Get and edit content controls starts to be supported in Spire.Doc 5.4.15

static void Main(string[] args)
        {
            Document document = new Document();
            document.LoadFromFile("..\\..\\sample.docx");
            foreach(Section section in document.Sections)
            {
                foreach(Body body in section.ChildObjects)
                {
                    ModifyBody(body);
                }
            }
            document.SaveToFile("result.docx",FileFormat.Docx);
            System.Diagnostics.Process.Start("result.docx");
        }
        static void ModifyBody(Body body)
        {
            if (body == null)
                return;
            foreach (DocumentObject docObj in body.ChildObjects)
            {
                if (docObj is StructureDocumentTag)
                {
                    SDTProperties SdtProperties = (docObj as StructureDocumentTag).SDTProperties;
                    ModifySdtProperties(SdtProperties);
                }
                else if (docObj is Table)
                {
                    ModifyTable(docObj as Table);
                }
                else if (docObj is Paragraph)
                {
                    ModifyParagraph(docObj as Paragraph);
                }
            }
        }

        static void ModifyTable(Table table)
        {
            if (table == null)
                return;
            foreach (TableRow row in table.Rows)
            {
                foreach (TableCell cell in row.Cells)
                {
                    if (cell is StructureDocumentTagCell)
                    {
                        SDTProperties SdtProperties = (cell as StructureDocumentTagCell).SDTProperties;
                        ModifySdtProperties(SdtProperties);   
                    }
                    else
                    {
                        ModifyBody(cell);
                    }
                }
            }
        }

        static void ModifyParagraph(Paragraph para)
        {
            if (para == null)
                return;

            foreach (DocumentObject docObj in para.ChildObjects)
            {
                if (docObj is StructureDocumentTagInline)
                {
                    SDTProperties SdtProperties = (docObj as StructureDocumentTagInline).SDTProperties;
                    ModifySdtProperties(SdtProperties);
                }
            }
        }
        public class StructureTags
        {
            List m_tagInlines;
            public List tagInlines
            {
                get
                {
                    if (m_tagInlines == null)
                        m_tagInlines = new List();
                    return m_tagInlines;
                }
                set
                {
                    m_tagInlines = value;
                }
            }
            List m_tags;
            public List tags
            {
                get
                {
                    if (m_tags == null)
                        m_tags = new List();
                    return m_tags;
                }
            }

            List m_sdtCell;
            public List SDTCell
            {
                get
                {
                    if (m_sdtCell == null)
                        m_sdtCell = new List();
                    return m_sdtCell;
                }
            }
        }
        static void ModifySdtProperties(SDTProperties properties)
        {
            if (properties == null)
                return;

            switch (properties.SDTType)
            {
                case SdtType.ComboBox:
                case SdtType.DropDownList:
                    {
                        SdtDropDownListBase dropDownListBase = (properties.ControlProperties as SdtDropDownListBase);
                        dropDownListBase.ListItems.SelectedValue = dropDownListBase.ListItems[1];
                        dropDownListBase.ListItems.Add(new SdtListItem("bill","apple"));
                    }
                    break;
                case SdtType.CheckBox:
                    {
                        SdtCheckBox sdtCheckBox = (properties.ControlProperties as SdtCheckBox);
                        sdtCheckBox.Checked = !sdtCheckBox.Checked;
                    }
                    break;
            }
        }