Register subclasses for factory methods

Class inheritance is a central concept in X++, as in other object-oriented languages. The X++ business logic uses the object-oriented strategy pattern. In this pattern, you encapsulate variations in behavior by using subclasses, and the business process uses an abstract base class or interface. A factory method determines the variation that it uses by creating an instance of a specific subclass.

This article describes how to register your own variations for the factories.

In X++, the factories use reflection to perform the following tasks:

• Find the correct subclass. The factory uses an extension framework to search all subclasses in a hierarchy for a specific set of attributes. If the attributes that decorate a subclass match the parameters that you pass to the factory, the factory uses that specific class.
• Create an instance. After the factory identifies the right type, it uses reflection to create an instance of the class.

The following illustration shows a typical decorated hierarchy.

In X++, two extension frameworks serve the same purpose. The implementer of the factory method determines which extension framework to use:

• SysExtension

  • This extension framework uses custom attributes that make it easier to consume.
  • It supports singletons that save a little performance when the same instance is created repeatedly. This extension framework is especially useful for stateless subclasses.
  • It seamlessly supports extensible enums that are often used to determine the variant.

• SysPlugin

  • This extension framework is based on the Managed Extension Framework. The Managed Extension Framework makes the SysPlugin extension framework available to non-X++ code.
  • It uses the ExportMetadataAttribute attribute to decorate the variants and is string-based.
  • It uses the ExportAttribute attribute to make the variant discoverable.

Introduce a new variant

1. Identify the base class or interface of the variant that you need to implement.

2. Create a new subclass of the base class, and implement your variation.

3. Identify which attribute is required to register your class. Use two approaches to find the attribute:

   • Look for attributes that other subclasses in the hierarchy define.
   • Look at the implementation of the factory method. That implementation contains the attributes that the factory method searches for.

4. Decorate your subclass with the attribute that matches your variation.

SysExtension example

[WHSWorkExecuteMode(WHSWorkExecuteMode::About)]
class WHSWorkExecuteDisplayAbout extends WHSWorkExecuteDisplay
{
    // Your code here.
}

SysPlugin example

[ExportMetadataAttribute('CaseIAssociation', 'Lead'),
ExportAttribute('Dynamics.AX.Application.CaseIAssociation')]
class smmLeadCaseAssociationProvider implements CaseIAssociation
{
    // Your code here.
}