How can interfaces and dependency injection patterns be applied to create testable Business Central extensions?

Interfaces and dependency injection can help Business Central developers reduce coupling between extension logic and concrete implementations. Instead of having business logic directly instantiate a specific service, developers can define an interface representing the required behavior and provide the implementation through an appropriate dependency mechanism.

For example, an extension may need to send notifications, calculate a business value, communicate with an external service, or retrieve exchange-rate information. Rather than embedding all of that logic directly inside a codeunit, the business process can depend on an abstraction. A concrete implementation can then be substituted during testing.

This becomes particularly valuable when testing codeunits that contain business logic. External dependencies can be replaced with test implementations or mocks, allowing developers to test business rules without repeatedly calling external systems or relying on unpredictable infrastructure.

However, dependency injection should not be introduced simply to increase the number of interfaces in a project. The abstraction should represent a meaningful boundary—for example, an external service, persistence dependency, or replaceable business capability.

A strong Business Central extension architecture therefore aims for:

Small responsibilities → clear interfaces → loosely coupled codeunits → replaceable dependencies → automated tests → easier maintenance.

For interview preparation, developers should be able to explain not only what dependency injection is, but also why it improves testability and where introducing an abstraction actually provides value.