Merge two web.config XML files

namespace XmlDocumentMerger
{
    using System;
    using System.Linq;
    using System.Text;
    using System.Xml;

    /*
     * Use this class for quickly merging two XML web.config files into one file.
     * 2015-06-30, Uwe Keim, http://www.zeta-test.com
    */
    public static class XmlDocumentMergeController
    {
        public static string MergeDocuments(
            string xmlToMergeFrom,
            string xmlToMergeInto)
        {
            if (string.IsNullOrEmpty(xmlToMergeFrom)) return xmlToMergeInto;
            else if (string.IsNullOrEmpty(xmlToMergeInto)) return xmlToMergeFrom;
            else if (string.IsNullOrEmpty(xmlToMergeFrom) &&
                     string.IsNullOrEmpty(xmlToMergeInto)) return xmlToMergeInto;

            // --

            var sourceDoc = new XmlDocument();
            var destDoc = new XmlDocument();

            sourceDoc.LoadXml(xmlToMergeFrom);
            destDoc.LoadXml(xmlToMergeInto);

            // --

            foreach (
                var sourceNode in sourceDoc.ChildNodes.Cast<XmlNode>().Where(n => n.NodeType == XmlNodeType.Element))
            {
                doProcessNode(sourceNode, destDoc);
            }

            // --

            return destDoc.OuterXml;
        }

        private static void doProcessNode(XmlNode sourceNode, XmlNode destParentNode)
        {
            var destNode = findNode(sourceNode, destParentNode.OwnerDocumentIntelligent());
            if (destNode == null)
            {
                // Gibt es noch nicht, einfach rüber kopieren.
                copyNode(sourceNode, destParentNode);
            }
            else
            {
                // Gibt es schon, Inhalt verarbeiten.
                foreach (
                    var childNode in sourceNode.ChildNodes.Cast<XmlNode>().Where(n => n.NodeType == XmlNodeType.Element)
                    )
                {
                    doProcessNode(childNode, destNode);
                }
            }
        }

        private static void copyNode(XmlNode sourceNode, XmlNode destParentNode)
        {
            // ReSharper disable once PossibleNullReferenceException
            var newNode = destParentNode.OwnerDocumentIntelligent().ImportNode(sourceNode, true);
            destParentNode.AppendChild(newNode);
        }

        private static XmlNode findNode(XmlNode sourceNode, XmlNode destDoc)
        {
            var xPath = findXPath(sourceNode);
            var destNode = destDoc.SelectSingleNode(xPath);

            return destNode;
        }

        // http://stackoverflow.com/a/241291/107625
        private static string findXPath(XmlNode node)
        {
            var builder = new StringBuilder();
            while (node != null)
            {
                switch (node.NodeType)
                {
                    case XmlNodeType.Attribute:
                        builder.Insert(0, string.Format(@"/@{0}", node.Name));
                        node = ((XmlAttribute) node).OwnerElement;
                        break;
                    case XmlNodeType.Element:
                        var index = findElementIndex((XmlElement) node);
                        builder.Insert(0, string.Format(@"/{0}[{1}]", node.Name, index));
                        node = node.ParentNode;
                        break;
                    case XmlNodeType.Document:
                        return builder.ToString();
                    default:
                        throw new ArgumentException("Only elements and attributes are supported");
                }
            }
            throw new ArgumentException("Node was not in a document");
        }

        private static int findElementIndex(XmlNode element)
        {
            var parentNode = element.ParentNode;
            if (parentNode is XmlDocument)
            {
                return 1;
            }

            var parent = (XmlElement) parentNode;
            var index = 1;

            if (parent != null)
            {
                foreach (XmlNode candidate in parent.ChildNodes)
                {
                    if (candidate is XmlElement && candidate.Name == element.Name)
                    {
                        if (candidate == element)
                        {
                            return index;
                        }
                        index++;
                    }
                }
            }
            throw new ArgumentException("Couldn't find element within parent");
        }
    }

    internal static class XmlExtensions
    {
        public static XmlDocument OwnerDocumentIntelligent(this XmlNode node)
        {
            if (node == null) return null;
            else
            {
                var document = node as XmlDocument;
                return document ?? node.OwnerDocument;
            }
        }
    }
}