I have a generic type that wraps a single primitive type to give it value equality semantics
public class ValueObject<T>
{
public T Value { get; }
public ValueObject(T value) => Value = value;
// various other equality members etc...
}
It is used like:
public class CustomerId : ValueObject<Guid>
{
public CustomerId(Guid value) : base(value) { }
}
public class EmailAddress : ValueObject<string>
{
public EmailAddress(string value) : base(value) { }
}
The issue is when serializing a type like:
public class Customer
{
public CustomerId Id { get; }
public EmailAddress Email { get; }
public Customer(CustomerId id, EmailAddress email)
{
Id = id;
Email = email;
}
}
Each object the inherits from ValueObject<T> is wrapped in a Value property (as expected). For example
var customerId = new CustomerId(Guid.NewGuid());
var emailAddress = new EmailAddress("some#email.com");
var customer = new Customer(customerId, emailAddress);
var customerAsJson = JsonConvert.SerializeObject(customer, Formatting.Indented, new JsonSerializerSettings
{
ContractResolver = new CamelCasePropertyNamesContractResolver()
})
Results in
{
"id": {
"value": "f5ce21a5-a0d1-4888-8d22-6f484794ac7c"
},
"email": {
"value": "some#email.com"
}
}
Is there a way to write a custom JsonConverter so the the Value property is excluded for types subclassing ValueObject<T> so that the above example would output
{
"id": "f5ce21a5-a0d1-4888-8d22-6f484794ac7c",
"email": "some#email.com"
}
I would prefer to have a single JsonConverter that can handle all ValueObject<T> rather than having to define a separate JsonConverter for each ValueObject<T> subclass
My first attempt was
public class ValueObjectOfTConverter : JsonConverter
{
private static readonly Type ValueObjectGenericType = typeof(ValueObject<>);
private static readonly string ValuePropertyName = nameof(ValueObject<object>.Value);
public override bool CanConvert(Type objectType) =>
IsSubclassOfGenericType(objectType, ValueObjectGenericType);
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
// converts "f5ce21a5-a0d1-4888-8d22-6f484794ac7c" => "value": "f5ce21a5-a0d1-4888-8d22-6f484794ac7c"
var existingJsonWrappedInValueProperty = new JObject(new JProperty(ValuePropertyName, JToken.Load(reader)));
return existingJsonWrappedInValueProperty.ToObject(objectType, serializer);
}
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
// to implement
}
private static bool IsSubclassOfGenericType(Type typeToCheck, Type openGenericType)
{
while (typeToCheck != null && typeToCheck != typeof(object))
{
var cur = typeToCheck.IsGenericType ? typeToCheck.GetGenericTypeDefinition() : typeToCheck;
if (openGenericType == cur) return true;
typeToCheck = typeToCheck.BaseType;
}
return false;
}
}
You can do this with a custom JsonConverter similar to the ones shown in Json.Net: Serialize/Deserialize property as a value, not as an object. However, since ValueObject<T> does not have a non-generic method to get and set the Value as an object, you will need to use reflection.
Here's one approach:
class ValueConverter : JsonConverter
{
static Type GetValueType(Type objectType)
{
return objectType
.BaseTypesAndSelf()
.Where(t => t.IsGenericType && t.GetGenericTypeDefinition() == typeof(ValueObject<>))
.Select(t => t.GetGenericArguments()[0])
.FirstOrDefault();
}
public override bool CanConvert(Type objectType)
{
return GetValueType(objectType) != null;
}
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
// You need to decide whether a null JSON token results in a null ValueObject<T> or
// an allocated ValueObject<T> with a null Value.
if (reader.SkipComments().TokenType == JsonToken.Null)
return null;
var valueType = GetValueType(objectType);
var value = serializer.Deserialize(reader, valueType);
// Here we assume that every subclass of ValueObject<T> has a constructor with a single argument, of type T.
