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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
1.7 KiB
Markdown
26 lines
1.7 KiB
Markdown
---
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bc-version: [18..]
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domain: interfaces
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keywords: [unknownvalueimplementation, unknown-enum-value, persisted-ordinal, enum-extension, extension-uninstall, interface-fallback]
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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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# Handle unknown enum ordinals with UnknownValueImplementation
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## Description
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An enum ordinal can remain in persisted data after the enum extension that declared it is uninstalled. The ordinal is then unknown: it matches no currently declared enum value. `DefaultImplementation` does not cover this case; it covers declared values that have no explicit interface implementation. `UnknownValueImplementation`, available from runtime 7.0 (Business Central 2021 release wave 1, BC18), provides the distinct interface implementation for an unknown ordinal.
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## Best Practice
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On BC18 or later, set `UnknownValueImplementation = <Interface> = <Codeunit>;` on an enum that implements an interface and can be persisted. Use an implementation that reports a clear domain error or safely contains the unknown state. Keep `DefaultImplementation` separately when declared but unmapped values also need a fallback.
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See sample: [`handle-unknown-enum-ordinals-with-unknownvalueimplementation.good.al`](handle-unknown-enum-ordinals-with-unknownvalueimplementation.good.al).
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## Anti Pattern
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Defining only `DefaultImplementation` and assuming it also handles a stored ordinal whose enum value has disappeared. After an enum extension is uninstalled, converting that unknown ordinal to the interface can produce a technical runtime error instead of controlled handling.
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See sample: [`handle-unknown-enum-ordinals-with-unknownvalueimplementation.bad.al`](handle-unknown-enum-ordinals-with-unknownvalueimplementation.bad.al).
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