using System;

using System.Linq;

using System.Xml;

using System.Xml.Linq;

​

namespace StackOverflow

{

    public class Program

    {

        public static void Main(string[] args)

        {

            string[] lines = new string[] {

                "root_node1_node2_node3=value",

                "root_node1_node2_node3=value",

                "root_node1_node2_node3_node4=value",

                "root_node5_node6=value"

            };

​

            XDocument xDoc = new XDocument();  

​

            for (int i = 0; i < lines.Length; i++) // read lines from streamreader instead of array

            {

                string[] nodes = lines[i].Split('_');               

                for (int j = 0; j < nodes.Length; j++)

                {

                    if(j == 0) // assume that all line should start with same root name

                    {

                        if (xDoc.Root == null)

                        {

                            var root = new XElement(nodes[j]);

                            xDoc.Add(root);

                        }

                    }

                    else

                    {

                        var previousNode = xDoc.Descendants(nodes[j - 1]).FirstOrDefault();

                        if (nodes[j].Contains('='))

                        {

                            var elementValues = nodes[j].Split('=');

​

                            if (previousNode.DescendantNodes().Count() == 1 && previousNode.Value != "")

                            {

                                previousNode.AddAfterSelf(new XElement(nodes[j - 1], new XElement(elementValues[0], elementValues[1])));                                    

                            }

                            else

                            {

                                previousNode.Add(new XElement(elementValues[0], elementValues[1]));

                            }

                        }

                        else

                        {

                            var node = xDoc.Descendants(nodes[j]);                            

                            if (node.Count() == 0)

                            {

                                previousNode.Add(new XElement(nodes[j]));

                            }

                        }

                    }

                }                

            }

            Console.WriteLine(xDoc);

        }

    }

}

​