FormFieldAt property

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Read or write the value of the form field located at the given page coordinate.

FormFieldAt looks up the AcroForm widget annotation that contains the page-space point (X, Y) and exposes its value as a Unicode string. Coordinates are in PDF user space, with the origin at the bottom-left corner of the page and units in points (1/72 inch). Reading the property returns the current field value (/V) decoded to WString; writing it updates the field and regenerates appearance streams so the new value renders on the next paint.

Returns an empty string when Active is False, when the document has no AcroForm, or when no widget covers the coordinate. The match policy mirrors FormFieldInfoAt: widget rectangles only (no appearance-bound hit testing), topmost widget wins for overlaps. Writes are no-ops when the coordinate misses every widget.

For checkbox and radio button widgets the value semantics follow the PDF spec — assigning 'Off' clears the field, assigning the value matching the widget’s /AP /N on-state name (commonly 'Yes' or the export value) ticks it. For text fields, anything you assign becomes the new /V contents. List / combo boxes accept the export value associated with one of the option entries. Use FormFieldInfoAt first to inspect the FieldType if the field type is uncertain.

• Writes are applied immediately but only persisted to disk by the next SaveToFile / SaveToStream call.
• To clear a text field set the value to an empty string; the field stays visible but renders with no glyphs.
• Concurrent writes through multiple FormFieldAt / FormField assignments are safe in sequence but never call them from background threads — PDFium form fields are not threadsafe.

FormFieldInfoAt, HasFormFieldAt, FormField, FormFieldCount