Opc data Access Automation Specification

6

 

Appendix A - OPC Automation Error Handling 

When a run-time error occurs, the properties of the Visual Basic 

Err

 object are filled with information that 

uniquely identifies the error.   

If your Visual Basic code is not set up to handle the error using the On Error mechanism, an exception will be 

generated, and depending on the context (Visual Basic in Debug Mode, or an executable), a message box 

will be invoked with the following information: 

•

  Runtime Error: decimal error number (hex error number) 

•

  Method “X” of Object “Y” Failed.  (Note when your application is an executable, no value for X and Y are 

displayed) 

Therefore, it is highly recommended by the OPC Foundation, that your application take appropriate steps to 

catch any OPC Automation errors that may occur as a result of setting properties or invoking methods on the 

OPC Data Access Automation Objects. 

An 

error handler

 is a routine for trapping and responding to errors in your application. An OPC Automation 

client should add error handlers for any application functionality that involves setting a property or calling a 

method of OPC Data Access Automation Objects. The process of designing an error handler involves three 

steps:

 

1.

 

Set, or 

enable

, an error trap by telling the application where to branch to (which error-handling routine to 

execute) when an error occurs.

 

The On Error statement enables the trap and directs the application to the label marking the beginning of 

the error-handling routine. 

 

2.

 

Write an error-handling routine that will handle errors from setting properties or from method invocation on 

OPC Data Access Automation objects.

 

3.

 

Exit the error-handling routine.

 

Decide what action your application should take as a result of the error.  For example, if you attempted to 

add a group with a duplicate name(provided from the end user), you could advise the end user that the 

group was not added, and to enter a different name.  Your application could also take the approach of 

adding the group again(with a “”), letting the server generate the name.