using System.Xml.Linq;

​

//XML example:

//<Lines>

//    <Line Id="1">one</Line>

//    <Line Id="2">two</Line>

//</Lines>

​

var filePath = @"\storage\emulated\0\0\data.xml";

var doc = new XDocument(

    new XElement("Lines",

        new XElement("Line", "one", 

            new XAttribute("Id", "1")),

        new XElement("Line", "two", 

            new XAttribute("Id", "2"))));

doc.Save(filePath);