How To Write To A OneNote 2013 Page Using C# and The OneNote Interop

I asked the same question on MSDN forums and was given this great answer. Below is a nice, clean example of how to write to OneNote using C# and the OneNote interop. I hope that this can help people in the future.

    static Application onenoteApp = new Application();
    static XNamespace ns = null;

    static void Main(string[] args)
    {
        GetNamespace();
        string notebookId = GetObjectId(null, OneNote.HierarchyScope.hsNotebooks, "MyNotebook");
        string sectionId = GetObjectId(notebookId, OneNote.HierarchyScope.hsSections, "Sample_Section");
        string firstPageId = GetObjectId(sectionId, OneNote.HierarchyScope.hsPages, "MyPage");
        GetPageContent(firstPageId);
        Console.Read();
    }
    static void GetNamespace()
    {
        string xml;

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

    static string GetObjectId(string parentId, OneNote.HierarchyScope scope, string objectName)
    {
        string xml;
        onenoteApp.GetHierarchy(parentId, scope, out xml);

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

        switch (scope)
        {
            case (OneNote.HierarchyScope.hsNotebooks): nodeName = "Notebook"; break;
            case (OneNote.HierarchyScope.hsPages): nodeName = "Page"; break;
            case (OneNote.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;
    }
    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();
        string contentVal = content.Value;
        content.Value = "modified";
        onenoteApp.UpdatePageContent(doc.ToString());
        return null;
    }

This is just what I've gleaned from reading examples on the web (of course, you've already read all of those) and peeking into the way OneNote stores its data in XML using ONOMspy (http://blogs.msdn.com/b/johnguin/archive/2011/07/28/onenote-spy-omspy-for-onenote-2010.aspx).

If you want to work with OneNote content, you'll need a basic understanding of XML. Writing text to a OneNote page involves creating an outline element, whose content will be contained in OEChildren elements. Within an OEChildren element, you can have many other child elements representing outline content. These can be of type OE or HTMLBlock, if I'm reading the schema correctly. Personally, I've only ever used OE, and in this case, you'll have an OE element containing a T (text) element. The following code will create an outline XElement and add text to it:

// Get info from OneNote
string xml;
onApp.GetHierarchy(null, OneNote.HierarchyScope.hsSections, out xml);
XDocument doc = XDocument.Parse(xml);
XNamespace ns = doc.Root.Name.Namespace;

// Assuming you have a notebook called "Test"
XElement notebook = doc.Root.Elements(ns + "Notebook").Where(x => x.Attribute("name").Value == "Test").FirstOrDefault();
if (notebook == null)
{
    Console.WriteLine("Did not find notebook titled 'Test'.  Aborting.");
    return;
}

// If there is a section, just use the first one we encounter
XElement section;
if (notebook.Elements(ns + "Section").Any())
{
    section = notebook.Elements(ns + "Section").FirstOrDefault();
}
else
{
    Console.WriteLine("No sections found.  Aborting");
    return;
}

// Create a page
string newPageID;
onApp.CreateNewPage(section.Attribute("ID").Value, out newPageID);

// Create the page element using the ID of the new page OneNote just created
XElement newPage = new XElement(ns + "Page");
newPage.SetAttributeValue("ID", newPageID);

// Add a title just for grins
newPage.Add(new XElement(ns + "Title",
    new XElement(ns + "OE",
        new XElement(ns + "T",
            new XCData("Test Page")))));

// Add an outline and text content
newPage.Add(new XElement(ns + "Outline",
    new XElement(ns + "OEChildren",
        new XElement(ns + "OE",
            new XElement(ns + "T",
                new XCData("Here is some new sample text."))))));

// Now update the page content
onApp.UpdatePageContent(newPage.ToString());

Here's what the actual XML you're sending to OneNote looks like:

<Page ID="{20A13151-AD1C-4944-A3D3-772025BB8084}{1}{A1954187212743991351891701718491104445838501}" xmlns="http://schemas.microsoft.com/office/onenote/2013/onenote">
  <Title>
    <OE>
      <T><![CDATA[Test Page]]></T>
    </OE>
  </Title>
  <Outline>
    <OEChildren>
      <OE>
        <T><![CDATA[Here is some new sample text.]]></T>
      </OE>
    </OEChildren>
  </Outline>
</Page>

Hope that helps get you started!