bcquality/microsoft/knowledge/interfaces/set-defaultimplementation-on-enum.md

bc-version	domain	keywords	technologies	countries	application-area
16..	interfaces	interface defaultimplementation enum-implements-interface fallback extensible-enum implementation-property	al	w1	all

Set DefaultImplementation on an enum so an unmapped value still resolves to an interface

Description

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.

Best Practice

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; declared values without one resolve to the default. For an ordinal that matches no currently declared value — for example persisted data left after an enum extension is uninstalled — runtime 7.0 and later can use UnknownValueImplementation as a distinct fallback. Do not recommend that property to apps targeting an earlier runtime.

See sample: set-defaultimplementation-on-enum.good.al.

Anti Pattern

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.

See sample: set-defaultimplementation-on-enum.bad.al.