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Add knowledge-backed AL development

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Add read-only planning and repository-changing development skills so BCQuality
knowledge can guide features, bug fixes, refactors, upgrades, and maintenance
before the existing AL review gate runs. Track Microsoft Learn ingestion and
add development and BCApps-shaped guidance evaluation fixtures.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 638b66d2-9f06-4f60-8781-808709e1485c
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---
bc-version: [25..]
domain: interfaces
keywords: [interface, is-operator, as-operator, type-test, cast, variant, runtime-error]
technologies: [al]
countries: [w1]
application-area: [all]
---
# Guard optional interface casts with `is`
## Description
From runtime 14.0, AL can type-test an interface or `Variant` with `is` and cast it to another interface with `as`. The test is non-throwing, but `as` raises a runtime error when the underlying codeunit does not implement the target interface. This matters when an extended capability is optional or implementations can come from other extensions.
## Best Practice
Use `is` to establish that the value supports the target interface before using `as`. Cast directly only where the target implementation is an invariant guaranteed by the surrounding contract.
See sample: `guard-interface-casts-with-is.good.al`.
## Anti Pattern
Using `as` unconditionally for an optional extended interface. An otherwise valid implementation of the base interface then fails at runtime merely because it does not implement the additional contract.
See sample: `guard-interface-casts-with-is.bad.al`.
## Reference
[Understand type testing and casting operators for interfaces](https://learn.microsoft.com/en-us/training/modules/business-central-interfaces/type-testing)