Tutorial 15
Automation and Error Handling
For Automation please refer to the notes in the Concept lesson for this tutorial.
Error Handling
In this tutorial selected errors that would usually cause an application to halt are taken
care of through code within the procedures/functions. Tutorial 14 introduces the
CreateObject method that will start another application from an existing application
(starting Access while in an Excel procedure). This process could result in several
instances of Access being started if the procedure is called several times. Using
GetObject, instead of CreateObject, will allow the existing Access application to be
reused rather than starting a new one. The problem is that GetObject will return an error
and halt the program if there is not another Access application running.
Is there another instance of the Server (second) application already running?
Rather than just creating an object, we can call GetObject(Class := …..) This function
will return the address of an existing instance of the Server application. Unfortunately if
the second application is not already running the function returns an Error, and the
procedure is halted.
Intercepting the Error returned by GetObject caused when an existing instance of the
class is not found.
A run-time error occurs when the procedure is running. We have seen our procedures
halted many times by syntax errors. The type of error is held in an object called: Err
object. We can trap this error before it halts the procedure. In this case, when GetObject
returns an error we can trap the error, and call CreateObject to create a new object.
When an error occurs, if it is trappable, the value retuned to Err object will allow the code
to recover from the error, rather than having the error halt the procedure. In the case of
GetObject used when the application is already running, the number 429 is returned to
Err.Number, and Err.Description = "ActiveX component can't create object" are both set.
Error Handling
The statement On Error placed in the code prior to GetObject will turn on the error
handling feature. This will make it possible to trap the error caused by the Server
application already having been created when GetObject was called.
Here is the syntax:
On Error Goto <line> , where <line> is a label of a line in the procedure
The Resume [ next | line ] statement is used to return to the procedure after the error has
been handled.
Here is the entire code that will call GetObject. If there is an error it will call
CreateObject, then continue processing:
'turn on error trapping
On Error GoTo NewWorkbookErrHandler
'declare object variable
Dim appExcel as Excel.Application
'assign Automation object's address
Set appExcel = GetObject(Class := "Excel.Application")
'add a new workbook
appExcel.Workbook.Add
'make if visible
Here:
appExcel.Visible = True
'release memory
Set appExcel = Nothing
Exit Sub 'Exit the subroutine
NewWorkbookErrHandler
'Err.Number will hold the number of the error generated
Select Case Err.Number
Case 429 'Automation does not exist, Create one
Set appExcel = _
CreateObject(Class:="Excel.Application")
Resume Next 'continue on line after GetObject
Case Else ' Unforeseen Error
MsgBox Prompt := "Please record number and info. " & _
Err.Description & " " & Err.Number, _
buttons:=vbOKOnly + vbInformation
Resume Here ' go to line labeled Here
End Select
End Sub