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VB Tech Tip: Undocumented - the Erl Function

Visual Basic 6.0 retains remnants of its BASIC origins, including line numbers, GoTo, and GoSub. Although most of these are now curiosities, some can be used effectively. Here, Scott Loban demonstrates a use for line numbers and the Erl function.
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Anyone besides me remember programming in BASIC with GoTo, GoSub, line numbers, and Returns? We wrote some great programs that way, but the difficult maintenance that followed is what gave birth to the term "spaghetti code." Well, Microsoft QuickBasic rescued us from this by offering BASIC programmers a way to do structured programming with functions. Then Visual Basic later brought us into the modern era with object-based, event-driven programming.

So all that old spaghetti code stuff is dead, right? Wrong! All four of these beasts—GoTo, GoSub, line numbers, and Returns—are still alive in VB6. Don't believe it? Copy and paste the code from Listing 1 into a button click handler in a VB6 application and try it!

Listing 1: VB Spaghetti Code

10 GoSub 30
15 MsgBox "2"
20 GoTo 50
30 MsgBox "1"
40 Return
50 End

It's a sad fact, but I ran into legacy code as part of a project in 2000 that made extensive use of GoSub and Return! GoSub and Return have finally been dropped in Visual Basic .NET, but line numbers are still around and kicking in VB (although they are automatically converted to labels in VB .NET.)

I'll be the last one to shed tears over the passing of GoSub and Return, but this article demonstrates a good use for line numbers and an accompanying function—Erl—that has been in VB (but has been undocumented) all along. By the way, Erl is still in VB .NET as of Beta 2 as a (still poorly documented) property of the Err object.

A Debugging Headache

A common problem when debugging VB components is unhelpful error messages. A classic example is "Runtime error 429: ActiveX component can't create object" being raised from a component's Class_Initialize method, which itself instantiates several other components (see Listing 2). This error is difficult to track down because the error could be coming from any of the Set statements—and, indeed, may well be originating from a lower-level dependency that you don't even know about.

Listing 2: Class_Initialize Code That Is Difficult to Debug

Private Sub Class_Initialize()
Dim ErNum, ErDesc, ErSrc

On Error GoTo ErrHandler

Set m_FormStr = CreateObject("FPIWebForms.CConCat")
Set FPIFormInfo = CreateObject("FPIFormInfo.CFormInfo")
Set FPIctrl = CreateObject("FPIFormInfo.CControlInfo")
Set FPIininfo = CreateObject("FPIInputInfo.CInputTableInfo")
Set InSymTab = CreateObject("FPISymTab.CSymTab")
Set FPIininfo.setSymTab = InSymTab
InSymTab.AliasPrefix = "in_"
Set CM = CreateObject("FPIClientMgr.CClientMgr")
Set CalcSymTab = CreateObject("FPISymTab.CSymTab")
CalcSymTab.AliasPrefix = "ca_"
CalcSymTab.InitializeSymbolTable 1000
Set DataSymTab = CreateObject("FPISymTab.CSymTab")
DataSymTab.AliasPrefix = "da_"
DataSymTab.InitializeSymbolTable 1000
Set Calc = CreateObject("FPIEval.CCalc")
Set Util = CreateObject("FPIUtility.CUtility")
Util.SuppressUI = True
Set Eval = CreateObject("FPIEval.CEval")
Set Eval.InputInfo = FPIininfo
Set Eval.CalcRef = Calc
Set Eval.Utility = Util
Eval.AddSymbolTable DataSymTab
Eval.AddSymbolTable CalcSymTab
Eval.AddSymbolTable InSymTab
Exit Sub
 ErNum = Err.Number
 ErDesc = Err.Description
 ErSrc = Err.Source & ", CFormHTML.Class_Initialize"
 Err.Raise ErNum, ErSrc, ErDesc
End Sub

However, the judicious use of VB line numbers and the Erl function can help make problems like this a little easier to solve.

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