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.IO.Compression;

using System.Reflection;

​

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

​

//https://stackoverflow.com/questions/72859465/how-can-i-change-to-in-xmldocument-c

​

    public class TestClass

    {

        public static void Test()

        {

            var xml = @"<foo Name=""hello""><bar Id=""101"">var value</bar></foo>";

            XmlDocument doc = new XmlDocument();

            doc.LoadXml(xml);

            Console.WriteLine(doc.GetOuterXml());

        }

    }

​

    public static partial class XmlSerializationHelper

    {

        public static string GetOuterXml(this XmlDocument doc, bool indent = true)

        {

            if (doc == null)

                return null;

            using (var textWriter = new StringWriterWithEncoding())

            {

                using (var xmlWriter = new CustomQuoteCharXmlTextWriter(textWriter) { Formatting = indent ? Formatting.Indented : Formatting.None })

                {

                    doc.Save(xmlWriter);

                }

                return textWriter.ToString();

            }

        }

    }

​

    public class CustomQuoteCharXmlTextWriter : XmlTextWriter

    {

        public CustomQuoteCharXmlTextWriter(Stream w, Encoding encoding) : base(w, encoding) => QuoteChar = '\'';

        public CustomQuoteCharXmlTextWriter(String filename, Encoding encoding) : base(filename, encoding) => QuoteChar = '\'';

        public CustomQuoteCharXmlTextWriter(TextWriter w) : base(w) => QuoteChar = '\'';

        public override void WriteStartDocument() 

        {

            base.WriteStartDocument();

            QuoteChar = '"';

        }

    }

    public sealed class StringWriterWithEncoding : StringWriter

    {

        // From this answer https://stackoverflow.com/a/42584394/3744182

        // To https://stackoverflow.com/questions/42583299/xmlwriter-encoding-utf-8-using-stringwriter-in-c-sharp

        public StringWriterWithEncoding() : this(Encoding.UTF8) { }

        public StringWriterWithEncoding(Encoding encoding) => this.Encoding = encoding;

        public override Encoding Encoding { get; }

    }

​

    public static partial class XmlSerializationHelper

    {

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

        {

            if (node == null)

                return null;

            using (var textWriter = new StringWriterWithEncoding())

            {

                using (var xmlWriter = new XmlTextWriter(textWriter) { Formatting = indent ? Formatting.Indented : Formatting.None })

                {

                    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;

            }

        }

    }

​