How to Protect Properties and Methods in a Class (145894)

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Within Visual FoxPro, the PROTECTED keyword in the class definition prevents access to these properties by any methods outside of its class. Therefore, the PROTECTED properties are invisible to outside methods. The ability to hide properties or methods when using an object-oriented programming approach in Visual FoxPro is a form of encapsulation.

The following example demonstrates the concept by creating a class called Protect_Globals. The PROTECTED keyword limits the scope of the property gd_sysdate to the Protect_Globals class. The property gc_day is not protected and can be accessed outside the class.

Create a Class with Protected and Unprotected Properties

1. On the File menu, click New, select Class, and then click the New File button.
2. For the Class Name, enter Protect_Globals. This will be Based On Custom and Store In g_class.
3. Within the Class Designer, click the Class menu, and select New Property. For the Name, type gd_sysdate, and select the Protected check box. Then click the OK button.
4. On the Properties sheet, locate the gd_sysdate property, and change its value to:

      =DATE()
   						

5. On the Class menu, click New Property. For the Name, type gc_day. Leave the Protected check box unchecked, and click the OK button.
6. On the Properties sheet, locate the gc_day property, and change its value to:

      =CDOW(DATE())
   						

7. On the Class menu, click New Method. For the Name, type getsysdate. Leave the Protected check box unchecked, and click the OK button.
8. On the Properties sheet, locate the getsysdate method, and enter the following code:

      WAIT WINDOW DTOC(This.gd_sysdate) NOWAIT
   						

9. Close the Class Designer, and save changes to the Protect_Globals class.

Place the Class within a Form

1. On the File menu, click New, and select Form, and then click the New File button.
2. Select the View Classes tool from the Form Controls, and Add the g_class that was created previously.
3. Locate and select the Protect_Globals tool, and then click any place on the form. This will create an instance of the Protect_Globals class with the name Protect_Globals1.
4. Select the View Classes tool from the Form Controls, and select Standard.
5. Add three command buttons (Command1, Command2, and Command3) to the new form.
6. Change the Caption of Command1 to GC_DAY. Within its Click event, enter the following:

      WAIT WINDOW ThisForm.Protect_Globals1.gc_day NOWAIT
   						

7. Change the Caption of Command2 to GETSYSDATE. Within its Click event, enter the following:

      ThisForm.Protect_Globals1.getsysdate
   						

8. Change the Caption of Command3 to GD_SYSDATE. Within its Click event, enter the following:

      WAIT WINDOW DTOC(ThisForm.Protect_Globals1.gd_sysdate) NOWAIT
   						

9. Save the form as g_form, and close the Form Designer.

Run the Form

1. Use the DO FORM command to run the g_form you just created.
2. Click the GC_DAY command button, and note that a WAIT WINDOW appears and displays the gd_day property of the Protect_Globals class.
3. Click the GETSYSDATE command button, and note that the gd_sysdate property of the Protect_Globals class is displayed by calling the getsysday method of that class.
4. Click the GD_SYSDATE command button. This results in a Program Error indicating that the gd_sysdate property cannot be found. This is because the gd_sysdate property is hidden to the form.
5. Select Ignore, and close the form.

In Step 3 it was possible for the value to be displayed by calling a method of the Protect_Globals class that passed the data contained within the gd_sysdate property. But because that property is protected, it is completely hidden from the command button of the form that attempted to call the gd_sysdate property directly.