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Lead with a complete plugin quick start and add task-oriented usage, troubleshooting, customization, and contribution guides. Preserve the broader plugin framing, correct conflicting contract guidance, support Agents folder reviews, and align repository validation. Convert existing sample references to clickable links without changing knowledge rules. Co-authored-by: Jesper Schulz-Wedde <jesper.schulzwedde@microsoft.com> Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
26 lines
2.5 KiB
Markdown
26 lines
2.5 KiB
Markdown
---
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bc-version: [all]
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domain: events
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keywords: [ishandled, overridable, onbefore, integration-event, extensibility, event-override, subscriber-hook]
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technologies: [al]
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countries: [w1]
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application-area: [all]
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---
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# Use the IsHandled pattern to make base behaviour overridable
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## Description
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AL has no method overriding, so a `procedure` that runs its body unconditionally cannot be replaced by an extension without editing base code. The established Business Central seam for substituting default behaviour is the `IsHandled` pattern: the routine raises an `OnBefore…` integration event carrying a `var IsHandled: Boolean`, then exits early when a subscriber has set it. This hands a partner a sanctioned hook to replace the logic instead of overwriting the routine. LLMs trained on languages with inheritance emit routines whose logic always runs and expose no `OnBefore`/`IsHandled` seam, so the behaviour silently cannot be overridden.
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## Best Practice
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Raise `OnBeforeX(…, IsHandled)` as the first step of the routine and guard with `if IsHandled then exit;` before any default logic runs. Declare the publisher `[IntegrationEvent(false, false)] local procedure OnBeforeX(…; var IsHandled: Boolean)` with an empty body, and keep `IsHandled` a `var` parameter so a subscriber can write to it. A subscriber that replaces the behaviour does its work and sets `IsHandled := true`; one that only augments leaves it untouched and guards with `if IsHandled then exit;` itself. Reserve the override hook for cases where a partner genuinely needs to replace logic — when the goal is only to react, a positive `OnAfter` event is the better seam.
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See sample: [`use-ishandled-to-make-base-behaviour-overridable.good.al`](use-ishandled-to-make-base-behaviour-overridable.good.al).
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## Anti Pattern
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Two shapes. First, a routine whose default logic always runs because there is no `OnBefore…`/`IsHandled` hook at all — extensions cannot change it without overwriting base code. Second, a routine that raises `OnBeforeX(IsHandled)` but omits the `if IsHandled then exit;` guard, so the default logic still executes after a subscriber set `IsHandled := true`, duplicating work and side effects. Detection: an `OnBefore` publisher with a `var IsHandled: Boolean` parameter whose caller never tests `IsHandled`, or a public routine doing non-trivial work with no overridable seam.
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See sample: [`use-ishandled-to-make-base-behaviour-overridable.bad.al`](use-ishandled-to-make-base-behaviour-overridable.bad.al).
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