Json.NET: Converting JSON with $ element to XML - json.net

Question

Json.NET seems to handle converting many names which would be invalid XML correctly, for example:
JsonConvert.DeserializeXmlNode("{"name!1": "test"}").OuterXml
Results in:
<name_x0021_1>test</name_x0021_1>
But attempting to convert the following, which passes JSONLint:
{"$": "test"}
Results in
Result Message: System.Xml.XmlException : The ':' character, hexadecimal value 0x3A, cannot be included in a name.
This error message in itself seems confusing as it suggests one of the names in the JSON has a : character. There might be a good reason for this but is there a way to get this to convert to XML without throwing an exception as some APIs seem to return the "$": "...." pair.

Answer 1

Update
This is fixed in Json.NET 9.0.1 in commit b71ca75.
Original Answer
This may be a bug in Json.NET's XmlNodeConverter.ReadElement(). Json.NET has several reserved property names, all of which begin with $, shown here:
public const string IdPropertyName = "$id";
public const string RefPropertyName = "$ref";
public const string TypePropertyName = "$type";
public const string ValuePropertyName = "$value";
public const string ArrayValuesPropertyName = "$values";
Then, in ReadElement(), there is special handling for property names that begin with a $:
private void ReadElement(JsonReader reader, IXmlDocument document, IXmlNode currentNode, string propertyName, XmlNamespaceManager manager)
{
if (string.IsNullOrEmpty(propertyName))
{
throw JsonSerializationException.Create(reader, "XmlNodeConverter cannot convert JSON with an empty property name to XML.");
}
Dictionary<string, string> attributeNameValues = ReadAttributeElements(reader, manager);
string elementPrefix = MiscellaneousUtils.GetPrefix(propertyName);
if (propertyName.StartsWith('#'))
{
string attributeName = propertyName.Substring(1);
string attributePrefix = MiscellaneousUtils.GetPrefix(attributeName);
AddAttribute(reader, document, currentNode, attributeName, manager, attributePrefix);
}
else if (propertyName.StartsWith('$'))
{
if (propertyName == JsonTypeReflector.ArrayValuesPropertyName)
{
propertyName = propertyName.Substring(1);
elementPrefix = manager.LookupPrefix(JsonNamespaceUri);
CreateElement(reader, document, currentNode, propertyName, manager, elementPrefix, attributeNameValues);
}
else
{
// Your code throws an exception going down this branch.
string attributeName = propertyName.Substring(1);
string attributePrefix = manager.LookupPrefix(JsonNamespaceUri);
AddAttribute(reader, document, currentNode, attributeName, manager, attributePrefix);
}
}
else
{
CreateElement(reader, document, currentNode, propertyName, manager, elementPrefix, attributeNameValues);
}
}
I suspect the propertyName.StartsWith('$') branch should only be taken for reserved property names, meaning the method should look like:
private void ReadElement(JsonReader reader, IXmlDocument document, IXmlNode currentNode, string propertyName, XmlNamespaceManager manager)
{
if (string.IsNullOrEmpty(propertyName))
{
throw JsonSerializationException.Create(reader, "XmlNodeConverter cannot convert JSON with an empty property name to XML.");
}
Dictionary<string, string> attributeNameValues = ReadAttributeElements(reader, manager);
if (propertyName.StartsWith('#'))
{
string attributeName = propertyName.Substring(1);
string attributePrefix = MiscellaneousUtils.GetPrefix(attributeName);
AddAttribute(reader, document, currentNode, attributeName, manager, attributePrefix);
}
else if (propertyName == JsonTypeReflector.ArrayValuesPropertyName)
{
propertyName = propertyName.Substring(1);
var elementPrefix = manager.LookupPrefix(JsonNamespaceUri);
CreateElement(reader, document, currentNode, propertyName, manager, elementPrefix, attributeNameValues);
}
else if (propertyName == JsonTypeReflector.IdPropertyName
|| propertyName == JsonTypeReflector.RefPropertyName
|| propertyName == JsonTypeReflector.TypePropertyName
|| propertyName == JsonTypeReflector.ValuePropertyName)
{
string attributeName = propertyName.Substring(1);
string attributePrefix = manager.LookupPrefix(JsonNamespaceUri);
AddAttribute(reader, document, currentNode, attributeName, manager, attributePrefix);
}
else
{
var elementPrefix = MiscellaneousUtils.GetPrefix(propertyName);
CreateElement(reader, document, currentNode, propertyName, manager, elementPrefix, attributeNameValues);
}
}
(Honestly, the handling of "$value" looks suspect also, since the value of a polymorphic property could be any JSON object.)
You might want to report an issue.
In the meantime, you could load your JSON into a JToken, manually remap names beginning with $, then convert to an XmlDocument like so:
var token = JToken.Parse(json);
token.RenameReplaceProperties(s => (s.StartsWith("$") && !JsonExtensions.IsReserved(s) ? XmlConvert.EncodeName(s) : s));
var xml = token.ToXmlNode();
Using the extension methods:
public static class JsonExtensions
{
const string IdPropertyName = "$id";
const string RefPropertyName = "$ref";
const string TypePropertyName = "$type";
const string ValuePropertyName = "$value";
const string ArrayValuesPropertyName = "$values";
public static bool IsReserved(string s)
{
return s == IdPropertyName || s == RefPropertyName || s == TypePropertyName || s == ValuePropertyName || s == ArrayValuesPropertyName;
}
public static IEnumerable<JToken> DescendantsAndSelf(this JToken root)
{
var container = root as JContainer;
if (container != null)
return container.DescendantsAndSelf();
else if (root != null)
return new[] { root };
else
return Enumerable.Empty<JToken>();
}
public static JProperty RenameReplace(this JProperty property, string name)
{
if (property == null || name == null)
throw new ArgumentNullException();
var value = property.Value;
property.Value = null;
var newProperty = new JProperty(name, value);
property.Replace(newProperty);
return newProperty;
}
public static JToken RenameReplaceProperties(this JToken root, Func<string, string> map)
{
var query = from property in root.DescendantsAndSelf().OfType<JProperty>()
let name = map(property.Name)
where name != property.Name && name != null
select new KeyValuePair<JProperty, string>(property, name);
foreach (var pair in query.ToList())
{
var newProperty = pair.Key.RenameReplace(pair.Value);
if (pair.Key == root)
root = newProperty;
}
return root;
}
public static XmlDocument ToXmlNode(this JToken root)
{
using (var reader = root.CreateReader())
return DeserializeXmlNode(reader);
}
public static XmlDocument DeserializeXmlNode(JsonReader reader)
{
return DeserializeXmlNode(reader, null, false);
}
public static XmlDocument DeserializeXmlNode(JsonReader reader, string deserializeRootElementName, bool writeArrayAttribute)
{
var converter = new Newtonsoft.Json.Converters.XmlNodeConverter() { DeserializeRootElementName = deserializeRootElementName, WriteArrayAttribute = writeArrayAttribute };
var jsonSerializer = JsonSerializer.CreateDefault(new JsonSerializerSettings { Converters = new JsonConverter[] { converter } });
return (XmlDocument)jsonSerializer.Deserialize(reader, typeof(XmlDocument));
}
}