Using a custom type discriminator to tell JSON.net which type of a class hierarchy to deserialize
In case you are still looking, here is an example: http://james.newtonking.com/archive/2011/11/19/json-net-4-0-release-4-bug-fixes.aspx
This will allow you to create a table based mapping:
public class TypeNameSerializationBinder : SerializationBinder
{
public TypeNameSerializationBinder(Dictionary<Type, string> typeNames = null)
{
if (typeNames != null)
{
foreach (var typeName in typeNames)
{
Map(typeName.Key, typeName.Value);
}
}
}
readonly Dictionary<Type, string> typeToName = new Dictionary<Type, string>();
readonly Dictionary<string, Type> nameToType = new Dictionary<string, Type>(StringComparer.OrdinalIgnoreCase);
public void Map(Type type, string name)
{
this.typeToName.Add(type, name);
this.nameToType.Add(name, type);
}
public override void BindToName(Type serializedType, out string assemblyName, out string typeName)
{
var name = typeToName.Get(serializedType);
if (name != null)
{
assemblyName = null;
typeName = name;
}
else
{
assemblyName = serializedType.Assembly.FullName;
typeName = serializedType.FullName;
}
}
public override Type BindToType(string assemblyName, string typeName)
{
if (assemblyName == null)
{
var type = this.nameToType.Get(typeName);
if (type != null)
{
return type;
}
}
return Type.GetType(string.Format("{0}, {1}", typeName, assemblyName), true);
}
}
The code has a slight defect in that if a type name mapping is attempted where the type is unique but the name is already used, the Map method will throw an exception after the type-to-name mapping is already added leaving the table in an inconsistent state.
To take eulerfx's answer further; I wanted to apply DisplayName attribute to a class and have that automatically become the type name used; to that end:
public class DisplayNameSerializationBinder : DefaultSerializationBinder
{
private Dictionary<string, Type> _nameToType;
private Dictionary<Type, string> _typeToName;
public DisplayNameSerializationBinder()
{
var customDisplayNameTypes =
this.GetType()
.Assembly
//concat with references if desired
.GetTypes()
.Where(x => x
.GetCustomAttributes(false)
.Any(y => y is DisplayNameAttribute));
_nameToType = customDisplayNameTypes.ToDictionary(
t => t.GetCustomAttributes(false).OfType<DisplayNameAttribute>().First().DisplayName,
t => t);
_typeToName = _nameToType.ToDictionary(
t => t.Value,
t => t.Key);
}
public override void BindToName(Type serializedType, out string assemblyName, out string typeName)
{
if (false == _typeToName.ContainsKey(serializedType))
{
base.BindToName(serializedType, out assemblyName, out typeName);
return;
}
var name = _typeToName[serializedType];
assemblyName = null;
typeName = name;
}
public override Type BindToType(string assemblyName, string typeName)
{
if (_nameToType.ContainsKey(typeName))
return _nameToType[typeName];
return base.BindToType(assemblyName, typeName);
}
}
and usage example:
public class Parameter
{
public string Name { get; set; }
};
[DisplayName("bool")]
public class BooleanParameter : Parameter
{
}
[DisplayName("string")]
public class StringParameter : Parameter
{
public int MinLength { get; set; }
public int MaxLength { get; set; }
}
[DisplayName("number")]
public class NumberParameter : Parameter
{
public double Min { get; set; }
public double Max { get; set; }
public string Unit { get; set; }
}
[DisplayName("enum")]
public class EnumParameter : Parameter
{
public string[] Values { get; set; }
}
internal class Program
{
private static void Main(string[] args)
{
var parameters = new Parameter[]
{
new BooleanParameter() {Name = "alive"},
new StringParameter() {Name = "name", MinLength = 0, MaxLength = 10},
new NumberParameter() {Name = "age", Min = 0, Max = 120},
new EnumParameter() {Name = "status", Values = new[] {"Single", "Married"}}
};
JsonConvert.DefaultSettings = () => new JsonSerializerSettings
{
Binder = new DisplayNameSerializationBinder(),
TypeNameHandling = TypeNameHandling.Auto,
NullValueHandling = NullValueHandling.Ignore,
DefaultValueHandling = DefaultValueHandling.Ignore,
Formatting = Formatting.Indented,
ContractResolver = new CamelCasePropertyNamesContractResolver()
};
var json = JsonConvert.SerializeObject(parameters);
var loadedParams = JsonConvert.DeserializeObject<Parameter[]>(json);
Console.WriteLine(JsonConvert.SerializeObject(loadedParams));
}
}
output:
[
{
"$type": "bool",
"name": "alive"
},
{
"$type": "string",
"maxLength": 10,
"name": "name"
},
{
"$type": "number",
"max": 120.0,
"name": "age"
},
{
"$type": "enum",
"values": [
"Single",
"Married"
],
"name": "status"
}
]
I've written purely declarative solution with ability to specify custom discriminator field, and provide scoped name handling per base class (as opposed to usecure global JsonSerializationSettings, especially on different Web-Api when we do not have ability to specify custom JsonSerializationSettings).
