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Only run JsonConverter on string tokens [duplicate]

This question already has answers here:
Best way to upgrade JSON field to a class structure
(1 answer)
Json.NET custom serialization with JsonConverter - how to get the "default" behavior
(1 answer)
Closed 1 year ago.
I'm trying to make a "reference" system, where JSON files can depend and reference other files. I know JSON.net has a built-in system like this but it doesn't fit my use case.
I implemented it like this:
public class ReferenceJsonConverter : JsonConverter<NamedType>
{
private readonly ContentLoader _contentLoader;
public ReferenceJsonConverter(ContentLoader contentLoader)
{
_contentLoader = contentLoader;
}
public override bool CanWrite => false;
public override void WriteJson(JsonWriter writer, NamedType value, JsonSerializer serializer)
{
throw new NotImplementedException();
}
public override NamedType ReadJson(JsonReader reader, Type objectType, NamedType existingValue, bool hasExistingValue,
JsonSerializer serializer)
{
switch (reader.TokenType)
{
case JsonToken.String:
{
string value = reader.Value!.ToString().ToLower();
return _contentLoader.Load<NamedType>(value);
}
default:
{
NamedType namedType = (NamedType?) serializer.Deserialize(reader, objectType);
return namedType;
}
}
}
}
So pretty simple, if the token is a string it gets it from memory instead of deserializing it. The problem is, since the token type is the same as the parent type, this will loop infinitely because it will keep trying to use ReferenceJsonConverter to deserialize non-strings.
I've tried the following:
Re-implement CanConvert to try and "skip" StartObject tokens (janky, didn't work)
Have a different JsonConverter do the StartObject, then deserialize using ReferenceJsonConverter on string tokens (loops infinitely because I can't "skip" the converter)
Re-implement the JObject convert method (wayy too much work, will break on updates, and it's all internal anyways)
For clarity, it would look like this:
{ <--- Serialized NamedType, should use default deserializer
"Name": "Test", <-- Field of NamedType, should use default deserializer
"ReferenceOther": "ReferencedType" <--- Referenced NamedType, should use ReferenceJsonConverter
}

Can I parse json either into a string or another concrete type as object?

I'd like to have property of type object that can be either a string or Template type.
Is it possible to tell Json.NET to parse something into one of several specified types?
class MyClass
{
public object Template { get; set; }
}
where Template = "TemplateName"
{
"Template": "TemplateName"
}
or Template = new Template()
{
"Template": { "Name": "OtherTamplate", ... }
}
UPDATE:
I tried to follow #krillgar' advice and create a custom JsonConverter but unfortunatelly the CanConvert method receives only the target type, in this case object. This information is not enough to tell wheter it can be deserialized (in case I had other object properties). I guess I need it to be a Template after all or create a derived type like TemplateReference or something:
class myconverter : JsonConverter
{
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
throw new NotImplementedException();
}
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
throw new NotImplementedException();
}
public override bool CanConvert(Type objectType)
{
// objectType = typeof(object)
throw new NotImplementedException();
}
}
Configuration = JsonConvert.DeserializeObject<MyClass>(text, new myconverter());
Disclaimer
This question has once been closed as a duplicate of How to deserialize a JSON property that can be two different data types using Json.NET. Because at the time of writing my question I hadn't known that there already was a similar one I'd like to clarify the difference between them to prevent it from being closed in future:
The other question is about how to deserialize different values into a concrete type whereas mine is about deserializing different values into an object. It might seem to be the same at the first look because in both examples only the type of the property is different but it has a huge impact on the overall application design. It's important for me that I can use an object to store different specialized types rather then one type having multiple responsibilities.

This problem can be solved by using a custom JsonConverter. Here is a generic version that should work for this situation:
class ObjectOrStringConverter<T> : JsonConverter
{
public override bool CanConvert(Type objectType)
{
// CanConvert is not called when the [JsonConverter] attribute is used
return false;
}
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
JToken token = JToken.Load(reader);
if (token.Type == JTokenType.Object)
{
return token.ToObject<T>(serializer);
}
return token.ToString();
}
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
serializer.Serialize(writer, value);
}
}
To use the converter, all you need to do is add a [JsonConverter] attribute to the property in your class that can be either a string or an object. The generic type parameter must match the type of non-string object you are expecting.
class MyClass
{
[JsonConverter(typeof(ObjectOrStringConverter<Template>))]
public object Template { get; set; }
}
Below is a demonstration of the converter in action:
class Program
{
static void Main(string[] args)
{
Console.WriteLine("--- first run ---");
string json = #"
{
""Template"": ""TemplateName""
}";
DeserializeAndDump(json);
Console.WriteLine("--- second run ---");
json = #"
{
""Template"": { ""Name"": ""OtherTemplate"" }
}";
DeserializeAndDump(json);
}
static void DeserializeAndDump(string json)
{
MyClass obj = JsonConvert.DeserializeObject<MyClass>(json);
if (obj.Template == null)
{
Console.WriteLine("Template property is null");
}
else
{
Console.WriteLine("Template property is a " + obj.Template.GetType().Name);
string name = "(unknown)";
if (obj.Template is Template) name = ((Template)obj.Template).Name;
else if (obj.Template is string) name = (string)obj.Template;
Console.WriteLine("Template name is \"" + name + "\"");
}
Console.WriteLine();
}
}
class Template
{
public string Name { get; set; }
}
And here is the output from the above:
--- first run ---
Template property is a String
Template name is "TemplateName"
--- second run ---
Template property is a Template
Template name is "OtherTemplate"
Fiddle: https://dotnetfiddle.net/Lw3RaN

