A wrapper DLL is a dynamic-link library that intercepts external library function calls made to an application. After a function call has been intercepted, the DLL controls the application or process that instantiated the DLL. The DLL can be designed to perform any task or set of tasks, or to do nothing at all. The developer can add to, change, or remove functionality or scope from the DLL without modifying the source code of the calling process or application.
For example, in the case of this conversion strategy, a wrapper DLL can intercept Btrieve calls made to an application and change them to use ODBC to access Microsoft SQL Server. This technique leaves the base application code intact while changing the scope and/or targets of the operation. The initial investment made in the application is preserved even though the application’s capabilities have been extended to access SQL Server data.
Alternatively, the wrapper DLL could retrieve data from SQL Server into buffers maintained on the client or another computer. The application then fetches data from the buffers instead of from SQL Server directly by using ISAM-like processing techniques. Although this implementation enables the unmodified base application to access SQL Server, it is complex and can be difficult to implement. It is best suited for those instances when you do not want to use set operations or to develop a full ODBC- and SQL-based application. This methodology is not presented in this chapter.