XmlWellFormedWriter ignores XmlWriterSettings.NewLineOnAttributes for attributes specifically named xmlns

[nitz]

Description

When writing an XDocument to a stream via XmlWriter, even if specifically requested, attributes specifically named xmlns will not write on their own line. Other attributes don't suffer this issue.

Reproduction Steps

See this .NET Fiddle: https://dotnetfiddle.net/PCyVdc (.NET 7)
(Similar fiddle for .NET Framework 4.7.2 here.)

Code from fiddle reproduced here.

using System;
using System.Xml;
using System.Xml.Linq;
using System.IO;
					
public class Program
{
	public static void Main()
	{
		XmlWriterSettings writerSettings = new() {
			// enable newlines on attributes
			NewLineOnAttributes = true,
			
			// value of NewLineHandling doesn't chnage behavior
			//NewLineHandling = NewLineHandling.None,
			//NewLineHandling = NewLineHandling.Entitize,
			NewLineHandling = NewLineHandling.Replace,
			
			// value of NewLineChars doesn't change behavior
			NewLineChars = "\n",
			
			// if Indent is false, no newlines will be written at all,
			// regardless of NewLineHandling; which seems improper 🤷🏻‍♀️
			Indent = true,
		};
		
		string[] tests = new[] {
			// this test will properly writes each attribute on their own lines
			"""<?xml version="1.0" encoding="utf-8"?><data attrib1="1" attrib2="2" attrib3="3" attrib4="4"><child /></data>""",
			// xmlns is written next to the element name, but other attributes are correctly written on their own line
			"""<?xml version="1.0" encoding="utf-8"?><data xmlns="1" attrib2="2" attrib3="3" attrib4="4"><child /></data>""",
			// no newline after attrib3, xmlns is written on the same line
			"""<?xml version="1.0" encoding="utf-8"?><data attrib1="1" attrib2="2" attrib3="3" xmlns="4"><child /></data>""",
			// no newline after attrib2, xmlns is written on the same line, but attrib4 is correctly written on it's own line
			"""<?xml version="1.0" encoding="utf-8"?><data attrib1="1" attrib2="2" xmlns="3" attrib4="4"><child /></data>""",
			// the same as above, just on an empty element
			"""<?xml version="1.0" encoding="utf-8"?><data attrib1="1" attrib2="2" xmlns="3" attrib4="4" />""",
		};
		
		foreach (string test in tests)
		{
			FormatAndPrint(test, writerSettings);
		}
	}
	
	public static void FormatAndPrint(string s, XmlWriterSettings writerSettings)
	{
		// load the xml string, and write it to memory
		XDocument xd = XDocument.Parse(s);
		using MemoryStream stream = new();
		using XmlWriter writer = XmlWriter.Create(stream, writerSettings);
		xd.WriteTo(writer);
		writer.Flush();

		// read the written xml back out out of memory, and write it to console
		stream.Position = 0;
		using StreamReader reader = new StreamReader(stream, writerSettings.Encoding);
		string xml = reader.ReadToEnd();
		Console.WriteLine("---");
		Console.WriteLine(xml);
		Console.WriteLine();
	}
}

Expected behavior

All written attributes should be on their own lines when XmlWriterSettings.NewLineOnAttributes is true.

Actual behavior

Attributes specifically named xmlns are always written on the same line the writer is currently on, regardless of XmlWriterSettings member values.

Regression?

Unlikely. I was able to reproduce it against the following runtimes: .NET 7, .NET 6, .NET 5, .NET Core 3.1, .NET Framework 4.7.2

Known Workarounds

Manually write the XML, or post-process the writer's output and manually insert a newline & indentation.

Configuration

Example code is running against .NET 7.

OS/Arch unlikely to change outcome.

Other information

None yet.

[ghost]

Tagging subscribers to this area: @dotnet/area-system-xml
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Description

When writing an XDocument to a stream via XmlWriter, even if specifically requested, attributes specifically named xmlns will not write on their own line. Other attributes don't suffer this issue.

