Table extensions in D365FO

 Source: Customize through extension and overlayering - Finance & Operations | Dynamics 365 | Microsoft Docs

You can create a table extension to extend a table's design and logic. You can add new fields, field groups, indexes, mappings and relations. You can also add new fields to existing field groups, change the label of a table field, change the Created By, Created Date Time, Modified By, Modified Date Time properties. Using table extensions, you can also change the Extended Data Type property on fields and set it to an EDT that is derived from the current EDT (This is available as of platform update 8).

In Microsoft Dynamics AX 2012, you could override the virtual methods of a table's base class to control the behavior that occurred during table operations, such as when creating, reading, updating, or deleting. In the current version, you instead use extensions to implement event handlers that are called from the base implementations of the table methods. The following table lists each table method and its events.

TABLE EXTENSIONS

Published Table method	Preceding event	Succeeding event
validateWrite	ValidatingWrite	ValidatedWrite
validateDelete	ValidatingDelete	ValidatedDelete
validateField	ValidatingField	ValidatedField
validateFieldValue	ValidatingFieldValue	ValidatedFieldValue
modifiedField	ModifyingField	ModifiedField
modifiedFieldValue	ModifyingFieldValue	ModifiedFieldValue
Insert	Inserting	Inserted
Update	Updating	Updated
Delete	Deleting	Deleted
Initvalue	InitializingRecord	InitializedRecord
FinalDeleteValidation	Executed when a delete operation is performed on a table object, before the operation is committed to the underlying database table	N/A
FinalInsertValidation	Executed when an insert operation is performed on a table object, before the operation is committed to the underlying database table	N/A
FinalReadValidation	Executed when a read operation is performed on a table object.	N/A
FinalUpdateValidation	Executed when an update operation is performed on a table object, before the operation is committed to the underlying database table.	N/A

Validation events capture and return results by using the DataEventArgs parameter. The display and edit method modifiers are supported on table extensions.