Error Handling

In a complex procedure, you will find there are lots of places where errors can occur, and quite often are the same errors. If this is the case putting code after each line is a waste of space and time. A much more efficient way is to use the statement:

On Error GoTo LineLabel

This statements tell visual basic, that if an error occurs after this statement, to continue code execution at the LineLabel specified. The following code attempts to delete A: est.txt (this assumes A: is a disk drive). If an error occurs, it will give the appropriate error message. It will then attempt to get the size of C:fred.txt . Again, if an error occurs it will give the appropriate message. However it all uses the same code to detect the error.

Sub Command1_Click()
   ' If an error occurs GoTo FileError
   On Error GoTo FileError
  
   ' Attempt to delete A: est.txt
   Kill "A: est.txt"

   ' Attempt to get the size of C: est.txt
   gFileSize = FileLen("A: est.txt")

   Msgbox "File Size is: " & gFileSize

   ' Exit the procedure as no error has occurred
   Exit Sub

FileError:
   Select Case Err
   Case 53
       ' The file does not exist. Inform the user
      Msgbox "The file does not exist"
      gFileSize = "<Unknown>"
   Case 5
      ' The disk is not ready. Prompt user for action
      Ans = Msgbox ("Insert a disk into Drive A", vbAbortRetryIgnore)
      If Ans = vbRetry Then
         ' The user chose Retry. Resume code execution
         ' at where the error occurred
         Resume
     ElseIf Ans=vbIgnore Then
       ' The user chose to ignore. Resume code execution
         ' at the line after where the error occurred
         Resume Next
      Else
         ' The user chose abort. Exit this procedure
         Exit Sub
      End If
   Case Else
        ' unexpected error
      Msgbox "An error occurred. " & Err & " : " & Error
   End Select
End Sub

FileLen returns the size of the specified file in bytes. Normally Err 75 would occur if the drive was not ready, however when using FileLen Err 5 occurs