If your application has requirements that cannot be met by the features already built into Visual FoxPro, you can extend the program by taking advantage of external libraries —ActiveX controls or dynamic-link libraries (DLLs). Using external libraries, you can add objects to your application, everything from enhanced text boxes to calendars and other full-featured applications, and you can take advantage of the functionality offered by other programs (including Windows itself) through their application programming interfaces (APIs).
Chapter 27 Extending Visual FoxPro with External Libraries
You can easily add ActiveX controls (.ocx files) to your application, which provides you with new objects to use in forms, to subclass, and to manage the way you work with native Visual FoxPro controls. In addition, you can link to an external library such as a DLL and call the library’s functions to use in your own programs. If you already have Visual FoxPro external libraries (.fll files), you can link those as well and call their functions.
Chapter 28 Accessing the Visual FoxPro API
If an external library is not available to suit your needs, you can write your own ActiveX control or Visual FoxPro-specific dynamic-link library (.fll file). By calling the functions available in the API available in Visual FoxPro, you can create controls or libraries that are tightly integrated with and optimized for use in Visual FoxPro.