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30 lines
1.4 KiB
Markdown
30 lines
1.4 KiB
Markdown
---
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bc-version: [25..]
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domain: interfaces
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keywords: [interface, is-operator, as-operator, type-test, cast, variant, runtime-error]
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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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# Guard optional interface casts with `is`
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## Description
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From runtime 15.0, 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.
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## Best Practice
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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.
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See sample: [`guard-interface-casts-with-is.good.al`](guard-interface-casts-with-is.good.al).
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## Anti Pattern
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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.
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See sample: [`guard-interface-casts-with-is.bad.al`](guard-interface-casts-with-is.bad.al).
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## Reference
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[Understand type testing and casting operators for interfaces](https://learn.microsoft.com/en-us/training/modules/business-central-interfaces/type-testing)
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