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23d5478ac6
Adds the interfaces knowledge domain covering AL interfaces and enum-with-implementation: three atomic articles with good/bad AL samples, a new al-interfaces-review leaf skill, and additive wiring into al-code-review and the README. Purely additive; no contract change. Part of #34. Co-authored-by: Jesper Schulz-Wedde <jesper.schulzwedde@microsoft.com> Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
26 lines
2.2 KiB
Markdown
26 lines
2.2 KiB
Markdown
---
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bc-version: [16..]
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domain: interfaces
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keywords: [interface, defaultimplementation, enum-implements-interface, fallback, extensible-enum, implementation-property]
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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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# Set DefaultImplementation on an enum so an unmapped value still resolves to an interface
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## Description
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An `enum` that `implements` an interface maps each value to a codeunit through the `Implementation` property. But an extensible enum can carry values that set no `Implementation` — values added later by an extension, or a value left intentionally blank. Assigning such a value to an interface variable and calling a method on it fails at runtime unless the enum provides a fallback. The enum-level `DefaultImplementation` property names the codeunit used whenever a value has no explicit `Implementation`, so resolution always yields a usable object. LLMs are generally unaware this property exists and leave the gap open.
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## Best Practice
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On any extensible enum that implements an interface, set `DefaultImplementation = <Interface> = <Codeunit>;` at the enum level, pointing at a safe implementation that does nothing harmful. Values with their own `Implementation` keep using it; every other value — including ones added later by extensions — resolves to the default instead of failing. For the distinct case of an out-of-range integer that matches no declared value, pair it with `UnknownValueImplementation`. The result is that a consumer can assign any enum value to the interface variable and call through it without a runtime guard.
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See sample: `set-defaultimplementation-on-enum.good.al`.
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## Anti Pattern
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An extensible `enum ... implements <Interface>` where at least one value sets no `Implementation` and the enum declares no `DefaultImplementation`. Code that assigns that value to an interface variable and invokes a method throws at the call site, and because the enum is extensible the failing value can be introduced by a third party long after the consumer ships. Detection signal: an enum that implements an interface, has a `value(...)` with no `Implementation`, and no enum-level `DefaultImplementation`. Add a `DefaultImplementation` mapping to close the gap.
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See sample: `set-defaultimplementation-on-enum.bad.al`.
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