Create OpenXML Spreadsheet's xml namespace in LINQ to XML.

xslxNamespace.cs

using System.Xml.Linq;

namespace XslxNameSpaceSample
{
    class Program
    {
        static void XNamespaceSample()
        {
            const string openXmlMarkupSchema = "http://schemas.openxmlformats.org/markup-compatibility/2006";
            const string spreadSheetSchema = "http://schemas.microsoft.com/office/spreadsheetml/2009/9/ac";
            const string relationshipSchema = "http://schemas.openxmlformats.org/officeDocument/2006/relationships";
            var sheetDocument = new XDocument();
            sheetDocument.Declaration = new XDeclaration("1.0", "utf-8", "yes");
            XNamespace xlsns = openXmlMarkupSchema;
            XName mcIgnorableName = XName.Get("Ignorable", openXmlMarkupSchema);
            XNamespace mcNameSpace = "mc";
            XNamespace rNameSpace = "r";
            XNamespace x14acNameSpace = "x14ac";
            sheetDocument.Add(
                new XElement("worksheet",
                new XAttribute(XNamespace.Xmlns + "r", relationshipSchema),
                new XAttribute(XNamespace.Xmlns + "x14ac", spreadSheetSchema)));
            var x14acPrefix = sheetDocument.Document.Root.GetPrefixOfNamespace(XNamespace.Get(spreadSheetSchema));
            var rPrefix = sheetDocument.Document.Root.GetPrefixOfNamespace(XNamespace.Get(relationshipSchema));
            var rNamePrefix = sheetDocument.Document.Root.GetNamespaceOfPrefix("r");
            var x14acNamePrefix = sheetDocument.Document.Root.GetNamespaceOfPrefix("x14ac");
            sheetDocument.Document.Root.Add(new XAttribute(XNamespace.Xmlns + "mc", mcIgnorableName.NamespaceName));
            sheetDocument.Document.Root.Add(new XAttribute(mcIgnorableName, x14acPrefix));
        }
    }
}