using System;

using System.Collections.Generic;

using System.Text;

using System.Linq;

using System.Xml.Serialization;

using System.Xml;

using System.Xml.Linq;

using System.IO;

using System.Data;

using System.Runtime.Serialization;

using System.Runtime.Serialization.Formatters.Binary;

using System.IO.Compression;

using System.Reflection;

​

using NUnit.Framework; //NUnit.Framework.AssertionException

​

    public class TestClass

    {

        public static void Test()

        {

            string outerXml = @"<kk:Root xmlns=""https://stackoverflow.com/questions/72740501/"" xmlns:kk=""kkNamespace""><Data></Data></kk:Root>";

            string innerXml = @"<ElementToAdd Id=""10101""><InnerValue>my inner value</InnerValue></ElementToAdd><ElementToAdd Id=""20202""><InnerValue>another inner value</InnerValue></ElementToAdd>";

            var doc = XDocument.Parse(outerXml);

            Console.WriteLine(outerXml);

            Console.WriteLine(innerXml);

            Console.WriteLine();

            var xmlElement = doc.Root.Elements().First();

​

            XmlNamespaceManager mgr = new XmlNamespaceManager(new NameTable());

            mgr.AddNamespace("", xmlElement.GetDefaultNamespace().ToString());

            XmlParserContext ctx = new XmlParserContext(null, mgr, null, XmlSpace.Default);

            using (var textReader = new StringReader(innerXml))

            using (var reader = XmlReader.Create(textReader, new XmlReaderSettings { ConformanceLevel = ConformanceLevel.Fragment }, ctx))

            using (var writer = xmlElement.CreateWriter())

            {

                writer.WriteNode(reader, true);

            }

            Console.WriteLine(doc);

        }

    }

​

    public static class XmlSerializationHelper

    {

        public static string GetOuterXml(this XmlNode node, bool indent = true)

        {

            if (node == null)

                return null;

            using (var textWriter = new StringWriter())

            {

                var settings = new XmlWriterSettings

                {

                    Indent = indent,

                    OmitXmlDeclaration = true,

                };

                using (var xmlWriter = XmlWriter.Create(textWriter, settings))

                {

                    node.WriteTo(xmlWriter);

                }

                return textWriter.ToString();

            }

        }

    }

​

    public class Program

    {

        public static void Main()

        {

            Console.WriteLine("Environment version: {0} ({1}, {2})", System.Runtime.InteropServices.RuntimeInformation.FrameworkDescription , Environment.Version, Environment.OSVersion);

            Console.WriteLine();

​

            try

            {

                TestClass.Test();

            }

            catch (Exception ex)

            {

                Console.WriteLine("Failed with unhandled exception: ");

                Console.WriteLine(ex);

                throw;

            }

        }

    }

​