Trapping VBScript Errors

'turn on error handling
On Error Resume Next

'try to do what you're doing - here I'm trying
'to open a connection to a database
objConn.Open "DSN=whatever;PWD=whatever;UID=whatever;DATABASE=whatever;"

'test for the error
If objConn.Errors.Count > 0 Then
    'do whatever you want
End If            

'turn error checking off
On Error Goto 0

As canadian5 said, you can also check ther Err object rather than the error collection of the object you're working with (I was using the connection object above). Hope that helps.

There is a great discussion of web page error handling at http://msdn.microsoft.com/workshop/c-frame.htm?939160113522#/workshop/author/script/weberrors.asp

I use a technique I found there.  It involves using On Error Resume Next, and then wherever an error might occur, I trap it by checking to see If Err.Number <> 0.  

Inside the trap, I call a subroutine that is included from a separate file.  In this subroutine (called TrapError), I first log all pertinent information to a text file on my server, and then redirect to a page that displays a user-friendly error message, depending on what the error was.  If the error was, say, a SQL timeout error, you could retry the SQL connection automatically instead of redirecting to an error message.

Also, canadian5 was correct in stating that On Error GoTo does not work in VBScript.

Your "answer" is nothing more than a "here is where you can do more research" as of yet canadian5 had the best "answer".  jrubin had a different approach that also seems to work.

canadian5 - post as answer and this question is over.

OK, here's more specific code -- I didn't put it all here last time because it's pretty long.  Sorry.  Hope this helps.


Step One:  Opening a connection to a database and Trapping the Error in our .asp file:

   On Error Resume Next      
      
   Set Conn = Server.CreateObject ("ADODB.Connection")
   Conn.Open Session("ConnectionString")
            
    'Check for errors opening the database connection      
    If err.number <> 0 Then                        
        sErrMsg = "[Error opening connection.]"
        trapError server.URLEncode(Request.ServerVariables("URL")), _
              Err.Number, _
              Err.Description, _
              sErrMsg            
    End If

Step 2:  The trapError sub routine (actually kept in a separate file and #included because it's accessed from many .asp files):

Sub trapError(docTitle, errNum, errDesc, msg)

    Dim errMsg
    Dim errLog
    Dim LogFile
    Dim FileSys
    Dim ShowID      
    Dim ErrorLogFullName
    Dim sLine
   
   On Error Resume Next
      
    ErrorLogFullName = "LOG_ERROR.txt"

    Err.Clear

      
Select Case errNum

      ' Call this subroutine with errNum=1 for all SQL timeout errors
     Case 1 ' SQL TIMEOUT
         ErrLog = "Error: " & errdesc
      ' Call this subroutine with errNum=2 when data objects are down
      Case 2 ' MSDATA objects error
          ErrLog = "MSCOM error - data objects are down"
      Case 5 ' Invalid procedure call or argument
      Case 6 ' Overflow
      Case 7 ' Out of memory
      Case 9 ' Subscript out of range
      Case 10 ' This array is fixed or temporarily locked
      Case 11 ' Division by zero
      Case 13 ' Type mismatch
      Case 14 ' Out of string space
      Case 17 ' Can't perform requested operation
      Case 28 ' Out of stack space
      Case 35 ' Sub or Function not defined
      Case 48 ' Error in loading DLL
      Case 51 ' Internal error
      Case 52 ' Bad file name or number
      Case 53 ' File not found
      Case 54 ' Bad file mode
      Case 55 ' File already open
      Case 57 ' Device I/O error
      Case 58 ' File already exists
      Case 61 ' Disk full
      Case 62 ' Input past end of file
      Case 67 ' Too many files
      Case 68 ' Device unavailable
      Case 70 ' Permission denied
      ErrMsg = "You do not have correct permissions to do that"            ErrLog = "I don't care about info on this one"
      Case 71 ' Disk not ready
      Case 74 ' Can't rename with different drive
      Case 75 ' Path/File access error
      Case 76 ' Path not found
      Case 91 ' Object variable not set
      Case 92 ' For loop not initialized
      Case 94 ' Invalid use of Null
      Case 322 ' Can't create necessary temporary file
      Case 424 ' Object required
      Case 429 ' ActiveX component can't create object
      Case 430 ' Class doesn't support Automation
      Case 432 ' File name or class name not found during Automation operation
      Case 438 ' Object doesn't support this property or method
      Case 440 ' Automation error
      Case 445 ' Object doesn't support this action
      Case 446 ' Object doesn't support named arguments
      Case 447 ' Object doesn't support current locale setting
      Case 448 ' Named argument not found
      Case 449 ' Argument not optional
      Case 450 ' Wrong number of arguments or invalid property assignment
      Case 451 ' Object not a collection
      Case 453 ' Specified DLL function not found
      Case 455 ' Code resource lock error
      Case 457 ' This key is already associated with an element of this collection
      Case 458 ' Variable uses an Automation type not supported in VBScript
      Case 500 ' Variable is undefined
      Case 501 ' Illegal assignment
      Case 502 ' Object not safe for scripting
      Case 503 ' Object not safe for initializing
      Case 504 ' Object not safe for creating
      Case 1001 ' Out of memory
      Case 1002 ' Syntax error
      Case 1003 ' Expected ':'
      Case 1005 ' Expected '('
      Case 1006 ' Expected ')'
      Case 1007 ' Expected ']'
      Case 1010 ' Expected identifier
      Case 1011 ' Expected '='
      Case 1012 ' Expected 'If'
      Case 1013 ' Expected 'To'
      Case 1014 ' Expected 'End'
      Case 1015 ' Expected 'Function'
      Case 1016 ' Expected 'Sub'
      Case 1017 ' Expected 'Then'
      Case 1018 ' Expected 'Wend'
      Case 1019 ' Expected 'Loop'
      Case 1020 ' Expected 'Next'
      Case 1021 ' Expected 'Case'
      Case 1022 ' Expected 'Select'
      Case 1023 ' Expected expression
      Case 1024 ' Expected statement
      Case 1025 ' Expected end of statement
      Case 1026 ' Expected integer constant
      Case 1027 ' Expected 'While' or 'Until'
      Case 1028 ' Expected 'While', 'Until' or end of statement
      Case 1030 ' Identifier too long
      Case 1031 ' Invalid number
      Case 1032 ' Invalid character
      Case 1033 ' Unterminated string constant
      Case 1034 ' Unterminated comment
      Case 1037 ' Invalid use of 'Me' keyword
      Case 1038 ' 'loop' without 'do'
      Case 1039 ' Invalid 'exit' statement
      Case 1040 ' Invalid 'for' loop control variable
      Case 1041 ' Name redefined
      Case 1042 ' Must be first statement on the line
      Case 1043 ' Cannot assign to non-ByVal argument
      Case 1044 ' Cannot use parens when calling a Sub
      Case 1045 ' Expected literal constant
      Case 1046 ' Expected 'In'
      Case 4096 ' Microsoft VBScript compilation error
      Case 4097 ' Microsoft VBScript runtime error
      Case -2147217900
      ErrLog = "Incorrect syntax near [SQL ERROR]:" & errdesc
      Case 3704 ' SQL Server not available or timed out      
      ErrLog = "SQL Server not available or timed out [SQL ERROR]:" & _
                    errdesc
      Case Else
End Select

