PRB: LCK Functions Do Not Work in a Private Data Session

• Microsoft Visual FoxPro for Windows, versions 5.0, 5.0a

SYMPTOMS

An ActiveX control (.ocx) has been written in Visual C++ that also makes use of the Visual FoxPro Library Construction Kit (LCK) functions. The control may work properly on forms that use the default DataSession but the control fails when the form uses a private DataSession.

CAUSE

Library Construction Kit function calls do not occur in the DataSession context of the form that made the call to the .ocx. There is only one global context in which the Library Construction Kit function can call into Visual FoxPro, no matter where the call originated.

RESOLUTION

If a private DataSession is necessary, write the ActiveX control completely in Visual C++ or Visual Basic and expose properties and methods that can be accessed by Visual FoxPro.

If it is absolutely necessary to use some of the Visual FoxPro LCK functions in an .ocx that will access tables in a private DataSession, it may be possible to get the .ocx to work by making the private DataSession the active DataSession by using the SET DATASESSION TO <nDataSessionNumber> command. However, this approach is not recommended. Writing the functionality into the control with another language is a better solution.

STATUS

This behavior is by design.

MORE INFORMATION

FoxPro and Visual FoxPro have included the Library Construction Kit for many versions. The Library Construction Kit functions allow developers to write libraries in C that can be loaded by FoxPro or Visual FoxPro to extend its functionality by accessing the FoxPro API. Starting with Visual FoxPro version 5.0, the Developer's Guide has included information to help developers get a start on creating ActiveX controls to access the Visual FoxPro API. ActiveX controls can make use of the LCK functions if used within the global context of Visual FoxPro.

Library Construction Kit functions that access tables that may be open in private DataSessions, such as _DBRead and the other _DBxxxx functions, _Load, _FindMemo and _FindVar, are some of the functions to avoid in ActiveX controls that need to work in private DataSessions. However, depending on what the ActiveX control does and how it does it, other LCK functions may also fail in private DataSessions. A good test is to try the control in the default DataSession. If it works there and not a private DataSession, this article probably applies.

REFERENCES

Microsoft Visual FoxPro "Developer's Guide," version 5.0, pages 645-647.

Examples.dbf located in the \API\Samples folder of Visual FoxPro.

Microsoft Visual FoxPro Help: Library Construction Kit.

Keywords          : FxprgGeneral FxtoolLck vfoxwin
Version           : WINDOWS:5.0,5.0a
Platform          : WINDOWS
Issue type        : kbprb
Solution Type     : kbnofix