vb6-trace-pattern

VB6 Trace Pattern

VB6 has no native call stack accessible to user code. When an error fires in production, Err.Description tells you what but not where in the chain of calls. This pattern solves that by maintaining a manual stack in a module-level array, pushed by EnterMethod on entry and popped by ExitMethod on exit.

The pattern is one of the most useful additions you can make to a long-lived VB6 codebase. The cost is one line at the top and one line in each exit path of every instrumented procedure.

1. When to instrument

Every non-trivial new procedure gets EnterMethod/ExitMethod. "Non-trivial" means any of:

• More than ~5 lines of logic
• Has On Error GoTo (CSEH style)
• Accesses database, file, or network
• Called from more than one place

Skip for:

• Trivial getters/setters (Property Get/Let that just reads/assigns a field)
• One-line utility functions
• Stateless pure helpers (string formatting, math)

When in doubt, instrument. The overhead is negligible compared to the diagnostic value when something fails in production.

2. Canonical placement

The pattern is paired with the CSEH error-handling pattern:

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

        '<EhHeader>
        On Error GoTo GetActiveCustomer_Err

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

        ' ... body ...

    '<EhFooter>
        On Error GoTo 0

GetActiveCustomer_Exit:
        ' cleanup
        ExitMethod "modDB", "GetActiveCustomer"
        Exit Function

GetActiveCustomer_Err:
        ' Capture Err 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 & ")"

        ' cleanup
        ExitMethod "modDB", "GetActiveCustomer"

        Err.Raise lGetActiveCustomer_ErrNum, "SampleApp.modDB.GetActiveCustomer", sGetActiveCustomer_Err
    '</EhFooter>
End Function