return Activator.CreateInstance(objectType, value);
}
const string ValuePropertyName = nameof(ValueObject<object>.Value);
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
var contract = (JsonObjectContract)serializer.ContractResolver.ResolveContract(value.GetType());
var valueProperty = contract.Properties.Where(p => p.UnderlyingName == ValuePropertyName).Single();
// You can simplify this to .Single() if ValueObject<T> has no other properties:
// var valueProperty = contract.Properties.Single();
serializer.Serialize(writer, valueProperty.ValueProvider.GetValue(value));
}
}
public static partial class JsonExtensions
{
public static JsonReader SkipComments(this JsonReader reader)
{
while (reader.TokenType == JsonToken.Comment && reader.Read())
;
return reader;
}
}
public static class TypeExtensions
{
public static IEnumerable<Type> BaseTypesAndSelf(this Type type)
{
while (type != null)
{
yield return type;
type = type.BaseType;
}
}
}
You could then apply the converter directly to ValueType<T> like so:
[JsonConverter(typeof(ValueConverter))]
public class ValueObject<T>
{
// Remainder unchanged
}
Or apply it in settings instead:
var settings = new JsonSerializerSettings
{
Converters = { new ValueConverter() },
ContractResolver = new CamelCasePropertyNamesContractResolver()
};
var customerAsJson = JsonConvert.SerializeObject(customer, Formatting.Indented, settings);
Working sample .Net fiddle #1 here.
Alternatively, you might consider adding a non-generic method to access the value as an object, e.g. like so:
public interface IHasValue
{
object GetValue(); // A method rather than a property to ensure the non-generic value is never serialized directly.
}
public class ValueObject<T> : IHasValue
{
public T Value { get; }
public ValueObject(T value) => Value = value;
// various other equality members etc...
#region IHasValue Members
object IHasValue.GetValue() => Value;
#endregion
}
With this addition, WriteJson() becomes much simpler:
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
serializer.Serialize(writer, ((IHasValue)value).GetValue());
}
Working sample .Net fiddle #2 here.
Notes:
ReadJson() assumes that every subclass of Value<T> has a public constructor taking a single argument of type T.
Applying the converter directly to ValueType<T> using [JsonConverter(typeof(ValueConverter))] will have slightly better performance, since CanConvert need never get called. See Performance Tips: JsonConverters for details.
You need to decide how to handle a null JSON token. Should it result in a null ValueType<T>, or an allocated ValueType<T> with a null Value?
In the second version of ValueType<T> I implemented IHasValue.GetValue() explicitly to discourage its use in cases where an instance of ValueType<T> is used in statically typed code.
If you really only want to apply the converter to types subclassing ValueObject<T> and not ValueObject<T> itself, in GetValueType(Type objectType) add a call to .Skip(1):
static Type GetValueType(Type objectType)
{
return objectType
.BaseTypesAndSelf()
.Skip(1) // Do not apply the converter to ValueObject<T> when not subclassed
.Where(t => t.IsGenericType && t.GetGenericTypeDefinition() == typeof(ValueObject<>))
.Select(t => t.GetGenericArguments()[0])
.FirstOrDefault();
}
And then apply the converter in JsonSerializerSettings.Converters rather than directly to ValueObject<T>.
Is it possible to serialize (and deserialize) a dictionary as an array with System.Text.Json?
Instead of { "hello": "world" } I would need my dictionary serialized as { "key": "hello", "value": "world" } preferably without having to set attributes on the dictionary property of my class.
