组合框未出现在打印的PDF上

Question

我正在尝试使用ComboBox将iTextSharp添加到打印的PDF的单元格中，但是它没有出现在生成的PDF中。相反，打印出来的只是一个没有下拉菜单的空白单元格。

我不确定是什么原因造成的，但是现在我很沮丧。我在网上也遵循了其他示例，例如this one here，但是我无法复制它。

我应该注意的一件事是，我们使用的iTextSharp的版本是1.4.3，它是iTextSharp 4.1.6的端口，这使其与.net内核兼容，因此不确定是否较旧版本可能是造成这种情况的原因。

这是我当前的代码。我在做什么错？

if (_dropDownCol)
{
    PdfFormField selectGroup = PdfFormField.CreateEmpty(_Writer);
    String[] options = { "Choose first option", "Choose second option", "Choose third option" };
    String[] exports = { "option1", "option2", "option3" };
    var comboBox = new PdfPCell();
    comboBox.CellEvent = new SelectCellEvent(GetTaskDropDownValueID(task, digitalDocument), selectGroup, Orientation, "combo1", exports, options);
    comboBox.MinimumHeight = 20;
    taskArea.AddCell(comboBox);
}

和SelectedCellEvent

public class SelectCellEvent : IPdfPCellEvent
{
    private string _ID { get; set; }
    protected PdfFormField selectGroup;
    protected String name;
    protected String[] exports;
    protected String[] options;
    protected BaseFont font;
    private int _Rotation { get; set; }

    public SelectCellEvent(string id, PdfFormField selectGroup, PDFOrientation orientation, string name, string[] exports, string[] options)
    {
        _ID = id;
        this.selectGroup = selectGroup;
        this.name = name;
        this.exports = exports;
        this.options = options;
        font = BaseFont.CreateFont(BaseFont.TIMES_ROMAN, BaseFont.CP1250, BaseFont.NOT_EMBEDDED);
        _Rotation = orientation == PDFOrientation.Portrait ? 0 : 90;
    }

    public void CellLayout(PdfPCell cell, Rectangle position, PdfContentByte[] canvases)
    {
        position.Left = position.Left + cell.PaddingLeft;
        PdfWriter writer = canvases[0].PdfWriter;
        TextField tf = new TextField(writer, position, _ID);
        tf.BorderStyle = PdfBorderDictionary.STYLE_BEVELED;
        tf.BorderColor = BaseColor.Gray;
        tf.ChoiceExports = exports;
        tf.Choices = options;
        tf.Alignment = Element.ALIGN_CENTER;
        selectGroup.AddKid(tf.GetComboField());
    }
}