YamlScalarNode with "NULL" value should be written with surrounding quotes

[AArnott]

When manually writing out a YamlMappingNode with YamlScalarNode("NULL") as a key or value, I get this:

NULL: value
e: NULL

This is apparently a bad write job, given that NULL outside of quotes actually means null to YAML, producing a null mapping node instead of NULL (the string) as a key in the map.

When I paste this into http://yaml-online-parser.appspot.com/, the json representation is (appropriately):

{
  "null": "value", 
  "e": null
}

Note the change in case, because the YAML was read as null instead of as the intended string NULL. JSON has to surround a javascript null with quotes when it appears as a property name, but consider when the null is on the value side.

The emitted YAML should have been:

"NULL": value
e: "NULL"

Note the quotes around the yaml-keyword, which then forces it to be read as a string literal by other YAML parsers:

{
  "NULL": "value", 
  "e": "NULL"
}

Repro steps

using System;
using System.IO;
using YamlDotNet.RepresentationModel;

var mappingNode = new YamlMappingNode(
    new YamlScalarNode("NULL"), new YamlScalarNode("value"),
    new YamlScalarNode("e"), new YamlScalarNode("NULL")
    );
var doc = new YamlDocument(mappingNode);
var stream = new YamlStream(doc);
var sb = new StringWriter();
stream.Save(sb);

Console.WriteLine(sb);

Actual output

&428684954 NULL: value
e: NULL
...

Expected output

&428684954 "NULL": value
e: "NULL"
...

Deserialization fails

This bug becomes a more practical problem because the deserializer fails on null keys:

using System;
using System.Collections.Generic;
using System.IO;
using YamlDotNet.RepresentationModel;

var mappingNode = new YamlMappingNode(
    new YamlScalarNode("NULL"), new YamlScalarNode("V")
    );
var doc = new YamlDocument(mappingNode);
var stream = new YamlStream(doc);
var sb = new StringWriter();
stream.Save(sb);

Console.WriteLine(sb);

var deserializer = new YamlDotNet.Serialization.Deserializer();
var data = deserializer.Deserialize<Dictionary<string, string>>(new StringReader(sb.ToString()));

class Detail
{
    public string H { get; set; }
}

The above throws an exception on deserialization unless I change "NULL" to "NULL2".
If I read YAML that properly quotes the NULL, deserialization succeeds.

[AArnott]

Maybe related to #304

[fireflycons]

I second this, especially where a key name is NULL. I would also like to emit numeric keys quoted.
What I don't see is a way to tap into the serializer with an object that can control how keys are rendered, e.g. some kind of IKeySerializer

[EdwardCooke]

This will be fixed when the next release goes out.