I don't know if you can do that, but you could go another way. Change your "Template" property to be a Template instead of an object and use a custom property of the Template class to know weither you want to serialize it as a Template or a string.
class MyClass
{
[JsonConverter(typeof(TemplateConverter))]
public Template Template { get; set; }
}
class Template
{
/* Your Template class */
public Type TypeToSerializeInto { get; private set; }
}
public class TemplateConverter : JsonConverter
{
public override bool CanConvert(Type objectType)
{
return objectType == typeof(Template);
}
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer) { }
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
Template val = value as Template;
writer.WriteStartObject();
writer.WritePropertyName("Template");
if (val.TypeToSerializeInto == typeof(Template))
serializer.Serialize(writer, val);
else if (val.TypeToSerializeInto == typeof(string))
serializer.Serialize(writer, val.Name);
writer.WriteEndObject();
}
}

If you have a property, which type is an abstract type - like object, on de-serialization, you can know the specific type that was serialized by serializing somewhere also the name of that specific type.
So your json should look like this:
{
"MyClass": {
"Template": "some name",
"type": "System.String"
}
}
This way on deserialization you can check what type was that property before the serialization (in this case String)
Another way to determine the type is by checking the json structure as you can see here:
C#: Deserializing JSON when one field can be different types

Creating a custom JsonConverter to handle System.Text.Encoding objects

I have written a custom JsonConverter which I am hoping will allow me to serialize and deserialize Encoding objects within my classes:
public class EncodingConverter : JsonConverter
{
public override bool CanConvert(Type objectType)
{
return objectType.IsSubclassOf(typeof(Encoding));
}
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
writer.WriteValue(((Encoding)value).EncodingName);
}
public override bool CanRead { get { return true; } }
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
var name = reader.ReadAsString();
return Encoding.GetEncoding(name);
}
}
However, when I run the following test code, I get an exception when calling DeserializeObject, and the ReadJson method never gets called.
class Program
{
private static void Main(string[] args)
{
var test = new TestClass();
var jsonSettings = new JsonSerializerSettings
{
Converters = new[] { new EncodingConverter(), }
};
var json = JsonConvert.SerializeObject(test, jsonSettings);
var test2 = JsonConvert.DeserializeObject<TestClass>(json, jsonSettings);
}
}
class TestClass
{
public string Property1;
public Encoding Encoding = Encoding.UTF8;
}
The exception message is:
Target type System.Text.Encoding is not a value type or a non-abstract class.
Am I missing something?

There are three problems with your converter that I see.
You are using the wrong check in CanConvert().
You are using the wrong name for the Encoding when serializing.
You are using the wrong method to get the value from the reader when deserializing.
Let's take these one at a time.
First, in your CanConvert method you are using objectType.IsSubclassOf(typeof(Encoding)) to determine whether the converter should handle the Encoding. This works fine on serialization because you have a concrete instance of the encoding (e.g. UTF8Encoding), which is indeed a subclass of Encoding. However, on deserialization, the deserializer doesn't know what concrete type of encoding you are going to make, so the type that is passed to the converter is just Encoding. Since Encoding is not a subclass of itself, CanConvert returns false, and your ReadJson method never gets called. That leaves Json.Net to try to instantiate the Encoding itself, which it can't do (because Encoding is abstract), so it throws the error you mentioned in your question. You should instead use typeof(Encoding).IsAssignableFrom(objectType) inside your CanConvert method.
Second, when serializing the Encoding inside WriteJson, you are outputting the EncodingName property, which is the human-readable display name of the encoding, not the code page name. If you look at the documentation for the Encoding.GetEncoding(string) method, it says:
Parameters
name
Type: System.String
The code page name of the preferred encoding. Any value returned by the WebName property is valid. Possible values are listed in the Name column of the table that appears in the Encoding class topic.
So, you should be outputting the value of the WebName property in your WriteJson method if you want to be able to use this value to later reconstruct the Encoding in ReadJson.
Third, in your ReadJson method you are using reader.ReadAsString() to attempt to get the encoding name from the JSON. This will not work as you expect. When ReadJson is called by Json.Net, the reader is already positioned at the current value. When you call ReadAsString(), that advances the reader to the next token and then attempts to interpret that token as a string. What you really want to do is just get the value of the current token, which you can do using the Value property. Because Value is of type object, you will need to cast it to a string.
Here is the corrected code for the converter:
public class EncodingConverter : JsonConverter
{
public override bool CanConvert(Type objectType)
{
return typeof(Encoding).IsAssignableFrom(objectType);
}
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
writer.WriteValue(((Encoding)value).WebName);
}
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
return Encoding.GetEncoding((string)reader.Value);
}
}
Fiddle: https://dotnetfiddle.net/UmLynX

