InvalidOperationException C＃Word加入

Question

我在代码中遇到此错误。我试图获取word文件的内容并将它们添加到现有的一个注释文件中。以下是我到目前为止的情况：

namespace WordAddIn1
{
    public partial class ThisAddIn
    {

        //creates a new OneNote Application object
        static OneNote.Application onenoteApp;
        static XNamespace ns = null;
        static string resultWordText = "";
        static Microsoft.Office.Interop.Word.Application word_app;

        private void ThisAddIn_Startup(object sender, System.EventArgs e)
        {
            onenoteApp = new OneNote.Application();
            getWordText();
            writeToOneNote();
        }

        public static void getWordText()
        {
            try
            {

                word_app = (Microsoft.Office.Interop.Word.Application)Marshal.GetActiveObject("Word.Application");
                word_app.Visible = true;

                object inputFile = "C:\\Users\\t-aaalle\\Desktop\\GreetingFile2.docx";// "selected_doc" contains the document name
                object confirmConversions = false;
                object readOnly = false;
                object missing = Type.Missing;

                Document doc = word_app.Documents.Open(ref inputFile, ref confirmConversions, ref readOnly, ref missing);
            }
            catch (Exception e)
            {
                var trace = new System.Diagnostics.StackTrace(e);
            }
        }

        public static void writeToOneNote()
        {
            GetNamespace();
            //Gets the notebook, section, and firstpage
            string notebookId = GetObjectId(null, OneNote.HierarchyScope.hsNotebooks, "Aaron @ Microsoft");
            string sectionId = GetObjectId(notebookId, OneNote.HierarchyScope.hsSections, "New Section 1");
            string firstPageId = GetObjectId(sectionId, OneNote.HierarchyScope.hsPages, "My Page");

            //Reads the contents of the first page
            GetPageContent(firstPageId);

            Console.Read();
        }

        //Gets the name of the notebook 
        static void GetNamespace()
        {
            string xml = "";

            onenoteApp.GetHierarchy(null, OneNote.HierarchyScope.hsNotebooks, out xml);
            var doc = XDocument.Parse(xml);
            ns = doc.Root.Name.Namespace;
        }


        //Gets the OneNote document
        static string GetObjectId(string parentId, HierarchyScope scope, string objectName)
        {
            string xml = "";

            onenoteApp.GetHierarchy(parentId, scope, out xml);

            var doc = XDocument.Parse(xml);
            var nodeName = "";

            switch (scope)
            {
                case (HierarchyScope.hsNotebooks): nodeName = "Notebook"; break;
                case (HierarchyScope.hsPages): nodeName = "Page"; break;
                case (HierarchyScope.hsSections): nodeName = "Section"; break;
                default:
                    return null;
            }

            var node = doc.Descendants(ns + nodeName).Where(n => n.Attribute("name").Value == objectName).FirstOrDefault();

            return node.Attribute("ID").Value;
        }

        //Get's the pages content 
        static string GetPageContent(string pageId)
        {
            string xml= "";
            onenoteApp.GetPageContent(pageId, out xml, OneNote.PageInfo.piAll);
            var doc = XDocument.Parse(xml);
            var outLine = doc.Descendants(ns + "Outline").First();
            var content = outLine.Descendants(ns + "T").First();
            //THE LINE ABOVE IS WHERE THE ERROR IS BEING THROWN
            string contentVal = content.Value;
            content.Value = resultWordText;
            onenoteApp.UpdatePageContent(doc.ToString());



            return null;
        }

        private void ThisAddIn_Shutdown(object sender, System.EventArgs e)
        {

        }

        #region VSTO generated code

        /// <summary>
        /// Required method for Designer support - do not modify
        /// the contents of this method with the code editor.
        /// </summary>
        private void InternalStartup()
        {
            this.Startup += new System.EventHandler(ThisAddIn_Startup);
            this.Shutdown += new System.EventHandler(ThisAddIn_Shutdown);
        }

        #endregion
    }
}

我是C＃的新手，完整的错误是：

An exception of type 'System.InvalidOperationException' occurred in System.Core.dll but was not handled in user code

这个错误令人困惑，因为我已经搜索过，无法找到任何可以帮助我的东西。帮助解决这个错误会很棒！