How to Verify If an Object Has a Particular Property or Method

• Microsoft Visual FoxPro for Windows, version 3.0

SUMMARY

There is no built-in function in Visual FoxPro to determine if an object has a particular property. This article shows by example how to do so programmatically.

MORE INFORMATION

For this example, the Visual FoxPro screen object _SCREEN is used. However, this example can be applied to any valid Visual FoxPro object. First the AMEMBERS() function is used to create an array containing all the properties of the _SCREEN object. Then the ASCAN() function is used to search the array for a specific property. If the property is found, the ASCAN() function returns the array element number of the property. If the property is not found, the ASCAN() function returns zero. Therefore, as long as the return value of the ASCAN() function is greater than zero, the object has the specified property.

The following code checks to see if the _SCREEN object has a Caption property or a RowSource property.

Code Sample

** Create an array of all the properties of the _SCREEN object

=AMEMBERS(aproplist,_SCREEN)

** Scan the array to determine if the object has a Caption property

IF ASCAN(aproplist,"CAPTION")>0

   WAIT WINDOW "The property exists"

   WAIT WINDOW "The property does not exist"

** Scan the array to determine if the object has a RowSource property

IF ASCAN(aproplist,"ROWSOURCE")>0

   WAIT WINDOW "The property exists"

   WAIT WINDOW "The property does not exist"

Running this program will show that the _SCREEN object has a Caption property but does not have a RowSource property. Note the array created by the AMEMBERS() function will be in all uppercase. Therefore, when using the ASCAN() function, be make to specify the property in uppercase.

You can also use the AMEMBERS() function to determine if an object has a particular method by using a third optional argument. If you include a 1 as the third argument, the array will contain the properties of the object as well as the methods and member objects. You can use this in a toolbar to determine if a form set has a particular method.

Code Sample

** Before you run this example, you need to call a form so ** that a form is active.

=AMEMBERS(amyarray, _screen.activeform, 1)

** Creates a 2 dimensional array with the properties, ** methods, and member objects of the active form.

** Scan the array to determine if a method called MYMETHOD ** exists

IF ASCAN(amyarray, "ACTIVATE") > 0

   WAIT WINDOW "The method exists"

   WAIT WINDOW "The method does not exist"

DISPLAY MEMO like amyarray