Development tutorial: your first AL extension for Business Central

Tools for this Business Central development tutorial

Install Visual Studio Code. Next, add the AL Language extension from its marketplace. You also need somewhere to publish. The simplest choice is an online sandbox environment. Any Business Central customer or trial tenant can create one in the admin center. Some developers use a local Docker container instead.

Add Git for source control from the first commit. It gives you history and lets two people work without overwriting each other. It is also the base for automated builds later. Microsoft offers AL-Go for GitHub templates. They give you ready builds once the team grows.

Create the project with AL: Go!

Open the command palette and run AL: Go!. Pick a folder, a platform version and the Microsoft cloud sandbox. It creates app.json. That file holds the name, publisher, version, dependencies and ID range. It also creates launch.json for the environment you publish to. A HelloWorld.al sample is added too.

Run AL: Download Symbols next and sign in when asked. Symbols are the definitions of the base application and system objects. Without them, the compiler cannot resolve names such as the Customer table. Then set the ID range in app.json to the numbers that Microsoft reserves for per-tenant extensions.

Add a field in this Business Central development tutorial

Create a file for a tableextension object that extends Customer. Declare a new field inside a fields block, for example a text field for a delivery note. Give it a Caption and a DataClassification. Snippets help. Type ttableext, and the editor offers a skeleton. Prefix your object and field names to avoid clashes with other extensions.

The field exists in the database, but no one can see it yet. Create a pageextension that extends Customer Card. In the layout block, use addlast or addafter. That places a control for your new field, with a ToolTip. Table and page extensions are the everyday building blocks. You cannot edit base objects directly.

Add logic with an event subscriber

Business logic goes into codeunits. To change standard behavior, you subscribe to events that Microsoft has published in the base code. Write a procedure with the EventSubscriber attribute, naming the object and the event. A validation trigger on a sales header field is a good first choice. Your code then runs whenever that point is reached.

Keep subscribers short. Move real work into separate procedures that you can test. Use the Error function to stop an invalid action. Use the Message or Notification features to inform users without blocking them. If no suitable event exists, developers can request one from Microsoft.

Finish the Business Central development tutorial: publish and deploy

Press F5. It compiles, publishes to the sandbox and opens the web client with the debugger on. Set a breakpoint in your subscriber and trigger the action. Then step through the code while you inspect variables. The Page Inspector in the client shows which extension added each field.

When the change is ready, build the .app file. Upload it to production from the Extension Management page. Choose the current or next version as the target. Test again after each Microsoft release wave. Keep the project in Git and tag each version you ship.

Business Central development tutorial questions

What language is Business Central developed in?

Customizations use AL. This language was made for Business Central, and you edit it in Visual Studio Code. It replaced the older C/AL language used in Dynamics NAV. AL code is packaged as extensions. They install and uninstall without altering the base application. So a Business Central development tutorial starts with AL and extensions, not with changes to Microsoft's code.

Do I need a paid license to learn AL development?

A trial tenant with a sandbox environment is enough to follow a beginner tutorial. A local Docker container is another option. Live use needs the customer's own licensed environment. Check Microsoft's current terms for developer and partner licenses. They change from time to time, so read the live page before you plan.

What is the difference between a per-tenant extension and an AppSource app?

A per-tenant extension is built for one customer and uploaded to that customer's environment. An AppSource app goes through Microsoft's marketplace to many customers. It goes through technical validation, and it uses an object ID range assigned to the publisher. So the first suits one business, and the second suits a product.

Can I modify standard Business Central code directly?

Not in the online service. You extend tables, pages and reports, and you subscribe to events to add logic. This split lets Microsoft update the base app often. Meanwhile, your custom work keeps running. It also means an update rarely breaks a well-built extension.

How do I test an extension before an update?

Create a sandbox on the upcoming version from the admin center. Publish your extension there and run your key processes. Automated test codeunits speed this up. Microsoft also notifies administrators when a per-tenant extension is not compatible with a coming release. So a regular sandbox check keeps surprises away from production.

Next steps after this Business Central development tutorial

After a first extension, most teams need environments to test in and a plan for bigger work. These pages cover sandboxes, integrations and extension work in Business Central. Read the sandbox guide next. Then return to this Business Central development tutorial with a safe place to publish. For the product detail, see Get started with AL on Microsoft Learn.