UK XL User Conference 2006 Excel VBA Best Practice Simon Murphy Developer – Codematic Ltd

UK XL User Conference 2006 Spreadsheet background Up to 200 Mb size Up to 1 Million formulas 1-10,000 unique formulas 5-10,000 lines of VBA £Billions in values Often linked to other technologies such as OLAP, ADO, COM or.net etc. Finance, Banking and Sales and Marketing areas Development cost up to $1M Active member of Eusprig – European Spreadsheet Risk group – dedicated to raising awareness of dangers and error rates in commercial spreadsheets

UK XL User Conference 2006 Agenda Security Fundamental Imperative Development process Development environment Design considerations High quality code –General –Modules –Procedures –Blocks –Variables –Excel Specific advice –Classes Avoiding common errors Debugging Testing Documentation Resources

UK XL User Conference 2006 Security Is everybodys concern Spreadsheets can be used as a staging board for privilege escalation (with your login details!) Consider SD 3 +C –Secure by Design Default Deployment Communication Threat Modelling – Assets, Threats Threat Types – STRIDE –(Spoofing, Tampering, Repudiation, Information Disclosure, Denial of Service, Elevation of Privilege) Threats – rate with DREAD –(Damage potential, Reproducibility, Exploitability, Affected Users, Discoverability) Spreadsheets (all flavours) are fairly insecure –Compiled UDFs (.net, COM, XLL) and Database servers can help Set macro security to high and use code signing certificates. See Microsoft MOC 2840A – Implementing security for more info.

UK XL User Conference 2006 Fundamental Imperative Manage complexity (McConnell) Solution complexity grows at 4 x the rate of problem complexity. (Glass) [Things] should be as simple as they can be, but no simpler (Einstein) K.I.S.S. This principle should drive all other work. Easier to build, easier to test, easier to document, easier to use, etc… [No conflict]

UK XL User Conference 2006 Development Process Systems Development lifecycle –Requirements, –Analysis, –Logical Design, –, –Physical Design, –Construction, –Test, –Release, –Maintain. –In some shape or form. Understanding the requirements is critical and difficult Some sort of structured approach is useful Build a library of useful code and references

UK XL User Conference 2006 Design – 2 Questions, 2 Approaches 2 Questions –What will happen when things go right? –What will happen when things go wrong? 2 Approaches: –What will the system do? –What real world objects am I modelling?

UK XL User Conference 2006 Coupling How strongly 2 separate elements depend on each other Low coupling is better, especially through clearly defined interfaces High coupling often means hidden dependencies which generally leads to incorrect modifications (side effects) Example: hard coded cell addresses in VBA code Example: Gobal/Public variables

UK XL User Conference 2006 Fan in / fan out Low fan in – a VBA routine depends on only a few other units –Good because it minimises dependencies and reduces complexity High fan out – a VBA routine is used by many others –Good because it minimises duplication -Good because routine is more thoroughly exercised

UK XL User Conference 2006 Development Environment Option explicit Dont warn about syntax errors Actual set up - which windows are visible etc Use a code library Use MZ tools Other tools – call tree, indenter, reference checker, BYG Source control, even just exporting as text

UK XL User Conference 2006 Design Considerations Simplicity Testability Cohesion and coupling Fan in fan out Encapsulation and separation Reusability What does it do, or what are you modelling? N tier – especially msgbox and error handling Consider future changes and/or enhancements Design patterns eg: singleton, factory, facade Design normal case and error case

UK XL User Conference 2006 High Quality Code – General 1 Naming convention: –Option 1 fit in with current standards (eg Reddick 3 letter) –Option 2 give everything a simple, distinctive, non abbreviated descriptive name Procedures – verb noun combination Variables – add scope prefix, data type maybe, usage more useful – eg m_rwCustomer for row in customer table or colMonth for month column. Only use a type prefix if you know what it is doing for you and you value that. (eg probably dont need obj) Avoid most comments – make meaning crystal clear in the code. Move non standard code and workarounds to named routines, and comment if necessary refactor early and often build test routines as you go, and at least step through in the debugger.

