VSTO : Creating and retrieving Custom XML part in Excel

Custom XML allows one to store XML in the desired format inside an Office document. For example, if we open Excel Workbook using 7-zip the custom XML folder is right at the root level. 

Default Folder Structure

string xmlString1 =
          "<?xml version=\"1.0\" encoding=\"utf-8\" ?>" +
          "<employees xmlns=\"http://schemas.microsoft.com/vsto/samplestest\">" +
              "<employee>" +
                  "<name>Surender G</name>" +
                  "<hireDate>1999-04-01</hireDate>" +
                  "<title>Manager</title>" +
              "</employee>" +
          "</employees>";
Office.CustomXMLPart employeeXMLPart = Globals.ThisAddIn.Application.ActiveWorkbook.CustomXMLParts.Add(xmlString1, missing);


After executing the above code two files (item4.xml, itemProps4.xml) get added to the folder.

itemProps4.xml
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<ds:datastoreItem ds:itemID="{8905F770-7F58-4172-B707-34A283DF8527}" xmlns:ds="http://schemas.openxmlformats.org/officeDocument/2006/customXml><ds:schemaRefs>
<ds:schemaRef ds:uri="http://schemas.microsoft.com/vsto/samplestest"/></ds:schemaRefs>
</ds:datastoreItem>

item4.xml
<?xml version="1.0" encoding="utf-8"?>
<employees xmlns="http://schemas.microsoft.com/vsto/samplestest">
<employee>
<name>Pradeep G</name>
<hireDate>1999-09-01</hireDate>
<title>Manager</title>
</employee>
</employees>

Folder structure after adding Custom XML

Now, we use the highlighted itemID in itemProps4.xml to retrieve the stored XML (Don't miss parenthesis).  

string str = "{8905F770-7F58-4172-B707-34A283DF8527}";
                    Office.CustomXMLPart obj = Globals.ThisAddIn.Application.ActiveWorkbook.CustomXMLParts.SelectByID(str);
MessageBox.Show("XML ::::"+obj.XML);

Result

We have successfully retrieved the XML.

References:
Using Custom XML Part as a data store