Using newtonsoft.json it was possible this way:
class DictionaryAsArrayResolver : DefaultContractResolver
{
protected override JsonContract CreateContract(Type objectType)
{
if (objectType.GetInterfaces().Any(i => i == typeof(IDictionary) ||
(i.IsGenericType && i.GetGenericTypeDefinition() == typeof(IDictionary<,>))))
{
return base.CreateArrayContract(objectType);
}
return base.CreateContract(objectType);
}
}
You can do this using a JsonConverterFactory that manufactures a specific JsonConverter<T> for every dictionary type that you want to serialize as an array. Here is one such converter that works for every class that implements IDictionary<TKey, TValue>:
public class DictionaryConverterFactory : JsonConverterFactory
{
public override bool CanConvert(Type typeToConvert)
{
return typeToConvert.IsClass && typeToConvert.GetDictionaryKeyValueType() != null && typeToConvert.GetConstructor(Type.EmptyTypes) != null;
}
public override JsonConverter CreateConverter(Type typeToConvert, JsonSerializerOptions options)
{
var keyValueTypes = typeToConvert.GetDictionaryKeyValueType();
var converterType = typeof(DictionaryAsArrayConverter<,,>).MakeGenericType(typeToConvert, keyValueTypes.Value.Key, keyValueTypes.Value.Value);
return (JsonConverter)Activator.CreateInstance(converterType);
}
}
public class DictionaryAsArrayConverter<TKey, TValue> : DictionaryAsArrayConverter<Dictionary<TKey, TValue>, TKey, TValue>
{
}
public class DictionaryAsArrayConverter<TDictionary, TKey, TValue> : JsonConverter<TDictionary> where TDictionary : class, IDictionary<TKey, TValue>, new()
{
struct KeyValueDTO
{
public TKey Key { get; set; }
public TValue Value { get; set; }
}
public override TDictionary Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
{
var list = JsonSerializer.Deserialize<List<KeyValueDTO>>(ref reader, options);
if (list == null)
return null;
var dictionary = typeToConvert == typeof(Dictionary<TKey, TValue>) ? (TDictionary)(object)new Dictionary<TKey, TValue>(list.Count) : new TDictionary();
foreach (var pair in list)
dictionary.Add(pair.Key, pair.Value);
return dictionary;
}
public override void Write(Utf8JsonWriter writer, TDictionary value, JsonSerializerOptions options)
{
JsonSerializer.Serialize(writer, value.Select(p => new KeyValueDTO { Key = p.Key, Value = p.Value }), options);
}
}
public static class TypeExtensions
{
public static IEnumerable<Type> GetInterfacesAndSelf(this Type type)
{
if (type == null)
throw new ArgumentNullException();
if (type.IsInterface)
return new[] { type }.Concat(type.GetInterfaces());
else
return type.GetInterfaces();
}
public static KeyValuePair<Type, Type>? GetDictionaryKeyValueType(this Type type)
{
KeyValuePair<Type, Type>? types = null;
foreach (var pair in type.GetDictionaryKeyValueTypes())
{
if (types == null)
types = pair;
else
return null;
}
return types;
}
public static IEnumerable<KeyValuePair<Type, Type>> GetDictionaryKeyValueTypes(this Type type)
{
foreach (Type intType in type.GetInterfacesAndSelf())
{
if (intType.IsGenericType
&& intType.GetGenericTypeDefinition() == typeof(IDictionary<,>))
{
var args = intType.GetGenericArguments();
if (args.Length == 2)
yield return new KeyValuePair<Type, Type>(args[0], args[1]);
}
}
}
}
Then add the factory to JsonSerializerOptions.Converters locally as follows:
var options = new JsonSerializerOptions
{
Converters = { new DictionaryConverterFactory() },
PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
};
var json = JsonSerializer.Serialize(dictionary, options);
var dictionary2 = JsonSerializer.Deserialize<TDictionary>(json, options);
Or globally in ASP.NET Core as shown in How to set json serializer settings in asp.net core 3?:
services.AddControllers().AddJsonOptions(options =>
{
options.JsonSerializerOptions.Converters.Add(new DictionaryConverterFactory());
options.PropertyNamingPolicy = JsonNamingPolicy.CamelCase;
});
The underlying individual converter DictionaryAsArrayConverter<TKey, TValue> can also be used directly if you only want to serialize certain dictionary types as arrays.
