iTextSharp :填充数据时，矩形单选按钮变为圆形按钮

using iTextSharp.text;
using iTextSharp.text.pdf;

// 创建PDF文档对象
Document document = new Document();
PdfWriter writer = PdfWriter.GetInstance(document, new FileStream("output.pdf", FileMode.Create));
document.Open();

// 创建表单对象
PdfContentByte cb = writer.DirectContent;
PdfFormField formField = PdfFormField.CreateRadioButton(writer, true);

// 设置单选按钮的位置、大小和名称
Rectangle rect = new Rectangle(100, 700, 120, 720);
PdfFormField radioField = PdfFormField.CreateEmpty(writer);
radioField.SetWidget(rect, PdfAnnotation.HIGHLIGHT_INVERT);

// 设置单选按钮的外观样式为圆形按钮
PdfAppearance appearance = PdfAppearance.CreateAppearance(writer, rect.Width, rect.Height);
appearance.SetColorFill(BaseColor.BLACK);
appearance.Circle(rect.Width / 2, rect.Height / 2, rect.Width / 2 - 1);
appearance.Fill();

// 将单选按钮字段添加到表单中
radioField.SetAppearance(PdfAnnotation.APPEARANCE_NORMAL, appearance);
formField.AddKid(radioField);

// 填充数据到表单中的其他字段
formField.SetField("name", "John Doe");

// 将表单添加到PDF文档中
writer.AddAnnotation(formField);

// 保存PDF文档
document.Close();