test.cs

private void FlattenFormFields(TextControl textControl) {
  int fieldCount = textControl1.FormFields.Count;

  for (int i = 0; i < fieldCount; i++) {
    TextFieldCollectionBase.TextFieldEnumerator fieldEnum =
      textControl1.FormFields.GetEnumerator();
    fieldEnum.MoveNext();

    FormField curField = (FormField)fieldEnum.Current;
    textControl1.FormFields.Remove(curField, true);
  }
}