Notes:
JsonSerializer currently does not respect PropertyNamingPolicy when serializing KeyValuePair<TKey, TValue> (see Issue #1197) so I had to introduce a KeyValueDTO to get the casing "key" and "value" as required in your question.
I did not implement a converter for non-generic IDictionary types. That could be done as an extension of the answer.
For more on the converter factory pattern see How to write custom converters for JSON serialization in .NET : Sample factory pattern converter
The types equivalent to DefaultContractResolver in System.Text.Json -- JsonClassInfo and JsonPropertyInfo -- are internal. There is an open enhancement Equivalent of DefaultContractResolver in System.Text.Json #42001 asking for a public equivalent.
Demo fiddle here.
If you want to keep it short and simple, you could consider projection via anonymous type:
var dictionary = new Dictionary<string, string>();
dictionary.Add("hello", "world");
dictionary.Add("how", "are you?");
var o = JsonSerializer.Serialize(dictionary.Select(x => new { key = x.Key, value = x.Value }));
// [{"key":"hello","value":"world"},{"key":"how","value":"are you?"}]
ed: of course, that's just if your feeling masochistic. If all you want is to just get the job done, just call .ToList()
JsonSerializer.Serialize(dictionary.ToList());
// [{"Key":"hello","Value":"world"},{"Key":"how","Value":"are you?"}]
I try to migrate from Newtonsoft.Json to System.Text.Json.
I want to deserialize abstract class. Newtonsoft.Json has TypeNameHandling for this.
Is there any way to deserialize abstract class via System.Text.Json on .net core 3.0?
Is polymorphic deserialization possible in System.Text.Json?
The answer is yes and no, depending on what you mean by "possible".
There is no polymorphic deserialization (equivalent to Newtonsoft.Json's TypeNameHandling) support built-in to System.Text.Json. This is because reading the .NET type name specified as a string within the JSON payload (such as $type metadata property) to create your objects is not recommended since it introduces potential security concerns (see https://github.com/dotnet/corefx/issues/41347#issuecomment-535779492 for more info).
Allowing the payload to specify its own type information is a common source of vulnerabilities in web applications.
However, there is a way to add your own support for polymorphic deserialization by creating a JsonConverter<T>, so in that sense, it is possible.
The docs show an example of how to do that using a type discriminator property:
https://learn.microsoft.com/en-us/dotnet/standard/serialization/system-text-json-converters-how-to#support-polymorphic-deserialization
Let's look at an example.
Say you have a base class and a couple of derived classes:
public class BaseClass
{
public int Int { get; set; }
}
public class DerivedA : BaseClass
{
public string Str { get; set; }
}
public class DerivedB : BaseClass
{
public bool Bool { get; set; }
}
You can create the following JsonConverter<BaseClass> that writes the type discriminator while serializing and reads it to figure out which type to deserialize. You can register that converter on the JsonSerializerOptions.
