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

Style	Marker	Handler behavior	Use for
ErrRaise	'CSEH: ErrRaise	Captures Err into locals, then re-raises the original Err.Number via Err.Raise ..., msg	Utility functions, library routines, any procedure called by other code
MsgBox	'CSEH: MsgBox	Displays critical dialog and returns to caller without re-raising	Top-level UI event handlers, command buttons, menu items

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