xpp-statement-and-type-rules

name	xpp-statement-and-type-rules
description	X++ statement-level and type-system rules — switch / break, ternary, no-DB-null sentinels, casting (`as` / `is`), `using` blocks, embedded function declarations. Invoke when writing any non-trivial X++ control flow or type conversion.
applies_when	User intent involves X++ control flow, switch statements, casting, null/empty checks on date/utcDateTime, or IDisposable resource handling.

⛔ NEVER write X++ AOT XML files directly via PowerShell, terminal file commands (Set-Content, Out-File, New-Item), editor write tools, or any raw text approach. The XML schema (<AxClass>, <AxTable>, <AxForm>, <Methods>, <SourceCode>) is proprietary — LLMs have not been trained on it reliably. ALWAYS use d365fo generate … commands to produce correct AOT XML. If d365fo is unavailable in PATH, stop and ask the user to install it.

X++ statement & type rules

Sources of truth: learn:xpp-conditional and learn:xpp-variables-data-types.

switch / break

• break is required at the end of every case. Implicit fall-through compiles but is misleading.
• To match multiple values to a single branch use the comma-list form — never empty fall-through:

// ✅ CORRECT
switch (mod(year, 4))
{
    case 13, 17, 21:
        result = "leap-adjacent";
        break;
    default:
        result = "ordinary";
        break;
}

Ternary

cond ? a : b — both branches must have the same type. No implicit widening of int ↔ real.

❗ X++ has NO database null

Each primitive has a "null-equivalent" sentinel:

Type	Null-equivalent value
int / int64	0
real	0.0
str	""
date	1900-01-01 (dateNull())
utcDateTime	date-part 1900-01-01 (utcDateTimeNull())
enum	element with value 0
boolean	false
RecId	0

In SQL where clauses these compare as false (rows with sentinel values are NOT returned by where field). In plain expressions they are ordinary values.

// ❌ WRONG — there is no null
if (myDate == null) { … }

// ✅ CORRECT
if (!myDate)              { … }   // boolean test on sentinel
if (myDate == dateNull()) { … }   // explicit

Same for utcDateTime — compare against utcDateTimeNull() or use if (!myUtc).

Casting

• Prefer as (returns null on type mismatch) and is (boolean test) over hard down-casts.
• Hard down-casts ((SubClass)objectExpr) on object-typed expressions throw InvalidCastException on mismatch.
• Late binding exists for Object and FormRun only — accept the runtime cost if you use it.

common = ledgerJournalTrans;
LedgerJournalTrans trans = common as LedgerJournalTrans;
if (trans) { trans.update(); }

using blocks for IDisposable

Equivalent to try + finally { x.Dispose(); } but shorter and exception-safe.

using (var reader = new StreamReader(path))
{
    line = reader.ReadLine();
}

Embedded function declarations

Local functions inside a method can read variables declared earlier in the enclosing method but cannot leak their own variables out. Prefer them over a private helper method only when the helper truly does not belong to the class API.

Hard "never" list

• Never test myDate == null — there is no null in X++.
• Never rely on switch fall-through — always break (or use the comma-list form).
• Never down-cast an Object without an is guard (or an as + null check).