public class BaseClassConverter : JsonConverter<BaseClass>
{
private enum TypeDiscriminator
{
BaseClass = 0,
DerivedA = 1,
DerivedB = 2
}
public override bool CanConvert(Type type)
{
return typeof(BaseClass).IsAssignableFrom(type);
}
public override BaseClass Read(
ref Utf8JsonReader reader,
Type typeToConvert,
JsonSerializerOptions options)
{
if (reader.TokenType != JsonTokenType.StartObject)
{
throw new JsonException();
}
if (!reader.Read()
|| reader.TokenType != JsonTokenType.PropertyName
|| reader.GetString() != "TypeDiscriminator")
{
throw new JsonException();
}
if (!reader.Read() || reader.TokenType != JsonTokenType.Number)
{
throw new JsonException();
}
BaseClass baseClass;
TypeDiscriminator typeDiscriminator = (TypeDiscriminator)reader.GetInt32();
switch (typeDiscriminator)
{
case TypeDiscriminator.DerivedA:
if (!reader.Read() || reader.GetString() != "TypeValue")
{
throw new JsonException();
}
if (!reader.Read() || reader.TokenType != JsonTokenType.StartObject)
{
throw new JsonException();
}
baseClass = (DerivedA)JsonSerializer.Deserialize(ref reader, typeof(DerivedA));
break;
case TypeDiscriminator.DerivedB:
if (!reader.Read() || reader.GetString() != "TypeValue")
{
throw new JsonException();
}
if (!reader.Read() || reader.TokenType != JsonTokenType.StartObject)
{
throw new JsonException();
}
baseClass = (DerivedB)JsonSerializer.Deserialize(ref reader, typeof(DerivedB));
break;
default:
throw new NotSupportedException();
}
if (!reader.Read() || reader.TokenType != JsonTokenType.EndObject)
{
throw new JsonException();
}
return baseClass;
}
public override void Write(
Utf8JsonWriter writer,
BaseClass value,
JsonSerializerOptions options)
{
writer.WriteStartObject();
if (value is DerivedA derivedA)
{
writer.WriteNumber("TypeDiscriminator", (int)TypeDiscriminator.DerivedA);
writer.WritePropertyName("TypeValue");
JsonSerializer.Serialize(writer, derivedA);
}
else if (value is DerivedB derivedB)
{
writer.WriteNumber("TypeDiscriminator", (int)TypeDiscriminator.DerivedB);
writer.WritePropertyName("TypeValue");
JsonSerializer.Serialize(writer, derivedB);
}
else
{
throw new NotSupportedException();
}
writer.WriteEndObject();
}
}
This is what serialization and deserialization would look like (including comparison with Newtonsoft.Json):
private static void PolymorphicSupportComparison()
{
var objects = new List<BaseClass> { new DerivedA(), new DerivedB() };
// Using: System.Text.Json
var options = new JsonSerializerOptions
{
Converters = { new BaseClassConverter() },
WriteIndented = true
};
string jsonString = JsonSerializer.Serialize(objects, options);
Console.WriteLine(jsonString);
/*
[
{
"TypeDiscriminator": 1,
"TypeValue": {
"Str": null,
"Int": 0
}
},
{
"TypeDiscriminator": 2,
"TypeValue": {
"Bool": false,
"Int": 0
}
}
]
*/
var roundTrip = JsonSerializer.Deserialize<List<BaseClass>>(jsonString, options);
// Using: Newtonsoft.Json
var settings = new Newtonsoft.Json.JsonSerializerSettings
{
TypeNameHandling = Newtonsoft.Json.TypeNameHandling.Objects,
Formatting = Newtonsoft.Json.Formatting.Indented
};
jsonString = Newtonsoft.Json.JsonConvert.SerializeObject(objects, settings);
Console.WriteLine(jsonString);
/*
[
{
"$type": "PolymorphicSerialization.DerivedA, PolymorphicSerialization",
"Str": null,
"Int": 0
},
{
"$type": "PolymorphicSerialization.DerivedB, PolymorphicSerialization",
"Bool": false,
"Int": 0
}
]
*/
var originalList = JsonConvert.DeserializeObject<List<BaseClass>>(jsonString, settings);
Debug.Assert(originalList[0].GetType() == roundTrip[0].GetType());
}
Here's another StackOverflow question that shows how to support polymorphic deserialization with interfaces (rather than abstract classes), but a similar solution would apply for any polymorphism:
Is there a simple way to manually serialize/deserialize child objects in a custom converter in System.Text.Json?
I ended up with that solution. It's lightwight and a generic enough for me.