    ' open up access to the logfile
    set FileSys = CreateObject("Scripting.FileSystemObject")
    set LogFile = FileSys.OpenTextFile (ErrorLogFullName, 8, True)

    ' generate a log entry for this error
    If len(errLog) = 0 Then      
    'Use a generic message (errDesc)
         sLine =  (Now & vbTab & msg & vbTab & errNum & vbTab &  _
               errDesc & vbTab & URLDecode(docTitle) & vbTab & _
               Request.ServerVariables("HTTP_USER_AGENT"))
    Else
    'Use a message defined above (errLog)
        sLine = (Now & vbTab & msg & vbTab & errNum & vbTab &  _
                     errLog & vbTab & URLDecode(docTitle) & vbTab & _
         Request.ServerVariables("HTTP_USER_AGENT"))
    End If

    LogFile.writeline sLine
    LogFile.Close
      
    ' display the user's error page
    ' NOTE:  We need an absolute URL here because we don't know which page
    '        (in which directory) is calling this method, so a relative
    '        URL is impossible to construct.
   Response.Redirect Request.ServerVariables("APPL_PHYSICAL_ADDR")  & _
                               "/includes/ShowError.asp?errorNum=" & errNum
                                                
End Sub

Step 3:  Helper function to decode URL

' helper function used to un-encode a URL string
Function URLDecode(strToDecode)
      Dim strIn      
              Dim strOut
      Dim intPos
      Dim strLeft
      Dim strRight
      Dim intLoop
      strIn = strToDecode
      strOut = ""                  'The result string
      intPos = instr(strIn, "+")      'look for + and replace with a space
      Do while intPos
            strLeft = ""
            strRight = ""
            If intPos > 1 Then strLeft = Left(strIn, intPos -1)
            If intPos < Len(strIn) Then strRight = Mid(strIn, intPos +1)
            strIn = strLeft & " " & strRight
            intPos = Instr(strIn, "+") ' and then look for next one
            intLoop = intLoop + 1
      Loop
      intPos = Instr(strIn, "%")      'Look for ASCII coded chars
      Do While intPos
            If intPos > 1 Then strOut = strOut & Left(strIn, intPos -1)
            strOut = strOut & Chr(Cint("&H" & Mid(strIn, intPos + 1, 2)))
            If intPos > (Len(strIn) - 3) Then
                  strIn = ""
            Else
                  strIn = Mid(strIn, intPos + 3)
            End If
            intPos = Instr(strIn, "%")      'and then look for next one
      Loop
      URLDecode = strOut & strIn
End Function

Step 3:  The ShowError.asp file:

    Dim ErrorType
    Dim ErrMessage

    ErrorType = clng(Request.QueryString("errorNum"))

    '----- Build Error Message to display based on the type of error -----'
    Select Case ErrorType
        Case SQL_OTHER
      ErrMessage = "We are sorry, but we are experiencing an " & _
                    "unusually high volume of traffic."
        Case else
      ErrMessage = "We are sorry, but we are experiencing " & _
                    "temporary problems with our web site."
    End Select

    <HTML>
        <HEAD></HEAD>
        <BODY>
            <% =ErrMessage %>
        </BODY>
    </HTML>