Choosing where to implement your logic in Microsoft Dynamics 365 Business Central depends on the specific requirements and behavior you want to achieve. Here are some guidelines to help you decide:

1. Use an Event Subscriber

When to use:

• Decoupled Logic: If the logic doesn’t need to modify the core behavior of the table but instead reacts to changes, like logging or updating another table.
• Modularity: When you want your logic to stay outside the base object to minimize direct dependency and allow easier upgrades.
• Extensibility Priority: If you want to easily enable/disable or extend the functionality without touching the core triggers.
• Standardized Events: Microsoft often provides predefined events in key places like table modifications, reports, or codeunit executions. Use these for consistency.

Example: Subscribing to an event like OnAfterValidateEvent for a field allows you to execute your logic after the standard validation logic is completed.

[EventSubscriber(ObjectType::Table, Table::"Sales Line", 'OnAfterValidateEvent', 'Quantity', true, false)]
local procedure MyCustomLogic(Rec: Record "Sales Line");
begin
    // Custom logic here
end;

2. Use Table Extensions (Field Triggers)

When to use:

• Direct Integration: If the logic needs to occur before or after the base functionality of the field (e.g., adding validation rules or custom calculations).
• Tight Coupling: When the logic is tightly tied to the functionality of the field and should always execute as part of the field’s trigger.
• No Available Event: If there’s no appropriate event exposed by Microsoft and you need to hook into the base trigger directly.

Example: Extending the Validate trigger of a field to add custom validation.

tableextension 50100 "My Sales Line Ext" extends "Sales Line"
{
    fields
    {
        field(50100; "My Custom Field"; Code[20])
        {
            trigger OnValidate()
            begin
                // Custom logic before or after the base logic
            end;
        }
    }
}

3. Key Decision Factors

Here’s how to decide between these approaches:

Factor	Event Subscriber	Field Trigger in Extension
Scope	Reacts to changes in a field or table globally.	Tightly coupled to a specific field in one table.
Upgrade Compatibility	Easier to upgrade as it’s decoupled from base logic.	More complex during upgrades if base logic changes.
Performance	Adds an additional layer, which might slightly impact performance.	Executes inline with field operations, often faster.
Reusability	Can be reused across multiple areas in the system.	Specific to the field in question.
Availability of Events	Depends on whether appropriate events are available.	Works even without predefined events.
Order?	Subscribers executed not in order?	Extensions executed not in order?

4. Best Practices

• Combine Approaches When Necessary: Use event subscribers for general or cross-functional logic and table extensions for tightly coupled field-specific logic.
• Avoid Code Duplication: Centralize shared logic in reusable codeunits.
• Performance Awareness: Ensure you’re not overloading event subscriptions with heavy logic, as they run alongside core processes.
• Stay Upgrade-Safe: Favor event subscribers over direct extensions unless field triggers are absolutely necessary.

By carefully analyzing your requirement and following these guidelines, you can decide the best place to implement your logic in Business Central.