using System;
using Newtonsoft.Json;
using Newtonsoft.Json.Linq;
using System.Reflection;
using System.Linq;
using System.Collections.Generic;
// Discriminated Json Converter (JsonSubtypes) implementation for .NET
//
// MIT License
//
// Copyright (c) 2016 Anatoly Ressin
// Permission is hereby granted, free of charge, to any person obtaining a
// copy of this software and associated documentation files (the "Software"),
// to deal in the Software without restriction, including without limitation
// the rights to use, copy, modify, merge, publish, distribute, sublicense,
// and/or sell copies of the Software, and to permit persons to whom the
// Software is furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
// FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
// DEALINGS IN THE SOFTWARE.
////////////////////// USAGE ////////////////////////////////////////////////////////////////////////////////
[JsonConverter(typeof(JsonSubtypes))] // Discriminated base class SHOULD NOT be abstract
public class ShapeBase {
[JsonTag, JsonProperty("@type")] // it SHOULD contain a property marked with [JsonTag]
public string Type {get;set;} // only one [JsonTag] annotation allowed per discriminated class
// it COULD contain other properties, however this is NOT RECOMMENDED
// Rationale: instances of this class will be created at deserialization
// only for tag sniffing, and then thrown away.
}
public abstract class Shape: ShapeBase { // If you want abstract parent - extend the root
public abstract double GetArea(); // with needed abstract stuff, then use this class everywhere (see DEMO below)
}
[JsonSubtype("circle")] // Every final class-case SHOULD be marked with [JsonSubtype(tagValue)]
public class Circle: Shape { // Two disctinct variant classes MUST have distinct tagValues
[JsonProperty("super-radius")] // You CAN use any Json-related annotation as well
public double Radius { get; set; }
public override double GetArea() {
return Radius * Radius * Math.PI;
}
}
[JsonSubtype("rectangle")]
public class Rectangle: Shape {
public double Height { get; set; }
public double Width { get; set; }
public override double GetArea() {
return Width * Height;
}
}
[JsonSubtype("group")]
public class Group: Shape {
[JsonProperty("shapes")]
public List<Shape> Items { get; set; }
public override double GetArea() {
return Items.Select(item => item.GetArea()).Sum();
}
}
// Every final class-case SHOULD be registered with JsonSubtypes.register(typeof(YourConcreteClass))
// either manually or with auto-register capability:
// You can auto-register all classes marked with [JsonSubtype(tag)] in given Assembly
// using JsonSubtypes.autoRegister(yourAssembly)
////////////////// DEMO /////////////////////////////////////////////////////////////////////////////////
public class Program
{
public static void Main()
{
JsonSubtypes.autoRegister(Assembly.GetExecutingAssembly());
Shape original = new Group() {
Items = new List<Shape> {
new Circle() { Radius = 5 },
new Rectangle() { Height = 10, Width = 20 }
}
};
string str = JsonConvert.SerializeObject(original);
Console.WriteLine(str);
var copy = JsonConvert.DeserializeObject(str,typeof(Shape)) as Shape;
// Note: we can deserialize object using any class from the hierarchy.
// Under the hood, anyway, it will be deserialized using the top-most
// base class annotated with [JsonConverter(typeof(JsonSubtypes))].
// Thus, only soft-casts ("as"-style) are safe here.
