UPDATE: This is now part of the EasyXml NuGet package. There is also an EasyXml.Sources NuGet package.

OK, so I had to output an Xml for something I am doing for work. I need the Xml to be pretty. I decided I wanted a C# class that would do the equivalent of what the Xml plugin in Notepad++ does. It should create Linear Xml, Pretty Xml, etc.

I found a bunch of problems when doing this in C#:

1. How to make an Xml linearized in C#?
2. How to make the Xml pretty, including indents and cleaned up spacing? I could get the indents, but the spacing was a problem until I solved how to linearize the Xml.
3. How to make the Xml declaration say UTF-8? It kept saying the Xml was in UTF-16, which was accurate because the file was UTF-16. Strings in C# are always Unicode (UTF-16).
4. How to make UTF-8 uppercase? Once I got the Xml declaration to say UTF-8, the UTF-8 text was lowercase, utf-8, instead of uppercase. Lowercase should work, but it turns out uppercase is preferred.
5. How to output the Xml as a file in actual UTF-8 format? It is one thing to have UTF-8 in the Xml declaration, it is quite another to actually output a file byte stream in UTF-8 vs UTF-16 (Unicode). A UTF-8 file should be 1/2 the size of a UTF-16 file.

So here is my class. I hope it helps you. All the questions are answered by this class.

using System.IO;
using System.Text;
using System.Text.RegularExpressions;
using System.Xml;
using System.Xml.Linq;

namespace Rhyous.EasyXml
{
    public class Xml
    {
        public Xml(string text)
        {
            Text = text;
        }

        /// <summary>
        /// And enumeration to select UTF8 or UTF16 encoding. This is used because the defautl UTF8 
        /// and Unicode encoding types don't capitalize the UTF characters in the Xml declaration.
        /// </summary>
        public enum XmlEncoding
        {
            UTF8, // By being first it is the default
            UTF16
        };

        /// <summary>
        /// The original Xml text as is.
        /// </summary>
        public string Text { get; private set; }

        /// <summary>
        /// The Xml in a single line (no new lines or carriage returns). The data is trimmed and no more than a single space anywhere.
        /// </summary>
        public string LinearizeXml
        {
            get { return _LinearizeXml ?? (_LinearizeXml = Clean(Document, LinearizedSettings)); }
        } private string _LinearizeXml;

        /// <summary>
        /// And XDocument representation of the Xml. It uses the Linearized Xml not the original text.
        /// </summary>
        public XDocument Document
        {
            get { return _Document ?? (_Document = XDocument.Parse(GetLinearizedXml(Text))); }
        } private XDocument _Document;

        /// <summary>
        /// The Xml with each element properly indented on a separate line. The data is trimmed and no more than a single space anywhere.
        /// </summary>
        public string PrettyXml
        {
            get { return _PrettyXml ?? (_PrettyXml = Clean(Document, PrettySettings)); }
        } private string _PrettyXml;

        /// <summary>
        /// An enum that specifies whether to use UTF-8 or Unicode (UTF-16).
        /// Changing the encoding shouldn't change the original Text but pretty and linearized 
        /// versions of the Xml should change as well as the stream.
        /// </summary>
        public XmlEncoding Encoding { get; set; }

        /// <summary>
        /// A method that outputs the Xml as a stream. It outputs using the correct Encoding.
        /// It isn't enough to write encoding="UTF-8" in the Xml declaration if the output file
        /// is still UTF-16. Botht the labeling and the actually bits in the file should match.
        /// </summary>
        /// <returns>A file stream in the configured encoding.</returns>
        public Stream ToStream()
        {
            return new MemoryStream(ToByteArray());
        }

        /// <summary>
        /// This creates a byte array using the correct encoding.
        /// 
        /// Note: Naturally, UTF-8 has half as manay bytes as UTF-16, however,
        /// if UTF-8 is n bytes, UTF-16 will be 2*N+2 bytes. This is because
        /// "UTF-8" is five characters and "UTF-16" is six characters. 
        /// So a 100 byte UTF-8 file would be 202 bytes in UTF-16. 
        /// </summary>
        /// <returns>A byte[] array of the Xml string in the configured encoding.</returns>
        public byte[] ToByteArray()
        {
            return GetEncoding().GetBytes(PrettyXml ?? "");
        }

        /// <summary>
        /// A method to get the current encoding based on the Enum value.
        /// </summary>
        /// <returns>The correct Encoding.</returns>
        private Encoding GetEncoding()
        {
            switch (Encoding)
            {
                case XmlEncoding.UTF8:
                    return XmlUTF8Encoding.Instance;
                case XmlEncoding.UTF16:
                    return XmlUnicode.Instance;
                default:
                    return XmlUnicode.Instance;
            }
        }

        /// <summary>
        /// XmlWriterSettings for linearized Xml.
        /// </summary>
        private XmlWriterSettings LinearizedSettings
        {
            get
            {
                return new XmlWriterSettings
                    {
                        Encoding = GetEncoding(),
                        Indent = false,
                        NewLineOnAttributes = false
                    };
            }
        }