UK XL User Conference 2006 High Quality Code – General 2 Recognise when you move from procedure based design to module/object based (eg procs need to return >1 value, lots of parameters get passed all around) Object Oriented designs can be useful, but can cost performance The biggest factor on performance is design Be aware of Excel/VBA limitations, and clear on its strengths Use TODO and maybe Enhancement comments (dont release until TODOs cleared – (use edit find in project) Use shift+F2 for definitions, F8, Shift+F8 and Ctrl+Shift+F8 (step out) and stop Note compiler is weak compared to C++ Use code cleaner regularly Differentiate style from quality Use the most restrictive scope that works for variables and routines

UK XL User Conference 2006 High Quality Code - Modules As solution complexity increases think in terms of modules rather than procedures Use enums to add understanding Could use types but probably better to go straight to classes. Module name should explain what it does, a comment at the top may be useful. May need 1 or 2 global variables, but may need lots of module level variables A module should represent a bunch of closely related things eg: use common data Use Option private module otherwise public routines will be listed in the user defined functions list. Forms should usually call straight out to a class or module to do the real work.

UK XL User Conference 2006 High Quality Code - Procedures Anatomy of a procedure/object – initialisation, process, clear up, error handling Short routines lines max (one screens worth) (easier to understand – but not proven to reduce errors) A procedure should do one thing well Prefer functions (Boolean success/failure) Use parameters rather than global variables (max 7+/- 2) Avoid application.run – breaks VBA error handling Use separate routines for separate error handling Consider error handling early, use break on all errors when testing Use environ rather than api where possible (simple) All call tree parents should have error handling Test performance dont guess (see perfmon) Protect procedures from bad input (including malicious input)

UK XL User Conference 2006 High Quality Code - Blocks Code block start and end together IF using If…then code an Else if, add a comment, only remove it if you are sure there is no else case (missing the else is a common error) Code the expected behavoir first, usually. Use do while and do until loops, be sure they will end, I generally mainly use for next and for each next, finding out the end first helps with status bar updates. In a Select Case always have a default case, possibly with an error warning.

UK XL User Conference 2006 High Quality Code - Variables Dont reuse variables for different purposes, avoid temp Use clear data types, and control them (watch out for implicit coercion) Variants are often easier to work with than arrays Pass parameters byVal if they are not to be changed (watch for coercion) Explicitly use byRef in an input parameter is to be changed, but watch for signs to redesign Dont implicitly use the default property, be explicit (eg range.value) Magic numbers and strings should be made into constants. Use explicit data casting Cstr() Cbool() etc Code with early binding if at all possible (new), change to late binding to support multiple versions or if component may not be installed. Late can be slower and may need better error handling. Can define all variables together at top or just before they are needed both have pros and cons. Sometimes a simple i, j, x or y is better than a long datatype prefixed descriptive name for a loop index Boolean variable should be named so True or False make sense – eg use done rather than status Booleans should be positive eg if not found then… rather than if not notFound then… Never mess with a loop counter within the loop.

UK XL User Conference 2006 High Quality Code – Excel 1 Its usually dangerous to hardcode a range reference in VBA. Use range names to connect code to worksheet ranges Choose a single cell range name as a start point Choose a multi-cell range to allow the user to insert/delete rows/columns Use in cell text where possible, closely ties the mechanics to what the user uses. Always check cell text thoroughly before depending on it Note data validation is easily bypassed Give sheets code names and use them Take the first few rows and columns for system use, hide them if necessary Check the selection type with typeOf if you are going to use it. Generally avoid selecting and copying – set values instead where possible. Be clear and explicit which workbook and worksheet code should operate on, especially ThisWorkbook v ActiveWorkbook for add-ins.