Console.WriteLine("original.area = {0}, copy.area = {1}", original.GetArea(), copy.GetArea());
}
}
//////////////////////// IMPLEMENTATION //////////////////////////////////////////////////////////////////
public class JsonSubtypeClashException: Exception {
public string TagValue { get; private set;}
public Type RootType { get; private set; }
public Type OldType { get; private set; }
public Type NewType { get; private set; }
public JsonSubtypeClashException(Type rootType, string tagValue, Type oldType, Type newType): base(
String.Format(
"JsonSubtype Clash for {0}[tag={1}]: oldType = {2}, newType = {3}",
rootType.FullName,
tagValue,
oldType.FullName,
newType.FullName
)
) {
TagValue = tagValue;
RootType = rootType;
OldType = oldType;
NewType = newType;
}
}
public class JsonSubtypeNoRootException: Exception {
public Type SubType { get; private set; }
public JsonSubtypeNoRootException(Type subType): base(
String.Format(
"{0} should be inherited from the class with the [JsonConverter(typeof(JsonSubtypes))] attribute",
subType.FullName
)
) {
SubType = subType;
}
}
public class JsonSubtypeNoTagException: Exception {
public Type SubType { get; private set; }
public JsonSubtypeNoTagException(Type subType): base(
String.Format(
@"{0} should have [JsonSubtype(""..."")] attribute",
subType.FullName
)
) {
SubType = subType;
}
}
public class JsonSubtypeNotRegisteredException: Exception {
public Type Root { get; private set; }
public string TagValue { get; private set; }
public JsonSubtypeNotRegisteredException(Type root, string tagValue): base(
String.Format(
@"Unknown tag={1} for class {0}",
root.FullName,
tagValue
)
) {
Root = root;
TagValue = tagValue;
}
}
[AttributeUsage(AttributeTargets.Class)]
public class JsonSubtypeAttribute: Attribute {
private string tagValue;
public JsonSubtypeAttribute(string tagValue) {
this.tagValue = tagValue;
}
public string TagValue {
get {
return tagValue;
}
}
}
public static class JsonSubtypesExtension {
public static bool TryGetAttribute<T>(this Type t, out T attribute) where T: Attribute {
attribute = t.GetCustomAttributes(typeof(T), false).Cast<T>().FirstOrDefault();
return attribute != null;
}
private static Dictionary<Type, PropertyInfo> tagProperties = new Dictionary<Type, PropertyInfo>();
public static bool TryGetTagProperty(this Type t, out PropertyInfo tagProperty) {
if (!tagProperties.TryGetValue(t, out tagProperty)) {
JsonConverterAttribute conv;
if (t.TryGetAttribute(out conv) && conv.ConverterType == typeof(JsonSubtypes)) {
var props = (from prop in t.GetProperties() where prop.GetCustomAttribute(typeof(JsonTagAttribute)) != null select prop).ToArray();
if (props.Length == 0) throw new Exception("No tag");
if (props.Length > 1) throw new Exception("Multiple tags");
tagProperty = props[0];
} else {
tagProperty = null;
}
tagProperties[t] = tagProperty;
}
return tagProperty != null;
}
public static bool TryGetTagValue(this Type t, out string tagValue) {
JsonSubtypeAttribute subtype;
if (t.TryGetAttribute(out subtype)) {
tagValue = subtype.TagValue;
return true;
} else {
tagValue = null;
return false;
}
}
public static bool TryGetJsonRoot(this Type t, out Type root, out PropertyInfo tagProperty) {
root = t;
do {
if (root.TryGetTagProperty(out tagProperty)) {
return true;
}
root = root.BaseType;
} while (t != null);
return false;
}
}
public class JsonTagAttribute: Attribute {
}
public class JsonTagInfo {
public PropertyInfo Property { get; set; }
public string Value { get; set; }
}
public class JsonRootInfo {
public PropertyInfo Property { get; set; }
public Type Root { get; set; }
}
public abstract class DefaultJsonConverter: JsonConverter {
[ThreadStatic]
private static bool silentWrite;
[ThreadStatic]
private static bool silentRead;
public sealed override bool CanWrite {
get {
var canWrite = !silentWrite;