Try:
public class CustomConverter : JsonConverter
{
public override bool CanConvert(System.Type objectType)
{
return true;// objectType.IsAssignableFrom(typeof(Encoding));
}
public override object ReadJson(JsonReader reader, System.Type objectType, object existingValue, JsonSerializer serializer)
{
return Encoding.GetEncoding(Convert.ToString(reader.Value));
}
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
var t = (Test)value;
var e = (Encoding)t.MyProperty;
writer.WriteValue(e.BodyName);
//serializer.Serialize(writer, e.BodyName);
}
}
And in Main:
var o = new Test { MyProperty = Encoding.UTF8 };
var s = new JsonSerializerSettings
{
Converters = new[] { new CustomConverter() }
};
var v = JsonConvert.SerializeObject(o, s);
var o2 = new Test();
o2.MyProperty = Encoding.GetEncoding(JsonConvert.DeserializeObject(v, s).ToString());

JObject ToObject - Crashes on a "bad date" conversions

I'm using a JObject to handle my client post's.
I convert the JObject into a strong type entity using the ToObject function.
When the datetime value isn't valid - let's say 29\05\2014(since there aren't 29 months), I get an exception:
Could not convert string to DateTime: 29/05/2014. Path 'PurchaseDate.Value'.
I understand the exception and I would like to prevent crashes in those kind of situations.
How can I tell the JObject to ignore invalid date values? In my specific case my entity is a nullable datetime object so I would like to keep in null if the parsing fails(rather then crash).
In this specific case I'm talking about a datetime, but if someone can give me a more generic answer on how I can prevent failures on "invalid parsing\conversions" that would be great, since all of my entities contain nullable fields and I don't want to handle validations on the client side.

You cannot disable them just for invalid dates, but you can stop the parsing of date values, storing them as strings and implement a custom parsing later.
jObject.ToObject<MyObject>( new JsonSerializer {
DateParseHandling = DateParseHandling.None
});

I found a work around - Adding a converter:
var js = new JsonSerializer
{
DateParseHandling = DateParseHandling.DateTime,
};
js.Converters.Add(new DateTimeConverter());
dynamic jsonObject = new JObject();
jsonObject.Date = "29/05/2014";
var entty = ((JObject)jsonObject).ToObject<Entity>(js);
Definitions:
public class Entity
{
public DateTime? Date { get; set; }
}
public class DateTimeConverter : DateTimeConverterBase
{
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
DateTime val;
if (reader.Value != null && DateTime.TryParse(reader.Value.ToString(), out val))
{
return val;
}
return null;
}
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
writer.WriteValue(((DateTime)value).ToString("MM/dd/yyyy"));
}
}

Serializing strings containing apostrophes with JSON.Net

I am using JSON.Net as my serializer for a large MVC 3 web application in c# and the Razor view engine. For the initial page load in one view, there is a large amount of JSON dumped inside a script tag using #Html.Raw(JsonConvert.SerializeObject(myObject)).
The problem is that some values of some objects contain apostrophes (think names like O'Brien), which JSON.Net is not escaping or encoding in any way.
It's not an option to pre-encode the values stored in the database because that vastly complicates various other processes.
Is there a way to force JSON.Net to HTML encode the values of the objects that it serializes, the same way that the built-in JavaScriptSerializer does when you call JavaScriptSerializer.Serialize(myObject)? Or, is there a way to deal with this in the view?

JsonSerializerSettings settings = new JsonSerializerSettings
{
StringEscapeHandling = StringEscapeHandling.EscapeHtml
};
JsonConvert.SerializeObject(obj, settings);

Though there are some cases wherein you might want to drop some JSON into your page as a JavaScript string, or an HTML attribute value, most often what you'd do is simply include it directly into JavaScript source, because JSON is valid JavaScript syntax after all.

You can create custom JsonConverter like this:
public class EscapeQuoteConverter : JsonConverter
{
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
writer.WriteValue(value.ToString().Replace("'", "\\'"));
}
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
var value = JToken.Load(reader).Value<string>();
return value.Replace("\\'", "'");
}
public override bool CanConvert(Type objectType)
{
return objectType == typeof(string);
}
}
To use this only for Name property specify it by attribute:
public class Person
{
[JsonConverter(typeof(EscapeQuoteConverter))]
public string Name { get; set; }
}
To apply Converter to all strings use:
JsonConvert.SerializeObject(person, Formatting.Indented, new EscapeQuoteConverter());

Use System.Web.HttpUtility.HtmlEncode
HttpUtility.HtmlEncode(JsonConvert.SerializeObject(myObject))