        /// <summary>
        /// XmlWriterSettings for Pretty Xml.
        /// </summary>
        private XmlWriterSettings PrettySettings
        {
            get
            {
                return new XmlWriterSettings
                {
                    Encoding = GetEncoding(),
                    Indent = true,
                    IndentChars = string.IsNullOrEmpty(IndentCharacters) ? "  " : IndentCharacters,
                    NewLineOnAttributes = false,
                    NewLineHandling = NewLineHandling.Replace
                };
            }
        }

        /// <summary>
        /// The characters to use for indenting Pretty Xml
        /// </summary>
        public string IndentCharacters { get; set; }

        /// <summary>
        /// The method that uses XDocument to do make clean (pretty or linearized) Xml
        /// </summary>
        /// <param name="doc">The XDcoument version of the Xml.</param>
        /// <param name="settings">The configured XmlWriterSettings.</param>
        /// <returns>A pretty Xml string.</returns>
        private string Clean(XDocument doc, XmlWriterSettings settings)
        {
            var sb = new StringBuilder();
            var stringWriter = new StringWriterWithEncoding(sb, GetEncoding());
            using (var xmlWriter = XmlWriter.Create(stringWriter, settings))
            {
                doc.Save(xmlWriter);
                xmlWriter.Flush();
                return sb.ToString();
            }
        }

        /// <summary>
        /// A method that uses Regex to linearize Xml. The regex replaces methods are used.
        /// </summary>
        /// <param name="text">The Xml text</param>
        /// <returns>Linearized Xml string.</returns>
        private string GetLinearizedXml(string text)
        {
            // Replace all white space with a single space
            var halfclean = Regex.Replace(text, @"\s+", " ", RegexOptions.Singleline);

            // Trim after >.
            var clean75 = Regex.Replace(halfclean, @">\s+", ">");

            // Trim before <
            var fullclean = Regex.Replace(clean75, @"\s+<", "<");

            return fullclean;
        }

        /// <summary>
        /// This clas allows for the Xml to be created with the Xml declaration saying UTF-8
        /// </summary>
        private sealed class StringWriterWithEncoding : StringWriter
        {
            private readonly Encoding _Encoding;

            public StringWriterWithEncoding(StringBuilder builder, Encoding encoding)
                : base(builder)
            {
                _Encoding = encoding;
            }

            public override Encoding Encoding
            {
                get { return _Encoding; }
            }
        }

        /// <summary>
        /// This class makes the UTF-8 text in the Xml declaration show up capitalized.
        /// </summary>
        private sealed class XmlUTF8Encoding : UTF8Encoding
        {
            public override string WebName
            {
                get { return base.WebName.ToUpper(); }
            }

            public override string HeaderName
            {
                get { return base.HeaderName.ToUpper(); }
            }

            public override string BodyName
            {
                get { return base.BodyName.ToUpper(); }
            }

            public static XmlUTF8Encoding Instance
            {
                get { return _XmlUTF8Encoding ?? (_XmlUTF8Encoding = new XmlUTF8Encoding()); }
            } private static XmlUTF8Encoding _XmlUTF8Encoding;
        }

        /// <summary>
        /// This class makes the UTF-16 text in the Xml declaration show up capitalized.
        /// </summary>
        private sealed class XmlUnicode : UnicodeEncoding
        {
            public override string WebName
            {
                get { return base.WebName.ToUpper(); }
            }

            public override string HeaderName
            {
                get { return base.HeaderName.ToUpper(); }
            }

            public override string BodyName
            {
                get { return base.BodyName.ToUpper(); }
            }

            public static XmlUnicode Instance
            {
                get { return _XmlUnicode ?? (_XmlUnicode = new XmlUnicode()); }
            } private static XmlUnicode _XmlUnicode;
        }
    }
}

And here are some Unit Tests.

using LANDesk.Licensing.WebServices.Model;
using Microsoft.VisualStudio.TestTools.UnitTesting;
using System.IO;
using System.Text;

namespace Rhyous.EasyXml.Tests
{
    [TestClass]
    public class XmlTests
    {
        public string LinearUtf8Xml = "<?xml version=\"1.0\" encoding=\"UTF-8\"?><Person><FirstName>John</FirstName><MiddleName>Al Leon</MiddleName><LastName>Doe</LastName></Person>";
        public string PrettyUtf8Xml =
@"<?xml version=""1.0"" encoding=""UTF-8""?>
<Person>
  <FirstName>John</FirstName>
  <MiddleName>Al Leon</MiddleName>
  <LastName>Doe</LastName>
</Person>";
        public string PrettyUtf8XmlWithTabs =
@"<?xml version=""1.0"" encoding=""UTF-8""?>
<Person>
	<FirstName>John</FirstName>
	<MiddleName>Al Leon</MiddleName>
	<LastName>Doe</LastName>
</Person>";
        public string UglyUtf8Xml =
@"<?xml version=""1.0""
encoding=""UTF-8""?>
<Person>

