I am trying to implement a custom JsonConverter for a struct, but I'm having a hard time getting it to work. I have previously implemented a custom converter for another class and that one works flawlessly, I thought I did the same thing for this one but the ReadJson method of my converter is never called.
This is the class:
public class TransformMatrixConverter : JsonConverter
{
public override bool CanConvert(Type objectType)
{
return typeof(TransformMatrix).IsAssignableFrom(objectType);
}
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
throw new NotImplementedException();
}
public override bool CanWrite
{
get { return false; }
}
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
throw new NotImplementedException();
}
}
And below is how I use it in my Item class. You can see my other converter, which works fine.
public static Item FromJSON(string json)
{
JsonConverter[] converters = { new LineConverter(), new TransformMatrixConverter() };
return JsonConvert.DeserializeObject<Item>(json, new JsonSerializerSettings()
{
Converters = converters
});
}
What happens: the CanConvert method of my converter is called and correctly returns true when appropriate; however, the ReadJson method is never hit, I have a breakpoint there and also that exception is never thrown. I have verified that the CanRead property of the converter is true. I am at a loss here, any ideas?
I think I solved it with the help of Brian, the problem was unrelated to the code above - the TransformMatrix I thought I was deserializing was just a read-only property. The solution I used was to expose another property which the deserializer can write.
Related
I need to apply a custom converter conditionally based on the depth of the reader. The root of the json object is a Def class that should deserialize like normal, however any Defs within the object should be resolved to a reference to that deserialized Def. My plan is to check the depth of the reader, and if we're not at the root, then create a skeleton Def and add it to a list to be resolved later once we've deserialized all the Defs.
public class DefConverter : JsonConverter {
public override bool CanConvert(Type objectType) {
return objectType == typeof(Def);
}
public override object? ReadJson(JsonReader reader, Type objectType, object? existingValue, JsonSerializer serializer) {
if (reader.Depth == 0) {
// Use the default serializer to read the Def
return serializer.Deserialize(reader, objectType); // ?
}
// Create skeleton Def with id
// Add to list of defs to be resolved later
}
public override bool CanWrite => false;
public override void WriteJson(JsonWriter writer, object? value, JsonSerializer serializer) {
throw new NotImplementedException();
}
}
The issue I'm running into is that there doesn't seem to be a way to call the Json.NET default converter, using serializer.Deserialize(reader, objectType) will just cause an infinite loop as it just calls the custom converter.
I managed to get it working using the solution here:
Json.NET Recursive Serialisation: Custom converter attribute seems to be being ignored
Toggling the converter off and on using the CanRead getter
For API purposes I need to ignore some fields based on criteria I receive. Usually I could use [ScriptIgnore] attribute to do this.
But how I can ignore fields dynamically (based on some conditionals)?
Use JsonIgnore attribute available in Newtonsoft.Json package.
then, if you want it to be dynamically conditional, see ShouldSerialize
Assuming you use Json.Net, you can create your own converter for a specific type by creating a class that inherits from JsonConverter.
public class MyJsonConverter : JsonConverter
{
public override bool CanConvert(Type objectType)
{
return objectType == typeof(MyType);
}
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
throw new NotImplementedException();
}
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
var objectToSerialize = new {}; //create the object you want to serialize here, based on your dynamic conditions
new JsonSerializer().Serialize(writer, objectToSerialize); //serialize the object to the current writer
}
}
Then you call JsonConvert.DeserializeObject and pass it your custom converter:
JsonConvert.DeserializeObject<MyType>(jsonString, new MyJsonConverter());
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());
Why my custom JsonConverter.WriteJson() method doesn't get called ?
