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bcquality/microsoft/knowledge/events/do-not-add-ishandled-to-an-existing-event.md
Jesper Schulz-Wedde faeacb2484 Add 12 general AL event-design articles to events domain 
Add 12 atomic knowledge articles under microsoft/knowledge/events covering
general AL event-design best practices: IsHandled initialization and OnAfter
preservation, appending new event parameters, position-based event naming,
reusing/extending events, avoiding per-iteration publishing, Temp-prefixing
temporary record parameters, unabbreviated parameter names, preferring the
this keyword over IncludeSender, avoiding loosely typed parameters, not
mutating existing event contracts, and not bypassing critical operations
with IsHandled. Each article ships a .good.al and .bad.al demonstration
sample (object IDs 50240-50296; not compiled by CI). Extend the
al-events-review leaf Worklist with one targeted check per new rule.

Additive only; no contract or wiring change (events leaf already wired).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
2026-06-25 11:58:59 +02:00

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bc-version domain keywords technologies countries application-area
all
events
ishandled
breaking-change
event-contract
backward-compatibility
onbefore
integration-event
subscribers
al
w1
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Do not add IsHandled to an existing event

Description

Adding a var IsHandled: Boolean parameter to an event that already shipped without one is a breaking contract change. The signature changes, so existing subscribers no longer match and silently stop firing until they are updated, and the event's meaning shifts from "notify" to "overridable" — a semantic the original subscribers never agreed to. The safe move is to leave the existing event untouched and introduce a new OnBefore… event carrying IsHandled at the point you want to make overridable. Existing subscribers keep working against the original event; new subscribers opt into the override seam through the new one.

Best Practice

Keep the existing event as-is and add a separate OnBeforeX(…; var IsHandled: Boolean) before the logic you want to make overridable. Two events with distinct, stable contracts are safer than one event whose meaning and signature were changed under its subscribers.

See sample: do-not-add-ishandled-to-an-existing-event.good.al.

Anti Pattern

Mutating a shipped event — for example adding var IsHandled to OnAfterCalculateTotal — to retrofit override behaviour, breaking every existing subscriber and overloading the event's meaning. Detection: an IsHandled parameter added to a pre-existing event signature rather than introduced through a new dedicated OnBefore publisher.

See sample: do-not-add-ishandled-to-an-existing-event.bad.al.