Where does FormFieldCollection live in the IronPDF API?

FormFieldCollection is in the IronSoftware namespace, shipped in IronPdf.dll. It derives from ObservableCollection of IFormField and implements IFormFieldCollection, which itself lives in the IronSoftware.Forms subnamespace. It is returned by the read-only PdfDocument.Form property.

What is the difference between FormFieldCollection and IFormFieldCollection?

FormFieldCollection is the concrete class in the IronSoftware namespace that derives from ObservableCollection of IFormField. IFormFieldCollection is the interface it implements, declared in the IronSoftware.Forms subnamespace. The two types live in different namespaces, which is unusual: the concrete class is in the assembly's shared parent namespace while the contract is grouped with the other forms types. Because PdfDocument.Form returns the concrete type, application code rarely needs to reference the interface directly. The interface itself declares only DisableFormFontFallback, FindFormField, and SetFormFont; everything else on the concrete class, including Add, Remove, RemoveAt, Clear, IsReadOnly, and the indexer, is inherited from ObservableCollection of IFormField.

How does FindFormField match field names?

FindFormField first tries to match a fully-qualified field name. If no fully-qualified match is found, it falls back to partial name matches. This matters for PDFs that group fields under a common prefix, such as applicant.fullName: calling FindFormField with the full dotted name returns the exact field, while calling it with just fullName matches by suffix if no fully-qualified field of that name exists.

How do you set a document-wide font for form fills?

Call SetFormFont on the collection with a PDF font name and the raw TrueType or OpenType bytes. When set, the rendering engine stops the automatic fallback embed for form values that contain non-ASCII characters and uses the registered font instead. Passing null or empty bytes activates name-only mode, which requires the named font to already be embedded in the document. DisableFormFontFallback offers the reverse option: suppress the automatic fallback without registering any replacement, which adds zero bytes to the document.

Class FormFieldCollection

Observable collection of form fields

Inheritance

System.Object

FormFieldCollection

Implements

IFormFieldCollection

System.Collections.Generic.IList<IFormField>

System.Collections.Generic.ICollection<IFormField>

System.Collections.Generic.IEnumerable<IFormField>

System.Collections.IEnumerable

Namespace: IronSoftware

Assembly: IronPdf.dll

Syntax

public class FormFieldCollection : ObservableCollection<IFormField>, IFormFieldCollection, IList<IFormField>, ICollection<IFormField>, IEnumerable<IFormField>, IEnumerable

The fillable form fields on an IronPDF document are reached through FormFieldCollection, the value returned by the read-only PdfDocument.Form property. It is the concrete, observable container of those fields. The interface it implements, IFormFieldCollection, lives in the IronSoftware.Forms subnamespace, so prose that mentions both types must qualify them.

FormFieldCollection matters whenever application code touches fillable PDF forms in IronPDF. Filling an existing form, removing or adding fields on a generated document, flattening forms before distribution, and embedding a document-wide font for non-ASCII fills are all routed through this collection. Because the property PdfDocument.Form returns the concrete FormFieldCollection, downstream code can call collection-level mutators directly without casting from the interface.

The headline surface groups into four buckets. Lookup and iteration: FindFormField(string) returns an IFormField by name (the implementation tries the fully-qualified name first, then falls back to partial matches), and the standard IList<IFormField> indexer plus IEnumerable<IFormField> iteration are inherited from the ObservableCollection<IFormField> base. Mutation: Add(IFormField), Remove(IFormField), RemoveAt(int), and Clear() operate on the field set. Font control: SetFormFont(string fontName, byte[] fontData, bool forceEmbed = false) registers a document-wide font for subsequent fills, and DisableFormFontFallback() suppresses the automatic fallback embed for non-ASCII values without registering any replacement font. State: the read-only IsReadOnly property reports whether the collection is mutable. The canonical idiom for fills is documented in the fill and edit PDF forms how-to.

using IronPdf;
using IronSoftware.Forms;

PdfDocument pdf = PdfDocument.FromFile("application.pdf");

// PdfDocument.Form returns the concrete FormFieldCollection.
FormFieldCollection fields = pdf.Form;

// Lookup is by name; fully-qualified matches win, partial matches fall back.
IFormField nameField = fields.FindFormField("applicant.fullName");
nameField.Value = "Jane Doe";

// Optional: register a document-wide font so non-ASCII fills are not
// auto-embedded with a fallback face.
// fields.SetFormFont("Helvetica", File.ReadAllBytes("Helvetica.ttf"));

// Collection-level mutators are inherited from ObservableCollection<IFormField>.
IFormField stale = fields.FindFormField("legacyConsent");
if (stale != null) fields.Remove(stale);

pdf.SaveAs("application-filled.pdf");

For building forms from scratch through HTML or programmatic construction, see the create PDF forms how-to. For the field-level contract returned by FindFormField, see IFormField.

Properties

IsReadOnly

Declaration

public bool IsReadOnly { get; }

Property Value

Type	Description
System.Boolean

Methods

Add(IFormField)

Declaration

public void Add(IFormField item)

Parameters

Type	Name	Description
IFormField	item

Clear()

Declaration

public void Clear()

DisableFormFontFallback()

Suppresses the automatic font fallback embed for non-ASCII form values without registering any replacement font. Zero bytes are added to the document.

Declaration

public void DisableFormFontFallback()

FindFormField(String)

Find a form field with the specified name. First tries to match a fully-qualified name. If a fully-qualified match cannot be found, then partial name matches are tried.

Declaration

public IFormField FindFormField(string Name)

Parameters

Type	Name	Description
System.String	Name	Form field name

Returns

Type	Description
IFormField	Form field with the specified name

GetHashCode()

Declaration

public override int GetHashCode()

Returns

Type	Description
System.Int32

Remove(IFormField)

Declaration

public bool Remove(IFormField item)

Parameters

Type	Name	Description
IFormField	item

Returns

Type	Description
System.Boolean

RemoveAt(Int32)

Declaration

public void RemoveAt(int index)

Parameters

Type	Name	Description
System.Int32	index

SetFormFont(String, Byte[], Boolean)

Sets the document-wide font for subsequent form field fills on this document. When set, the engine skips the automatic font fallback embed for form values that contain non-ASCII characters.

Declaration

public void SetFormFont(string fontName, byte[] fontData, bool forceEmbed = false)

Parameters

Type	Name	Description
System.String	fontName	PDF font name as it appears in the document's /DR /Font dictionary
System.Byte[]	fontData	Raw TrueType/OpenType bytes; pass null or empty for name-only mode (the font must already be embedded in the document)
System.Boolean	forceEmbed	When true, embed `fontData` even if a font with the same name and usable glyph data is already present. Has no effect in name-only mode.

Implements

IFormFieldCollection

System.Collections.Generic.IList<>

System.Collections.Generic.ICollection<>

System.Collections.Generic.IEnumerable<>

System.Collections.IEnumerable