Reproduction Steps

See this .NET Fiddle: https://dotnetfiddle.net/PCyVdc

Code from fiddle reproduced here.

using System;
using System.Xml;
using System.Xml.Linq;
using System.IO;
					
public class Program
{
	public static void Main()
	{
		XmlWriterSettings writerSettings = new() {
			// enable newlines on attributes
			NewLineOnAttributes = true,
			
			// value of NewLineHandling doesn't chnage behavior
			//NewLineHandling = NewLineHandling.None,
			//NewLineHandling = NewLineHandling.Entitize,
			NewLineHandling = NewLineHandling.Replace,
			
			// value of NewLineChars doesn't change behavior
			NewLineChars = "\n",
			
			// if Indent is false, no newlines will be written at all,
			// regardless of NewLineHandling; which seems improper 🤷🏻‍♀️
			Indent = true,
		};
		
		string[] tests = new[] {
			// this test will properly writes each attribute on their own lines
			"""<?xml version="1.0" encoding="utf-8"?><data attrib1="1" attrib2="2" attrib3="3" attrib4="4"><child /></data>""",
			// xmlns is written next to the element name, but other attributes are correctly written on their own line
			"""<?xml version="1.0" encoding="utf-8"?><data xmlns="1" attrib2="2" attrib3="3" attrib4="4"><child /></data>""",
			// no newline after attrib3, xmlns is written on the same line
			"""<?xml version="1.0" encoding="utf-8"?><data attrib1="1" attrib2="2" attrib3="3" xmlns="4"><child /></data>""",
			// no newline after attrib2, xmlns is written on the same line, but attrib4 is correctly written on it's own line
			"""<?xml version="1.0" encoding="utf-8"?><data attrib1="1" attrib2="2" xmlns="3" attrib4="4"><child /></data>""",
			// the same as above, just on an empty element
			"""<?xml version="1.0" encoding="utf-8"?><data attrib1="1" attrib2="2" xmlns="3" attrib4="4" />""",
		};
		
		foreach (string test in tests)
		{
			FormatAndPrint(test, writerSettings);
		}
	}
	
	public static void FormatAndPrint(string s, XmlWriterSettings writerSettings)
	{
		// load the xml string, and write it to memory
		XDocument xd = XDocument.Parse(s);
		using MemoryStream stream = new();
		using XmlWriter writer = XmlWriter.Create(stream, writerSettings);
		xd.WriteTo(writer);
		writer.Flush();

		// read the written xml back out out of memory, and write it to console
		stream.Position = 0;
		using StreamReader reader = new StreamReader(stream, writerSettings.Encoding);
		string xml = reader.ReadToEnd();
		Console.WriteLine("---");
		Console.WriteLine(xml);
		Console.WriteLine();
	}
}

Expected behavior

All written attributes should be on their own lines when XmlWriterSettings.NewLineOnAttributes is true.

Actual behavior

Attributes specifically named xmlns are always written on the same line the writer is currently on, regardless of XmlWriterSettings member values.

Regression?

Unlikely. I was able to reproduce it against the following runtimes: .NET 7, .NET 6, .NET 5, .NET Core 3.1, .NET Framework 4.7.2

Known Workarounds

Manually write the XML, or post-process the writer's output and manually insert a newline & indentation.

Configuration

Example code is running against .NET 7.

OS/Arch unlikely to change outcome.

Other information

None yet.

Author:	nitz
Assignees:	-
Labels:	area-System.Xml, untriaged
Milestone:	-

[krwq]

Hey @nitz, thanks for reporting this. Note that we're currently not actively investing in XML area (JSON is our main focus) but we're open for contributions if they're relatively low-risk. We can assist if you need any help with making contribution.

[jeffhandley]

I wanted to follow up to clarify the statement above.

As @krwq said, we appreciate the issue being filed. We have moved this issue into Future as it's unlikely our team will get to this during .NET 8. We'd consider a community contribution though, and we'd be happy to review one and merge it if the risk of the change is low.

Thanks again!

[nitz]

Oh no worries at all; was just a really odd one that I stumbled across and was surprised to see no one else had written it down yet, so I figured that meant it was my duty :)