XmlWriterSettings.NewLineOnAttributes is ignored on writing xmlns:* attributes

[smdn]

Description

As described in the title.
XmlWriterSettings.NewLineOnAttributes is applied to normal attributes properly, but seems not to be applied to xmlns attributes.

This problem seems to occur with both case of xmlns="..." and xmlns:nsname="...".

Reproduction Steps

Code for reproduction:

using System.Xml;
using System.Xml.Linq;

var nsSvg = (XNamespace)"http://www.w3.org/2000/svg";
var nsXLink = (XNamespace)"http://www.w3.org/1999/xlink";
var doc = new XDocument(
  new XDeclaration("1.0", "utf-8", null),
  new XElement(
    nsSvg + "svg",
    new XAttribute("version", "1.1"),
    new XAttribute("width", "10"),
    new XAttribute(XNamespace.Xmlns + "xlink", nsXLink.NamespaceName),
    new XAttribute("height", "10"),
    new XElement(nsSvg + "g")
  )
);

var settings = new XmlWriterSettings() {
  CloseOutput = false,
  Indent = true,
  IndentChars = " ",
  NewLineChars = "\n",
  NewLineOnAttributes = true
};

using (var writer = XmlWriter.Create(Console.Out, settings)) {
  doc.Save(writer);
}

Console.WriteLine();

Note: The same problem occurs with XDocument created from XDocument.Load.

var reader = new StringReader(@"<?xml version=""1.0"" encoding=""utf-8""?>
<svg
 version=""1.1""
 xmlns=""http://www.w3.org/2000/svg""
 width=""10""
 xmlns:xlink=""http://www.w3.org/1999/xlink""
 height=""10""
>
 <g />
</svg>
");
var doc = XDocument.Load(reader);

Expected behavior

<?xml version="1.0" encoding="utf-8"?>
<svg
 version="1.1"
 width="10"
 xmlns:xlink="http://www.w3.org/1999/xlink"
 height="10"
 xmlns="http://www.w3.org/2000/svg">
 <g />
</svg>

Actual behavior

<?xml version="1.0" encoding="utf-8"?>
<svg
 version="1.1"
 width="10" xmlns:xlink="http://www.w3.org/1999/xlink"
 height="10" xmlns="http://www.w3.org/2000/svg">
 <g />
</svg>

Regression?

No response

Known Workarounds

No response

Configuration

$ dotnet --info
.NET SDK (reflecting any global.json):
 Version:   6.0.400
 Commit:    7771abd614

Runtime Environment:
 OS Name:     ubuntu
 OS Version:  20.04
 OS Platform: Linux
 RID:         ubuntu.20.04-x64
 Base Path:   /usr/share/dotnet/sdk/6.0.400/

global.json file:
  Not found

Host:
  Version:      6.0.8
  Architecture: x64
  Commit:       55fb7ef977

*snip*

Other information

No response

[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

As described in the title.
XmlWriterSettings.NewLineOnAttributes is applied to normal attributes properly, but seems not to be applied to xmlns attributes.

This problem seems to occur with both case of xmlns="..." and xmlns:nsname="...".

Reproduction Steps

Code for reproduction:

using System.Xml;
using System.Xml.Linq;

var nsSvg = (XNamespace)"http://www.w3.org/2000/svg";
var nsXLink = (XNamespace)"http://www.w3.org/1999/xlink";
var doc = new XDocument(
  new XDeclaration("1.0", "utf-8", null),
  new XElement(
    nsSvg + "svg",
    new XAttribute("version", "1.1"),
    new XAttribute("width", "10"),
    new XAttribute(XNamespace.Xmlns + "xlink", nsXLink.NamespaceName),
    new XAttribute("height", "10"),
    new XElement(nsSvg + "g")
  )
);

var settings = new XmlWriterSettings() {
  CloseOutput = false,
  Indent = true,
  IndentChars = " ",
  NewLineChars = "\n",
  NewLineOnAttributes = true
};

using (var writer = XmlWriter.Create(Console.Out, settings)) {
  doc.Save(writer);
}

Console.WriteLine();

Note: The same problem occurs with XDocument created from XDocument.Load.

var reader = new StringReader(@"<?xml version=""1.0"" encoding=""utf-8""?>
<svg
 version=""1.1""
 xmlns=""http://www.w3.org/2000/svg""
 width=""10""
 xmlns:xlink=""http://www.w3.org/1999/xlink""
 height=""10""
>
 <g />
</svg>
");
var doc = XDocument.Load(reader);

Expected behavior

<?xml version="1.0" encoding="utf-8"?>
<svg
 version="1.1"
 width="10"
 xmlns:xlink="http://www.w3.org/1999/xlink"
 height="10"
 xmlns="http://www.w3.org/2000/svg">
 <g />
</svg>

Actual behavior

<?xml version="1.0" encoding="utf-8"?>
<svg
 version="1.1"
 width="10" xmlns:xlink="http://www.w3.org/1999/xlink"
 height="10" xmlns="http://www.w3.org/2000/svg">
 <g />
</svg>

Regression?

No response

Known Workarounds

No response

Configuration

$ dotnet --info
.NET SDK (reflecting any global.json):
 Version:   6.0.400
 Commit:    7771abd614

Runtime Environment:
 OS Name:     ubuntu
 OS Version:  20.04
 OS Platform: Linux
 RID:         ubuntu.20.04-x64
 Base Path:   /usr/share/dotnet/sdk/6.0.400/

global.json file:
  Not found

Host:
  Version:      6.0.8
  Architecture: x64
  Commit:       55fb7ef977

*snip*

Other information

No response

Author:	smdn
Assignees:	-
Labels:	area-System.Xml
Milestone:	-

[krwq]

Hey @smdn, it does looks like a bug although since this behavior hasn't changed probably for at least 10 years we probably won't pick this up. I'll recommend to create a PR if you're interested in this. I'd also recommend starting out testing directly with XmlWriter since it's most likely where this originates from.

[smdn]

it does looks like a bug although since this behavior hasn't changed probably for at least 10 years we probably won't pick this up.

@krwq, thank you for telling me the background.

For now, I will try to handle this by creating custom XmlWriter or something.
After that, I might try to fix this.

Thanks!