Knowledge Base

PRB: AddProperty Does Not Work in OLE DLL

Article ID: 193944

Article Last Modified on 8/14/1999

Microsoft Visual FoxPro 6.0 Professional Edition

This article was previously published under Q193944

SYMPTOMS

Running a custom Component Object Model (COM) server that uses the AddProperty method of the same object, returns an OLE Automation error when the property is referenced.

CAUSE

Microsoft Visual FoxPro allows creation of the property, although it does not expose it publicly. You can access the added property within the class definition. For example, the following code sample shows assignment of the initial value of the added property to an existing property.

Modify the custom COM server created in the MORE INFORMATION section to the following code: DEFINE CLASS addcomprop AS LINE OLEPUBLIC errbyte=0 mytest="" PROCEDURE INIT THIS.ADDPROPERTY("NewProp", "Test") THIS.mytest=THIS.NewProp *No Error, from preceding line, Shows NewProp's Existence ENDPROC PROCEDURE ERROR LPARAMETERS nError, cMethod, nLine THIS.errbyte=STRTOFILE(STR(nError) + " " + cMethod + " " ; + STR(nLine),"c:\errfile.txt") ENDPROC ENDDEFINE

Call the preceding server with the following client code: x = CREATEOBJECT("PropTest1.addcomprop") ? x.mytest && Returns Value 'Test' rather than '' ? x.errbyte && Returns 0 for no errors. ? x.NewProp && Returns Unknown Name Error.

RESOLUTION

Reference the property internally in the object, and expose it through a public property, or a return value of a method.

A better way to create a property that you do not publicly expose in a COM server is to use the PROTECTED or HIDDEN keyword when defining the property. For example, you could use the following code in the class definition (not in the Init event or any other procedure) in place of the This.AddProperty("NewProp","Test") code that is in the Init event of the class definition:

   PROTECTED NewProp
   NewProp="Test"

STATUS

This behavior is by design.

MORE INFORMATION

The following error is not trappable by the Error event of the class set as OLEPUBLIC:

Error Text: "Program Error OLE error code 0x80020006: Unknown Name"

NOTE: We do not recommend that you create properties programmatically in COM servers because the type library file, PropTest1.tlb, will not reflect the properties.

Steps to Reproduce Behavior

Place the following code in a program file, attach it to a project, and build it into PropTest1.exe: DEFINE CLASS addcomprop AS LINE OLEPUBLIC errbyte=0 PROCEDURE INIT THIS.ADDPROPERTY("NewProp", "Test") ENDPROC PROCEDURE ERROR LPARAMETERS nError, cMethod, nLine THIS.errbyte=STRTOFILE(STR(nError) + " " + cMethod + " " ; + STR(nLine),"c:\errfile.txt") ENDPROC ENDDEFINE

Use the following code in a program file to call that custom COM server: x = CREATEOBJECT("PropTest1.addcomprop") ? x.errbyte && Returns 0 for no errors. ? x.NewProp && Returns Unknown Name Error.

Additional query words: kbCOMt kbCtrl kbVFp600 kbOOP

Keywords: kbprb kbcode KB193944