bcquality/microsoft/knowledge/error-handling/fielderror-default-message-logic.md

bc-version	domain	keywords	technologies	countries	application-area
all	error-handling	fielderror testfield error-message field-caption lowercase-convention record-context validation	al	w1	all

Rely On FieldError's Auto-Generated Context And Pass Only A Lowercase Predicate

Description

Rec.FieldError(FieldNo) does not just print the text you give it. Business Central automatically prepends the field caption, appends the current field value (when non-blank), and suffixes the table name and primary-key values for record identification. The optional second argument is only the middle predicate of that sentence — e.g. "must be unique", not a whole self-contained message. Misunderstanding this leads to messages that duplicate the caption and value or read as broken grammar, because the framework's surrounding text is built to join a lowercase fragment.

Best Practice

For a plain required-field check, prefer TestField, which tests the condition and raises the error in one call. When the condition is non-trivial and has already been evaluated, call FieldError(FieldNo) with no message to get the localized default (must have a value, is not valid, etc.), or pass a short lowercase predicate such as FieldError(FieldNo, 'must be a positive number'). Start the custom text with a lowercase letter so it reads as one sentence with the auto-inserted caption, and use a field-number reference (or the field token) rather than a hard-coded field name so captions and translations stay correct. Let the framework supply the caption, value, table, and key context for you.

See sample: fielderror-default-message-logic.good.al.

Anti Pattern

Re-testing a condition you already evaluated, or passing a fully formed sentence like 'The Amount field must be positive.' to FieldError. The result reads as Amount The Amount field must be positive. in Gen. Journal Line ... — capital letter mid-sentence, caption and value repeated, and a stray trailing clause. Reviewer signals: a FieldError argument that names the field, restates the current value, starts with a capital letter, or ends with a period. Each is a sign the author treated FieldError like Error instead of as a predicate slotted into framework-generated context.

See sample: fielderror-default-message-logic.bad.al.