what is the better way to handle errors in VB6

Question

I have VB6 application , I want to put some good error handling finction in it which can tell me what was the error and exact place when it happened , can anyone suggest the good way to do this

Answer 1

First of all, go get MZTools for Visual Basic 6, its free and invaluable. Second add a custom error handler on every function (yes, every function). The error handler we use looks something like this:

On Error GoTo {PROCEDURE_NAME}_Error

{PROCEDURE_BODY}

    On Error GoTo 0
    Exit {PROCEDURE_TYPE}

{PROCEDURE_NAME}_Error:

   LogError "Error " & Err.Number & " (" & Err.Description & ") in line " & Erl & _
            ", in procedure {PROCEDURE_NAME} of {MODULE_TYPE} {MODULE_NAME}"

Then create a LogError function that logs the error to disc. Next, before you release code add Line Numbers to every function (this is also built into MZTools). From now on you will know from the Error Logs everything that happens. If possible, also, upload the error logs and actually examine them live from the field.

This is about the best you can do for unexpected global error handling in VB6 (one of its many defects), and really this should only be used to find unexpected errors. If you know that if there is the possibility of an error occurring in a certain situation, you should catch that particular error and handle for it. If you know that an error occurring in a certain section is going to cause instability (File IO, Memory Issues, etc) warn the user and know that you are in an "unknown state" and that "bad things" are probably going happen. Obviously use friendly terms to keep the user informed, but not frightened.

Answer 2

a simple way without additional modules, useful for class modules:

pre-empt each function/subs:

On Error Goto Handler

handler/bubbleup:

Handler:
  Err.Raise Err.Number, "(function_name)->" & Err.source, Err.Description

voila, ghetto stack trace.

Answer 3

I use a home-grown Error.bas module to make reporting and re-raising less cumbersome.

Here's its contents (edited for length):

Option Explicit

Public Sub ReportFrom(Source As Variant, Optional Procedure As String)
    If Err.Number Then
        'Backup Error Contents'
        Dim ErrNumber As Long: ErrNumber = Err.Number
        Dim ErrSource As String: ErrSource = Err.Source
        Dim ErrDescription As String: ErrDescription = Err.Description
        Dim ErrHelpFile As String: ErrHelpFile = Err.HelpFile
        Dim ErrHelpContext As Long: ErrHelpContext = Err.HelpContext
        Dim ErrLastDllError As Long: ErrLastDllError = Err.LastDllError
    On Error Resume Next
        'Retrieve Source Name'
        Dim SourceName As String
        If VarType(Source) = vbObject Then
            SourceName = TypeName(Source)
        Else
            SourceName = CStr(Source)
        End If
        If LenB(Procedure) Then
            SourceName = SourceName & "." & Procedure
        End If
        Err.Clear
        'Do your normal error reporting including logging, etc'
        MsgBox "Error " & CStr(ErrNumber) & vbLf & "Source: " & ErrSource & vbCrLf & "Procedure: " & SourceName & vbLf & "Description: " & ErrDescription & vbLf & "Last DLL Error: " & Hex$(ErrLastDllError)
        'Report failure in logging'
        If Err.Number Then
            MsgBox "Additionally, the error failed to be logged properly"
            Err.Clear
        End If
    End If
End Sub

Public Sub Reraise(Optional ByVal NewSource As String)
    If LenB(NewSource) Then
        NewSource = NewSource & " -> " & Err.Source
    Else
        NewSource = Err.Source
    End If
    Err.Raise Err.Number, NewSource, Err.Description, Err.HelpFile, Err.HelpContext
End Sub

Reporting an error is as simple as:

Public Sub Form_Load()
On Error Goto HError
    MsgBox 1/0
    Exit Sub
HError:
    Error.ReportFrom Me, "Form_Load"
End Sub

Reraising an error is as simple as calling Error.Reraise with the new source.

Although it is possible to retrieve the Source and Procedure parameters from the call stack if you compile with symbolic debug info, it's not reliable enough to use in production applications

Answer 4

ON ERROR GOTO

and the

Err

object.

There is a tutorial here.

Answer 5

Yes, take Kris's advice and get MZTools.

You can add line numbers to section off areas of complex procedures, which ERL will report in the error handler, to track down which area is causing the error.

10
    ...group of statements
20
    ...group of statements
30
    ...and so on

Answer 6

Use on

dim errhndl as string
on error goto errhndl
errhndl:
msgbox "Error"

Answer 7

Use the On Error statement and the Err object.