bcquality/microsoft/knowledge/interfaces/guard-interface-casts-with-is.md

bc-version	domain	keywords	technologies	countries	application-area
25..	interfaces	interface is-operator as-operator type-test cast variant runtime-error	al	w1	all

Guard optional interface casts with is

Description

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.

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