using System;

using System.Collections.Generic;

using System.Text;

using System.Linq;

using System.Xml;

using System.Xml.Linq;

using System.IO;

​

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

​

//https://stackoverflow.com/questions/20762/how-do-you-remove-invalid-hexadecimal-characters-from-an-xml-based-data-source-p

​

#nullable enable

​

    public static class XmlExtensions

    {

        public static string RemoveInvalidXmlCharacters(this string xmlText, char? fallback = null)

        {

            ArgumentNullException.ThrowIfNull(xmlText);

​

            StringBuilder? sb = null;

​

            for (int i = 0; i < xmlText.Length; i++)

            {

                if (XmlConvert.IsXmlChar(xmlText[i]))

                {

                    if (sb != null)

                        sb.Append(xmlText[i]);

                }

                else if (i < xmlText.Length - 1 && XmlConvert.IsXmlSurrogatePair(xmlText[i+1], xmlText[i])) // Yes this order is correct.

                {

                    if (sb != null)

                        sb.Append(xmlText, i, 2);

                    i++;

                }

                else

                {

                    if (sb == null)

                    {

                        sb = new();

                        sb.Append(xmlText, 0, i);

                    }

                    if (fallback != null)

                        sb.Append(fallback.Value);

                }

            }

            return sb?.ToString() ?? xmlText;

        }

    }

​

    public class TestClass

    {

        public static void Test()

        {

            var xmlWithInvalidCharacters = new []

            {

                // https://learn.microsoft.com/en-us/dotnet/standard/base-types/character-encoding-introduction#grapheme-clusters

                (bad : "👩🏽•🚒 \x01Hello, \x01\x1EWorld! H\u0302\U00029E3D", fallback: (char?) null, good : "👩🏽•🚒 Hello, World! Ĥ𩸽"),

                (bad: "\x0\x1\x2\x3", fallback: (char?) null, good : ""),

                (bad: "\x0\x1\x2\x3", fallback: (char?) '?', good : "????"),

            };

            foreach ((var bad, var fallback, var good) in xmlWithInvalidCharacters)

            {

                Test(bad, fallback, good);

            }

        }

​

        static void Test(string badXmlText, char? fallback, string expectedFixedXmlText)

        {

            Assert.Throws(Is.InstanceOf(typeof(Exception)), () => XmlConvert.VerifyXmlChars(badXmlText));

​

            var @fixed = fallback == null ? badXmlText.RemoveInvalidXmlCharacters() : badXmlText.RemoveInvalidXmlCharacters(fallback);

​

            Assert.DoesNotThrow(() => XmlConvert.VerifyXmlChars(@fixed));

            Console.WriteLine("Original: {0}\n  Fixed: {1}", badXmlText, @fixed);

            Assert.That(expectedFixedXmlText == @fixed);

        }

    }

​

    public class Program

    {

        public static void Main()

        {

            Console.WriteLine("Environment version: {0} ({1}, {2}, NewLine: {3}).", 

                              System.Runtime.InteropServices.RuntimeInformation.FrameworkDescription , Environment.Version, Environment.OSVersion,

                              System.Text.Json.JsonSerializer.Serialize(Environment.NewLine));

            Console.WriteLine();

​

            try

            {

                TestClass.Test();

            }

            catch (Exception ex)

            {

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

                Console.WriteLine(ex);

                throw;

            }

        }

    }

​