Vb6 Error Handling

VB6 Error Handling — CSEH Pattern

VB6 has no try/catch. The model is On Error GoTo <label> with structured cleanup. The CSEH (Code Style for Error Handler) convention standardizes this pattern across the codebase so error paths are predictable, log output is consistent, and Erl (error line) is meaningful in production logs.

CSEH style is declared by a marker comment above the procedure signature:

'CSEH: ErrRaise
Public Function GetActiveCustomer(...) As Object

The <EhHeader> and <EhFooter> tags delimit regions that may be regenerated by external tooling (MZ-Tools or similar). Never alter the delimiter format or remove the tags — automation depends on them.

1. Two CSEH styles

Default for new code: ErrRaise. The MsgBox style is reserved for the outermost layer (event handlers) where errors must be surfaced to the user and the call stack ends.

2. ErrRaise template

Canonical structure for a function that propagates errors to its caller:

'CSEH: ErrRaise
Public Function GetActiveCustomer(ByVal lngCustomerID As Long) As Object

        '<EhHeader>
        On Error GoTo GetActiveCustomer_Err

        EnterMethod "modDB", "GetActiveCustomer"
        '</EhHeader>

        Dim objCmd As Object
        Dim objRs  As Object
        Dim strSQL As String

100     InitializeDBConnection

105     Set objCmd = CreateObject("ADODB.Command")
110     Set objCmd.ActiveConnection = mobjCnn
115     objCmd.CommandType = adCmdText

120     strSQL = "SELECT * FROM Customer WHERE idCustomer = ? AND custActive = 1"
125     objCmd.CommandText = strSQL

130     objCmd.Parameters.Append objCmd.CreateParameter("idCustomer", adInteger, adParamInput, , lngCustomerID)

135     Set objRs = objCmd.Execute

140     Set GetActiveCustomer = objRs

    '<EhFooter>

        On Error GoTo 0

GetActiveCustomer_Exit:
        Set objCmd = Nothing
        ExitMethod "modDB", "GetActiveCustomer"
        Exit Function

GetActiveCustomer_Err:
        ' Capture Err state FIRST — the cleanup and ExitMethod below reset the Err object
        Dim lGetActiveCustomer_ErrNum As Long
        Dim sGetActiveCustomer_Err    As String

        lGetActiveCustomer_ErrNum = Err.Number
        sGetActiveCustomer_Err = "Error: " & Err.Number & " - " & Err.Description & " (" & Erl & ")"

        ' clean up resources here
        Set objCmd = Nothing
        Set objRs = Nothing

        ExitMethod "modDB", "GetActiveCustomer"

        Err.Raise lGetActiveCustomer_ErrNum, "SampleApp.modDB.GetActiveCustomer", sGetActiveCustomer_Err

    '</EhFooter>
End Function

Invariants

1. Marker above the signature: 'CSEH: ErrRaise
2. <EhHeader> region contains exactly On Error GoTo <Func>_Err and EnterMethod "<module>", "<Func>"
3. Two labels in the footer:
   • <Func>_Exit (success path)
   • <Func>_Err (error path)
4. <Func>_Exit falls through to Exit Sub/Exit Function — never to the _Err label
5. Capture Err state first: the opening statements of <Func>_Err copy Err.Number into a local and build the message string before any other statement runs. Cleanup, ExitMethod, object teardown, or any procedure call resets Err (see "Capture Err before cleanup" below)
6. Error message format: "Error: " & Err.Number & " - " & Err.Description & " (" & Erl & ")"
7. Cleanup before raise: every Set ... = Nothing happens before Err.Raise, not after (after is unreachable)
8. ExitMethod is called on both paths — success and error — to keep the trace stack consistent
9. Err.Raise source: qualified as <Application>.<Module>.<Procedure>
10. Err.Raise number: when propagating a caught error, re-raise the original Err.Number captured in invariant 5; reserve vbObjectError + <offset> for errors the procedure originates itself (see section 5)

Capture Err before cleanup

The opening statements of the _Err handler must copy Err.Number and Err.Description into locals. Any statement that runs afterwards — a Set obj = Nothing (which fires the object's Class_Terminate), ExitMethod, closing a recordset, or any call to a procedure that does its own error handling — can reset the Err object to 0 / "". Referencing Err.Number or Err.Description directly in the final Err.Raise, after cleanup has run, re-raises a blank error and the real failure is lost.

❌ Wrong — Err is read in Err.Raise after cleanup has already cleared it:

BuildPagamentos_Err:
        Set rsPag = Nothing
        ExitMethod "clsFiscalApiEmitterNFe", "BuildPagamentos"
        ' Err.Number / Err.Description are 0 / "" by now — a blank error is re-raised
        Err.Raise Err.Number, "AutomatizeSis.clsFiscalApiEmitterNFe.BuildPagamentos", Err.Description

✅ Right — capture into locals first, then clean up, then raise:

BuildPagamentos_Err:
        Dim lBuildPagamentos_ErrNum As Long
        Dim sBuildPagamentos_Err    As String

        lBuildPagamentos_ErrNum = Err.Number
        sBuildPagamentos_Err = "Error: " & Err.Number & " - " & Err.Description & " (" & Erl & ")"

        Set rsPag = Nothing
        ExitMethod "clsFiscalApiEmitterNFe", "BuildPagamentos"

        Err.Raise lBuildPagamentos_ErrNum, "AutomatizeSis.clsFiscalApiEmitterNFe.BuildPagamentos", sBuildPagamentos_Err

The same rule applies to the MsgBox style: build the message string from Err before MsgBoxCritical and before falling through to _Exit (where ExitMethod runs).