UK XL User Conference 2006 High Quality Code – Excel 2 Pulling Range.Value or Range.Formula into a variant is much faster than looping the cells. Use worksheet.cells(r,c) structure rather than range(A & someNumber) Be consistent how you use ranges (I usually use sheet.[rangeName].value) Remember the power of R1C1 notation Its often easier to set the formulaR1C1 than the formula of a range. If distributing code with references, consider making it late bound. Use standard environment management routines Use Excel functionality wherever possible Use doEvents where needed Work upwards when deleting rows, otherwise counters get messed up Watch for inadvertently firing events, dont hog the onCalculate event

UK XL User Conference 2006 High Quality Code - Classes Classes result from asking what am I modelling? rather than what is this system to do? Full class based design may be overkill for most VBA applications Hard/impossible to combine the best of spreadsheets with proper OO – data hiding is not possible. Classes may be useful when a set of routines share lots of data and it becomes cumbersome to pass around as parameters Or when you need to return a complex type from a routine

UK XL User Conference 2006 Avoiding Common Errors Consider lifetime – initialise, set, use, destroy (CRUD – Create, read, update, delete) Off by one errors, especially ranges into variants, arrays, loops and ubound() Watch for index cross talk Dim x, y as integer => x is a variant StrComp, InStr – check the documentation

UK XL User Conference 2006 Debugging and Testing Debugging –Immediate window –Locals –Debug.print debug.assert Testing –Pre-conditions and post conditions –Use a test module, and application.run for private routines –Test early and test often –Keep all test code – it can act as a specification, and can give comfort that later changes do not break anything –Get good test data – realistic, test full range including boundary conditions, but weight testing towards real world use. –Execution testing v static testing and code inspection/review. –Unit testing, system testing, user acceptance testing.

UK XL User Conference 2006 Documentation If you design for simplicity only very minor additional documentation is needed Design and build the user Interface with the Users needs primary, the documentation will automatically be there Design and build the business logic parts with the maintainers needs primary, the main documentation will automatically be there, expand as required. Integral tests should explicitly clarify intent. Reports should contain enough description to be meaningful. External documentation is almost always so out of date its worse than useless. Excessive documentation is too hard to plod through Poorly targeted documentation is pointless Working software is more useful than documentation Sometimes documentation is important

UK XL User Conference 2006 Documentation Call tree print out should be enough for most uses Be clear who the target audience is Should be auto generated from source code if it is to be up to date. Stepping through code is often the easiest way to understand it. Download a VBA to HTML addin to pretty up code for printing. PUP provides a nice summary, MZ Tools creates a weighty document.

UK XL User Conference 2006 Extending Excel Excel is powerful not perfect Leverage benefits whilst managing weaknesses by using complimentary technologies Data: VBA, ADO, ODBC, OLAP,.net, COM, Info Bridge, XML, DDE, Web Queries, SOX Solution Accelerator Logic: VBA, COM,.net, xll, Pivots, Filters Search the web for vast array of free or cheap tips and tools.

UK XL User Conference 2006 Summary Manage complexity Consider security carefully Excel/VBA not the best tool for everything Be defensive, especially with inputs Understand your data Aim for cohesive models with low coupling Use names to connect VBA to worksheets Use complimentary technologies where appropriate Manage documentation

UK XL User Conference 2006 Resources Books –Code complete 2 – Steve McConnell –Professional Excel Development – Stephen Bullen, Rob Bovey & John Green –VBA Developers Handbook – Ken Getz Tools – (Stephen Bullen), Call Tree, Smart Indenterwww.oaltd.co.uk – Toolbar creator and back up. – (Rob Bovey) code cleanerwww.appspro.com – – (John W), PUPwww.j-walk.com – MZ tools VBA IDE addin. – reference checker.

UK XL User Conference 2006 Questions? –Spreadsheet consulting, reviewing, maintaining, rescuing, migrating, add-in development etc. –Staff coaching, mentoring and training Websites – –