silentWrite = false;
return canWrite;
}
}
public sealed override bool CanRead {
get {
var canRead = !silentRead;
silentRead = false;
return canRead;
}
}
protected void _WriteJson(JsonWriter writer, Object value, JsonSerializer serializer) {
silentWrite = true;
serializer.Serialize(writer, value);
}
protected Object _ReadJson(JsonReader reader, Type objectType, Object existingValue, JsonSerializer serializer) {
silentRead = true;
return serializer.Deserialize(reader, objectType);
}
}
public class JsonSubtypes: DefaultJsonConverter {
private static Dictionary<Type, Dictionary<string, Type>> implementations = new Dictionary<Type, Dictionary<string, Type>>();
private static Dictionary<Type, JsonTagInfo> tags = new Dictionary<Type, JsonTagInfo>();
private static Dictionary<Type, JsonRootInfo> roots = new Dictionary<Type, JsonRootInfo>();
public static void register(Type newType) {
PropertyInfo tagProperty;
Type root;
if (newType.TryGetJsonRoot(out root, out tagProperty)) {
for(var t = newType; t != root; t = t.BaseType) {
roots[t] = new JsonRootInfo() {
Property = tagProperty,
Root = root
};
}
roots[root] = new JsonRootInfo() {
Property = tagProperty,
Root = root
};
Dictionary<string, Type> implementationMap;
if (!implementations.TryGetValue(root, out implementationMap)) {
implementationMap = new Dictionary<string, Type>();
implementations[root] = implementationMap;
}
JsonSubtypeAttribute attr;
if (!newType.TryGetAttribute(out attr)) {
throw new JsonSubtypeNoTagException(newType);
}
var tagValue = attr.TagValue;
Type oldType;
if (implementationMap.TryGetValue(tagValue, out oldType)) {
throw new JsonSubtypeClashException(root, tagValue, oldType, newType);
}
implementationMap[tagValue] = newType;
tags[newType] = new JsonTagInfo() {
Property = tagProperty,
Value = tagValue
};
} else {
throw new JsonSubtypeNoRootException(newType);
}
}
public static void autoRegister(Assembly assembly) {
foreach(var type in assembly.GetTypes().Where(type => type.GetCustomAttribute<JsonSubtypeAttribute>() != null)) {
register(type);
}
}
public override bool CanConvert(Type t) {
return true;
}
public static T EnsureTag<T>(T value) {
JsonTagInfo tagInfo;
if (tags.TryGetValue(value.GetType(), out tagInfo)) {
tagInfo.Property.SetValue(value, tagInfo.Value);
}
return value;
}
public override void WriteJson(JsonWriter writer, Object value, JsonSerializer serializer) {
_WriteJson(writer, EnsureTag(value), serializer);
}
public override Object ReadJson(JsonReader reader, Type objectType, Object existingValue, JsonSerializer serializer) {
JsonTagInfo tagInfo;
if (tags.TryGetValue(objectType, out tagInfo)) {
return _ReadJson(reader, objectType, existingValue, serializer);
} else {
JsonRootInfo rootInfo;
if (roots.TryGetValue(objectType, out rootInfo)) {
JToken t = JToken.ReadFrom(reader);
var stub = _ReadJson(t.CreateReader(), rootInfo.Root, existingValue, serializer);
var tagValue = rootInfo.Property.GetValue(stub) as string;
var implementationMap = implementations[rootInfo.Root];
Type implementation;
if (implementationMap.TryGetValue(tagValue, out implementation)) {
return ReadJson(t.CreateReader(), implementation, null, serializer);
} else {
throw new JsonSubtypeNotRegisteredException(rootInfo.Root, tagValue);
}
} else {
return _ReadJson(reader, objectType, existingValue, serializer);
}
}
}
public static T Deserialize<T>(string s) where T: class {
return JsonConvert.DeserializeObject(s, typeof(T)) as T;
}
public static string Serialize<T>(T value) where T: class {
return JsonConvert.SerializeObject(value);
}
}
output:
{"shapes":[{"super-radius":5.0,"@type":"circle"},{"Height":10.0,"Width":20.0,"@type":"rectangle"}],"@type":"group"}
original.area = 278.539816339745, copy.area = 278.539816339745
You can grab it here:
https://dotnetfiddle.net/ELcvnk