3. MsgBox template

For top-level procedures where errors terminate at the UI:

'CSEH: MsgBox
Public Sub cmdSave_Click()

        '<EhHeader>
        On Error GoTo cmdSave_Click_Err

        EnterMethod "frmCustomer", "cmdSave_Click"
        '</EhHeader>

        ' ... UI logic that calls other CSEH:ErrRaise functions ...

    '<EhFooter>

cmdSave_Click_Exit:
        ExitMethod "frmCustomer", "cmdSave_Click"
        Exit Sub

cmdSave_Click_Err:
        Dim scmdSave_Click_Err As String

        scmdSave_Click_Err = Err.Description & vbCrLf & _
                             "in SampleApp.frmCustomer.cmdSave_Click at line " & Erl

        MsgBoxCritical scmdSave_Click_Err

        GoTo cmdSave_Click_Exit

    '</EhFooter>
End Sub

Differences from ErrRaise

• MsgBoxCritical (or equivalent project wrapper) displays the error to the user
• No Err.Raise at the end — the error stops here
• GoTo <Func>_Exit after the dialog so cleanup and ExitMethod still run

4. Line numbering and Erl

Erl is a built-in function returning the line number where the last error occurred — but only if lines are numbered. Without numbering, Erl returns 0 and traceability is lost.

Numbering convention

• Start at 100, increment by 5
• Number only executable lines (no Dim, no comments, no labels, no <EhHeader>/<EhFooter> tags)
• Multi-line SQL concatenation also gets numbered
• Do not number trivial getters/setters or one-liner functions

        Dim strSQL As String

100     strSQL = ""
105     strSQL = strSQL & "SELECT idCustomer, custName " & vbNewLine
110     strSQL = strSQL & "  FROM Customer " & vbNewLine
115     strSQL = strSQL & " WHERE custActive = 1 " & vbNewLine

120     objCmd.CommandText = strSQL
125     Set objRs = objCmd.Execute

Which procedures get numbered

Number any procedure with:

• On Error GoTo (CSEH style)
• More than ~10 executable lines
• Database access
• File or COM-object interaction

Skip numbering for:

• Property Get/Let that just reads/assigns a field
• One-liner utility functions
• Computed-only functions with no side effects and minimal logic

5. Err.Raise number — propagate vs originate

Two different situations call Err.Raise, and they choose the number differently:

• Propagating a caught error (the _Err handler re-raising what it caught): re-raise the original Err.Number, captured into a local before cleanup (see section 2). This keeps the true error code intact as it travels up the stack, so the outermost handler logs the real failure — not a re-stamped offset.
• Originating an error (the procedure itself detects a fault and raises, e.g. a failed validation or an empty mandatory recordset): use vbObjectError + <offset>, where <offset> identifies the condition. Here the procedure is setting Err, so there is nothing to capture first.

' Originating — the procedure decides this is an error:
125     If objRs.EOF Then Err.Raise vbObjectError + 101, "SampleApp.modDB.GetActiveCustomer", "Customer not found"

For originated errors, the offset is chosen per procedure (or per error category, if the codebase catalogs them):

In all cases the qualified source string (SampleApp.modDB.GetActiveCustomer) carries the procedure identity, and the EnterMethod/ExitMethod trace stack records the full call chain independently.

6. On Error Resume Next — only in cleanup

On Error Resume Next is forbidden in domain code. It silently swallows errors and produces invisible failures. One legitimate exception: cleanup and dispose routines, where the operation must complete even when individual steps fail.

Public Sub CloseDBConnection()
    On Error Resume Next                    ' OK: cleanup
    If Not mobjCnn Is Nothing Then
        If mobjCnn.State <> 0 Then mobjCnn.Close
    End If
    Set mobjCnn = Nothing
End Sub

Other acceptable uses:

• Closing file handles in an error handler
• Releasing COM objects when their state may be invalid
• Best-effort log writes (failing to log should not crash the app)

Not acceptable:

• Around business logic
• "To suppress a warning I do not want to deal with"
• Wrapping an entire procedure body

7. EnterMethod / ExitMethod placement

Every CSEH procedure calls both:

EnterMethod "modDB", "GetActiveCustomer"  ' in <EhHeader>
ExitMethod  "modDB", "GetActiveCustomer"  ' in both _Exit and _Err

Calling ExitMethod only on the success path leaves the trace stack with the function "still open" after an error, corrupting future trace output. The error label must also call ExitMethod before the Err.Raise.

See vb6-trace-pattern for the trace mechanism details.

8. Cleanup discipline

Resource cleanup (Set objX = Nothing, closing recordsets, closing files) appears in both the _Exit and _Err labels. Code paths must converge on freed resources regardless of outcome.

Common pattern: extract cleanup into a helper or duplicate the lines in both labels. The CSEH _Exit/_Err structure does not provide an automatic finally — discipline is on the author.

9. Anti-patterns

10. Pre-flight check

• CSEH marker present above the signature ('CSEH: ErrRaise or 'CSEH: MsgBox)
• <EhHeader> and <EhFooter> tags present and unmodified
• On Error GoTo <Func>_Err is the first line inside <EhHeader>
• EnterMethod called in <EhHeader>
• Both _Exit and _Err labels exist
• _Exit falls through to Exit Sub/Exit Function, not into _Err
• Error message includes Err.Number, Err.Description, and Erl
• Err.Number/Err.Description captured into locals before any cleanup or ExitMethod; the re-raise uses the captured values (the original number when propagating), never Err.Number/Err.Description read after cleanup
• Cleanup is performed before Err.Raise (or before MsgBoxCritical in MsgBox style)
• ExitMethod is called in both labels
• Err.Raise source string is <App>.<Module>.<Procedure>
• Executable lines are numbered (for procedures large enough to warrant it)
• No On Error Resume Next outside cleanup/dispose