bcquality/microsoft/knowledge/interfaces/assign-codeunit-to-interface-for-testability.md

bc-version	domain	keywords	technologies	countries	application-area
16..	interfaces	interface dependency-injection testability test-double codeunit polymorphism mocking	al	w1	all

Assign a codeunit to an interface variable for injectable, testable dependencies

Description

An interface variable can hold any codeunit that implements the interface, assigned directly — no enum is required. That is the lever for dependency injection in AL: a consumer depends on the interface, production code injects the real codeunit, and a test injects a lightweight double that returns predictable values. A consumer that instead var-declares a concrete Codeunit type hardwires the dependency, so a test is forced to exercise the real logic — external calls, posting, and all. Interfaces arrived in Business Central 2020 release wave 1; LLMs still default to concrete codeunit variables and miss the seam that makes code testable.

Best Practice

Declare the dependency as an Interface variable on the consumer and supply the implementation from outside — typically setter injection through a procedure that takes an Interface parameter, or a parameter on the entry method. Production passes the real implementation codeunit; a test passes a test-double codeunit that implements the same interface with deterministic behaviour. Because a codeunit assigns to an interface variable directly, no enum or factory is needed for the injectable case. The consumer's logic is then verifiable in isolation.

See sample: assign-codeunit-to-interface-for-testability.good.al.

Anti Pattern

A consumer that declares its dependency as a concrete Codeunit "..." variable and calls it directly. The collaborator cannot be substituted, so a unit test either runs the production side effects or cannot cover the consumer at all. Detection signal: a var of type Codeunit "<concrete impl>" used for a collaborator that has — or could have — an interface, especially one that performs I/O, posting, or external calls. Extract an interface, depend on the interface variable, and inject the implementation.

See sample: assign-codeunit-to-interface-for-testability.bad.al.