* being accessed through a method of the custom class. When   *
* this fails, choose IGNORE and the program will continue.    *

? oMyObject.ProtProperty("ONE")
? oMyObject.ProtProperty("TWO")
? oMyObject.ProtProperty("THREE")

DEFINE CLASS MyClass AS Custom
  PROTECTED One, Two, Three
  One   = "ONE"
  Two   = 2
  Three = "THREE"

  FUNCTION ProtProperty(lcProperty)
    RETURN EVAL("THIS."+lcProperty)
  ENDFUNC
ENDDEFINE
*--------------------------------------------------------------*