INF: Error Handling in Visual Basic Data Access Layer (95023)

MORE INFORMATION

There are 49 trappable errors for Visual Basic ODBC object language elements (error numbers 600 through 648). A list of these messages and their meanings can be found in the online help (Data Access Help). In Visual Basic, all runtime errors are fatal errors; Visual Basic displays the error message and terminates the program. However, it is possible to trap these error messages and transfer control to an error handling routine. The "On Error Go To <linenumber>" function accomplishes this. Control is transferred to <linenumber> where the error handling routine begins. Within this routine, the ERR function can be used to obtain the error number of the current error message, and based on this, appropriate action can be taken. Some of the Visual Basic ODBC Object layer (VB/ODBC) error messages use internal variables to provide specific information about an error. This is referred to as the "variable portion of the error message" in the online Data Access Help. The variable portion can be retrieved by using the ERROR$ function. So, the error handler would typically contain a call to the ERR function to get the error code; if the error has a variable portion, it would contain a call to the ERROR$ function.

Because of the architecture of the VB/ODBC, there are four places where the actual error may have occurred:

1. Visual Basic
2. The ODBC driver manager
3. The ODBC driver
4. The data source

The ODBC API defines a standard set of error codes that are returned for errors that occur in 2, 3, or 4. Visual Basic maps these error codes to one of its own 49 trappable errors. In cases where it makes sense to provide the native ODBC error code back to the VB application, VB does so - and the variable portion of the error message contains the native error. Eight of the 49 trappable errors (ERR numbers 602, 606, 611, 613, 615, 618, 619, and 644) return the native error messages in the ERROR$ function when these errors occur. You can use a general error handler containing a Select Case statement to return the ERROR$ function for those ERR numbers. Native ODBC errors contain a lot of information, including the name of the data source, the driver name, the vendor of the driver, and the error text. For example: The important thing to note, especially for programmers transitioning from VBSQL, is that there are no callback error handler controls available.