Create a custom number sequence in X++ (D365 F&O)

A custom number sequence follows the number sequence framework: an extended data type (EDT) for the number; a "reference" declared in the loadModule method of a number sequence module class (NumberSeqDatatype + create); a static method returning that reference, usually on the module's parameters table; then, in code, NumberSeq::newGetNum(reference).num() to get the next number. In the application, the sequence is then created and linked to the reference (wizard or manual setup).

In D365 F&O you do not over-layer the standard module class: you add your reference through an extension (Chain of Command on loadModule) or create your own module class. Microsoft has opened standard modules' loadModule methods to extension (for example NumberSeqModuleProject in version 8.1.2).

The framework documentation (Dynamics AX 2012, still the reference for the pattern) shows the declaration in loadModule:

// In the module class's loadModule method (AX 2012 example)
NumberSeqDatatype datatype = NumberSeqDatatype::construct();

datatype.parmDatatypeId(extendedTypeNum(WorkOrderNum));
// ... other properties: label, scope, length, defaults
this.create(datatype);

In a D365 F&O extension, this code goes in an extension class of the module class, after the call to next loadModule(). Loading references is a one-time operation: once the module is loaded, the reference appears in the module parameters and in the number sequence wizard.

1. In the application, generate the sequence with the wizard (Organization administration > Number sequences > Number sequences > Generate) or create it manually, then link it to the reference and scope (shared, company…).
2. Expose the reference through a static method on the parameters table, following the standard numRef… methods.
3. In code (initValue, insert, service…), get the number:

// Get the next number from the reference
MyWorkOrderNum workOrderNum = NumberSeq::newGetNum(MyParameters::numRefWorkOrderNum()).num();

• Over-layering the standard module class instead of an extension.
• Forgetting to reload references after deployment: the reference does not show in the parameters.
• Not setting up the sequence in each company concerned (company scope): "number sequence not set up" error.
• Taking the number too early (when the form opens) then abandoning the record: gaps in a continuous sequence.
• Using a continuous sequence where it is not required: it is more expensive.

EDT, reference, accessor, initialisation: a custom sequence is a short ticket. Skalp AI writes the specification, generates the X++ code and unit tests, compiles with a best-practice check, then delivers a pull request to your Azure DevOps: nothing is merged without your technical architect's approval. See custom D365 F&O extensions, the bigger picture in building X++ without a consultant in 2026, and the Basic, Pro and Max plans (1 credit, ≈ €400 on Basic, €200 on Pro, consumed only when the code is generated).

• Microsoft Learn (Dynamics AX 2012), "Number Sequence Framework", accessed 18 Oct 2026.
• Microsoft Learn, "Set up number sequences by using a wizard", updated 10 Mar 2026, accessed 18 Oct 2026.
• Microsoft Learn, "Extensibility changes in Dynamics 365 for Finance and Operations version 8.1.2", updated 20 Jan 2026, accessed 18 Oct 2026.