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bcquality/microsoft/knowledge/error-handling/defensive-vs-offensive-code-must-match-blast-radius.md
Michael Dieringer 3842ef7138 Address second round of Jesper Schulz-Wedde's review on PR #157 
- log-writes-must-survive-rollback.good.al: fixed invalid trigger
  OnRun(var Rec: ...) declaration; Rec is implicit when TableNo is set.
- exposed-objects-must-be-in-a-permission-set.md: distinguished the three
  exposure mechanisms (page/query web service or API, codeunit published
  as a web service, [ServiceEnabled] bound action on a page) and their
  actual permission targets (page/query "..." = X vs codeunit "..." = X).
- code-must-not-change-workdate.md: scoped from an absolute "never" to
  "not as a side effect of unrelated logic" - verified real WorkDate(x)
  setter usage in BCApps demo-data generators and test codeunits.
- bcpt-scenarios-must-be-app-specific.md: SingleInstance and
  StartScenario/EndScenario reframed as context-dependent patterns, not
  mandatory requirements - BCPT Create Customer uses neither.
- test-feature-scenario-tags.good.al/.bad.al: replaced the invented
  LibrarySales.CreateCustomerWithPrice/"Item Price Mgt." calls with a real,
  verified price-list-line test using Library - Sales/Library - Inventory/
  Library - Price Calculation.
- page-design-must-match-bc-page-type-conventions.md: scoped the missing
  UsageCategory anti-pattern to pages intended as searchable entry points.
- defensive-vs-offensive-code-must-match-blast-radius.md/.good.al/.bad.al:
  replaced the VAT registration number "low blast radius" example with a
  genuinely cosmetic field (customer home page URL).
- source-organized-by-feature-not-object-type.md: anti-pattern reframed as
  inconsistency with a repo's own convention, not the object-type scheme
  itself.
- pictures-must-use-media-not-blob.md: removed leftover "image variants"
  wording contradicting the already-corrected MediaSet description.

Proactively fixed while sweeping all fixtures for invented APIs:
- given-blocks-must-cover-full-precondition-chain.bad.al: PostSalesOrder
  called with wrong arity and referenced an undeclared variable.
- ui-test-codeunit-naming.good.al/.bad.al: replaced the same fake
  "Item Price Mgt."/TestPage "Item Price" with real Library - Sales calls
  and the real Customer Card TestPage.

Worklist completeness: added review-skill cues for the 12 of 18 new rules
that had none (al-appsource-review.md, al-data-modeling-review.md,
al-error-handling-review.md, al-security-review.md, al-style-review.md x3,
al-testing-review.md x2, al-ui-review.md, al-upgrade-review.md,
al-web-services-review.md), and fixed test-feature-scenario-tags' cue,
which only matched the compliant (tagged) shape instead of the anti-pattern
(untagged/generic-named test).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
2026-09-21 22:43:46 +02:00

2.8 KiB
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bc-version domain keywords technologies countries application-area
all
error-handling
defensive-programming
offensive-programming
fail-fast
blast-radius
guarded-lookup
al
w1
all

Match defensive vs. offensive error handling to the blast radius of being wrong

Description

Whether code should guard gracefully (defensive) or fail loudly (offensive/fail-fast) is not a matter of habit or a blanket house style — it depends on what happens downstream if the guarded condition is silently defaulted or skipped. Treating every missing value the same way, defensively or offensively, is itself the anti-pattern: uniform defensiveness hides the failures that matter most, while uniform fail-fast turns ordinary, expected absence into unnecessary crashes. Two fields can look structurally identical — both read from a related record, both potentially missing — and still deserve opposite treatment depending on what they feed.

Best Practice

Trace what a silently-defaulted or skipped value actually reaches before deciding how to guard it. If it reaches a posted ledger amount, a tax/VAT calculation, a quantity or price actually used in a transaction, or a legally/compliance-facing output, code offensively: let the lookup fail loud (TestField, an unguarded Get() expected to always succeed, or an explicit Error) so a human sees the problem before anything posts. If it is cosmetic, informational, or easily corrected after the fact (a display field, an optional UI enhancement, a report not yet run), code defensively — but the fallback must be an explicit, deliberately-chosen, named business value, never a blank or zero that is merely the datatype default. When genuinely unsure which category a field falls into, that is a question to resolve explicitly with whoever owns the requirement, not a coin flip.

See sample: defensive-vs-offensive-code-must-match-blast-radius.good.al.

Anti Pattern

Guarding two fields the same way purely out of habit, without analyzing what each one feeds. A low-blast-radius field, such as a customer's home page URL shown only for convenience on a printed document, and a high-blast-radius field, such as the VAT posting group that determines VAT actually applied to a posted transaction, are both wrapped in the same if Header.Get(...) then ... else pattern with a blank/zero fallback — leaving the posting-critical field free to post with a silently wrong value. A VAT registration number is not a safe stand-in for the low-risk side of this example: it is legally relevant, often validated, and can feed external VAT services or mandated document output, so it belongs on the offensive/fail-fast side alongside the posting group, not next to it as the "safe" contrast.

See sample: defensive-vs-offensive-code-must-match-blast-radius.bad.al.