<FirstName>
    John
        </FirstName>

<MiddleName>
    Al
    Leon
                </MiddleName>
  <LastName>
    


Doe
        </LastName>


</Person>";

        public string LinearUtf16Xml = "<?xml version=\"1.0\" encoding=\"UTF-16\"?><Person><FirstName>John</FirstName><MiddleName>Al Leon</MiddleName><LastName>Doe</LastName></Person>";
        public string PrettyUtf16Xml =
@"<?xml version=""1.0"" encoding=""UTF-16""?>
<Person>
  <FirstName>John</FirstName>
  <MiddleName>Al Leon</MiddleName>
  <LastName>Doe</LastName>
</Person>";
        public string UglyUtf16Xml =
@"<?xml version=""1.0""
encoding=""UTF-16""?>
<Person>

<FirstName>
    John
        </FirstName>

<MiddleName>
    Al
    Leon
                </MiddleName>
  <LastName>
    


Doe
        </LastName>


</Person>";

        [TestMethod]
        public void TestMethodLinearize()
        {
            // Arrange
            Xml xml = new Xml(PrettyUtf8Xml);

            // Act
            var actual = xml.LinearizeXml;

            // Assert
            Assert.AreEqual(LinearUtf8Xml, actual);
        }

        [TestMethod]
        public void TestMethodPretty()
        {
            // Arrange
            Xml xml = new Xml(LinearUtf8Xml);

            // Act
            var actual = xml.PrettyXml;

            // Assert
            Assert.AreEqual(PrettyUtf8Xml, actual);
        }

        [TestMethod]
        public void TestMethodLinearizeUgly()
        {
            // Arrange
            Xml xml = new Xml(UglyUtf8Xml);

            // Act
            var actual = xml.LinearizeXml;

            // Assert
            Assert.AreEqual(LinearUtf8Xml, actual);
        }

        [TestMethod]
        public void TestMethodMakeUglyPretty()
        {
            // Arrange
            Xml xml = new Xml(UglyUtf8Xml);

            // Act
            var actual = xml.PrettyXml;

            // Assert
            Assert.AreEqual(PrettyUtf8Xml, actual);
        }

        [TestMethod]
        public void TestMethodLinearizeUglyUtf16()
        {
            // Arrange
            Xml xml = new Xml(UglyUtf16Xml)
            {
                Encoding = Xml.XmlEncoding.UTF16
            };

            // Act
            var actual = xml.LinearizeXml;

            // Assert
            Assert.AreEqual(LinearUtf16Xml, actual);
        }

        [TestMethod]
        public void TestMethodMakeUglyPrettyUtf16()
        {
            // Arrange
            Xml xml = new Xml(UglyUtf16Xml)
            {
                Encoding = Xml.XmlEncoding.UTF16
            };

            // Act
            var actual = xml.PrettyXml;

            // Assert
            Assert.AreEqual(PrettyUtf16Xml, actual);
        }

        [TestMethod]
        public void TestMethodStreamIsUtf8()
        {
            // Arrange
            Xml xml = new Xml(UglyUtf8Xml)
            {
                Encoding = Xml.XmlEncoding.UTF8
            };

            // Act
            var actual = xml.ToStream();
            using (var memoryStream = new MemoryStream())
            {
                actual.CopyTo(memoryStream);
                var bytes = memoryStream.ToArray();
                // Assert
                Assert.AreEqual(154, bytes.Length);
            }
        }

        [TestMethod]
        public void TestMethodStreamIsUtf16()
        {
            // Arrange
            Xml xml = new Xml(UglyUtf16Xml)
            {
                Encoding = Xml.XmlEncoding.UTF16
            };

            // Act
            var actual = xml.ToStream();
            using (var memoryStream = new MemoryStream())
            {
                actual.CopyTo(memoryStream);
                var bytes = memoryStream.ToArray();

                // Assert
                // 310 is twice the size of 154, 308, but add 2 bytes because
                // UTF-8 is 5 characters but UTF-16 is 6 characters so it is 
                // one character longer.John
                Assert.AreEqual(310, bytes.Length);
            }
        }

        [TestMethod]
        public void TestMethodPrettyWuthTabs()
        {
            // Arrange
            Xml xml = new Xml(LinearUtf8Xml)
            {
                IndentCharacters = "\t"
            };

            // Act
            var actual = xml.PrettyXml;

            // Assert
            Assert.AreEqual(PrettyUtf8XmlWithTabs, actual);
        }

        [TestMethod]
        public void TestMethodStreamUtf8IsDifferentThanStreamUtf16()
        {
            const string text = "Hello, world!";

            var utf8 = Encoding.UTF8.GetBytes(text);
            var utf16 = Encoding.Unicode.GetBytes(text);

            Assert.AreNotEqual(utf8.Length, utf16.Length);
        }
    }
}