How can developers use interfaces and events together to create extensible Business Central applications?

One of the most important architectural concepts in modern Dynamics 365 Business Central development is designing extensions that are loosely coupled and easier to evolve.

Two powerful mechanisms are:

Interfaces — define a contract for behavior.

Events — provide extension points where other components can react without modifying the original implementation.

Used together, they can support flexible application architectures.

Start with the interface

Suppose an application needs multiple ways to calculate a discount.

Instead of writing:

IF Customer Type = A
   ...
ELSE IF Customer Type = B
   ...
ELSE IF Customer Type = C
   ...

you could define a common interface representing the behavior.

Conceptually:

IDiscountCalculator
        │
 ┌──────┼────────┐
 ↓      ↓        ↓
Standard Customer VIP Customer Partner

Each implementation follows the same contract but can contain different logic.

This is useful when the business expects the behavior to evolve.

Where events enter the architecture

Business Central events allow extensions to subscribe to defined events rather than modifying the base application's source code.

For example:

Business Process
      ↓
Publisher Event
      ↓
Subscriber
      ↓
Custom Business Logic

The publisher doesn't need to know every subscriber.

That creates a looser relationship between components.

Combine the two

A more extensible architecture could look like:

Business Central Process
          ↓
      Event Publisher
          ↓
    Event Subscriber
          ↓
   Interface Contract
          ↓
 ┌────────┼─────────┐
 ↓        ↓         ↓
Impl A   Impl B    Impl C

The event determines when something happens.

The interface determines how a particular behavior is implemented.

That distinction is extremely useful.

Example: notification strategy

Imagine an organization wants different notification mechanisms after a Business Central transaction.

You could have:

INotificationProvider

with implementations such as:

EmailNotificationProvider
TeamsNotificationProvider
ExternalNotificationProvider

An event can identify the appropriate business moment:

Sales Order Released
       ↓
Event Subscriber
       ↓
Notification Provider
       ↓
Selected Implementation

Now the core business process doesn't need to contain every notification implementation.

Why this matters

Without extensibility, developers often create large blocks of tightly coupled code.

That can lead to:

• Difficult testing
• Difficult upgrades
• High maintenance cost
• Complex dependencies
• Repeated customizations

Interfaces and events can help establish cleaner boundaries.

Don't overuse interfaces and events

Extensibility does not mean every function needs an interface and every process needs a custom event.

Overengineering creates its own problems.

Use an interface when you genuinely have interchangeable behavior or a stable contract.

Use events when you need a clean extension point or decoupled reaction to an event.

A practical project for students

Build a Business Central extension for an order-processing scenario.

Create:

Sales Order
    ↓
Release
    ↓
Event
    ↓
Subscriber
    ↓
Interface
    ↓
Notification / Validation Implementation

Then create a second implementation without modifying the original business process.

That exercise demonstrates a fundamental enterprise-development principle:

Design the core application for extension instead of repeatedly modifying the core application itself.

For working professionals, this concept becomes especially important when designing Business Central extensions that need to coexist with standard functionality and other extensions.