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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: Copilot App <223556219+Copilot@users.noreply.github.com>
26 lines
2 KiB
Markdown
26 lines
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 declared value to a codeunit through the `Implementation` property. A declared value, including one supplied by an enum extension, can omit that mapping. Assigning that value to an interface variable then fails at runtime unless the enum provides `DefaultImplementation`. This property is for declared but unmapped values; an ordinal that is no longer declared is a different case covered by `handle-unknown-enum-ordinals-with-unknownvalueimplementation`.
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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. Values with their own `Implementation` keep using it; declared values without one resolve to the default. Do not rely on this property for persisted ordinals that match no declared enum value.
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See sample: [`set-defaultimplementation-on-enum.good.al`](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`](set-defaultimplementation-on-enum.bad.al).
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