class MyType{
[JsonConverter(typeof(DocumentXamlDeserializer))]
public string GuiData { get; set; }
public string SimpleString;
}
Although the ReadJson does get called:
public class DocumentXamlDeserializer : JsonConverter
{
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
writer.WriteValue(Gui.Handler.SerializeData());
}
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
var xaml = reader.Value as string;
Gui.Handler.DeserializeData(xaml);
return xaml;
}
public override bool CanConvert(Type objectType) { return typeof(string).IsAssignableFrom(objectType); }
}
The serialization call: JsonConvert.SerializeObject(dataModel, Formatting.Indented);
The deserialization call: JsonConvert.DeserializeObject<Model>(raw);
Apparently this is because GuiData is null... I guess I could specify:
TypeNameHandling = TypeNameHandling.Objects
But I want to serialize only GuiData even if it's null (I set its value during serialization), without serializing all null-properties... well, if I don't find better way, I guess I'd have to suffice with it...
I notice there are some other results on stackoverflow for this question but they don't seem to work or are vague. Using the most popular result, I have put together this:
The problem is that when JSON comes back and is being serialised into one of my custom types, one of the bits of JSON is sometimes an array, and sometimes just a string. If my custom type has a string, and the JSON is an array, I get an error. The same happens the other way around, if the JSON is an object and my custom type is an array. It just can't map it to the property.
I decided to solve this, I want to override the deserialisation of this particular property, and if it's an object, I want to convert it into an array of 1 object.
In the object I am serialising to, I added a JsonConverter which I think is going to override the way it's deserialised.
[JsonConverter(typeof(ArrayOrSingleObjectConverter<string>))]
public List<string> person { get; set; }
The idea is that the custom converter will convert a single object to an array. So if the JSON is "Hello" it will set person to be a List containing "Hello" instead of throwing an exception saying cannot convert string to List.
If it's already an array, it should just leave it alone.
The converter looks like this:
public class ArrayOrSingleObjectConverter<T> : JsonConverter
{
public override bool CanConvert(Type objectType)
{
return true; // Not sure about this but technically it can accept an array or an object, so everything is game.
}
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
if (objectType == typeof(List<T>))
{
return serializer.Deserialize<List<T>>(reader);
}
else
{
var singleObject = serializer.Deserialize<T>(reader);
var objectAsList = new List<T>();
objectAsList.Add(singleObject);
return objectAsList;
}
}
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
throw new NotImplementedException();
}
}
It doesn't work. The above code throws an exception trying to deserialize a a single string saying it can't cast it into a List inside the if statement (the 'objectype' is however a List).
I'm struggling to understand exactly what the read and write methods are doing. In the other answer on stackoverflow, it suggests throwing a NotImplementedException in the read method. But if I do that, the read method is called and the exception throws.
I think I'm on the right track, but I need a nudge in the right direction. I think I'm a little confused about what the ReadJSon method is doing and what its parameters mean.
I don't really understand where the value is coming from that it's deserializing since I didn't specify it in the Deserialize method call.
I'm a bit out of my depth on this one.
I had to do something similar last week and I came up with the following, which works fine for a List rather than an array
internal class GenericListCreationJsonConverter<T> : JsonConverter
{
public override bool CanConvert(Type objectType)
{
return true;
}
public override bool CanRead
{
get { return true; }
}
public override bool CanWrite
{
get { return false; }
}
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
if (reader.TokenType == JsonToken.StartArray)
{
return serializer.Deserialize<List<T>>(reader);
}
else
{
T t = serializer.Deserialize<T>(reader);
return new List<T>(new[] { t });
}
}
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
throw new NotImplementedException();
}
I like this method which makes Json.NET do all the heavy lifting. And as a result, it supports anything that Json.NET supports (List<>, ArrayList, strongly-typed arrays, etc).
public class FlexibleCollectionConverter : JsonConverter
{
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
serializer.Serialize(writer, value);
}
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
if (reader.TokenType == JsonToken.StartArray)
{
return serializer.Deserialize(reader, objectType);
}
var array = new JArray(JToken.ReadFrom(reader));
return array.ToObject(objectType);
}
public override bool CanConvert(Type objectType)
{
return typeof (IEnumerable).IsAssignableFrom(objectType);
}
}