The type discriminator converter
public class TypeDiscriminatorConverter<T> : JsonConverter<T> where T : ITypeDiscriminator
{
private readonly IEnumerable<Type> _types;
public TypeDiscriminatorConverter()
{
var type = typeof(T);
_types = AppDomain.CurrentDomain.GetAssemblies()
.SelectMany(s => s.GetTypes())
.Where(p => type.IsAssignableFrom(p) && p.IsClass && !p.IsAbstract)
.ToList();
}
public override T Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
{
if (reader.TokenType != JsonTokenType.StartObject)
{
throw new JsonException();
}
using (var jsonDocument = JsonDocument.ParseValue(ref reader))
{
if (!jsonDocument.RootElement.TryGetProperty(nameof(ITypeDiscriminator.TypeDiscriminator), out var typeProperty))
{
throw new JsonException();
}
var type = _types.FirstOrDefault(x => x.Name == typeProperty.GetString());
if (type == null)
{
throw new JsonException();
}
var jsonObject = jsonDocument.RootElement.GetRawText();
var result = (T) JsonSerializer.Deserialize(jsonObject, type, options);
return result;
}
}
public override void Write(Utf8JsonWriter writer, T value, JsonSerializerOptions options)
{
JsonSerializer.Serialize(writer, (object)value, options);
}
}
The interface
public interface ITypeDiscriminator
{
string TypeDiscriminator { get; }
}
And the example models
public interface ISurveyStepResult : ITypeDiscriminator
{
string Id { get; set; }
}
public class BoolStepResult : ISurveyStepResult
{
public string Id { get; set; }
public string TypeDiscriminator => nameof(BoolStepResult);
public bool Value { get; set; }
}
public class TextStepResult : ISurveyStepResult
{
public string Id { get; set; }
public string TypeDiscriminator => nameof(TextStepResult);
public string Value { get; set; }
}
public class StarsStepResult : ISurveyStepResult
{
public string Id { get; set; }
public string TypeDiscriminator => nameof(StarsStepResult);
public int Value { get; set; }
}
And here is the test method
public void SerializeAndDeserializeTest()
{
var surveyResult = new SurveyResultModel()
{
Id = "id",
SurveyId = "surveyId",
Steps = new List<ISurveyStepResult>()
{
new BoolStepResult(){ Id = "1", Value = true},
new TextStepResult(){ Id = "2", Value = "some text"},
new StarsStepResult(){ Id = "3", Value = 5},
}
};
var jsonSerializerOptions = new JsonSerializerOptions()
{
Converters = { new TypeDiscriminatorConverter<ISurveyStepResult>()},
WriteIndented = true
};
var result = JsonSerializer.Serialize(surveyResult, jsonSerializerOptions);
var back = JsonSerializer.Deserialize<SurveyResultModel>(result, jsonSerializerOptions);
var result2 = JsonSerializer.Serialize(back, jsonSerializerOptions);
Assert.IsTrue(back.Steps.Count == 3
&& back.Steps.Any(x => x is BoolStepResult)
&& back.Steps.Any(x => x is TextStepResult)
&& back.Steps.Any(x => x is StarsStepResult)
);
Assert.AreEqual(result2, result);
}
Please try this library I wrote as an extension to System.Text.Json to offer polymorphism:
https://github.com/dahomey-technologies/Dahomey.Json
If the actual type of a reference instance differs from the declared type, the discriminator property will be automatically added to the output json:
public class WeatherForecast
{
public DateTimeOffset Date { get; set; }
public int TemperatureCelsius { get; set; }
public string Summary { get; set; }
}
public class WeatherForecastDerived : WeatherForecast
{
public int WindSpeed { get; set; }
}
Inherited classes must be manually registered to the discriminator convention registry in order to let the framework know about the mapping between a discriminator value and a type:
JsonSerializerOptions options = new JsonSerializerOptions();
options.SetupExtensions();
DiscriminatorConventionRegistry registry = options.GetDiscriminatorConventionRegistry();
registry.RegisterType<WeatherForecastDerived>();
string json = JsonSerializer.Serialize<WeatherForecast>(weatherForecastDerived, options);
Result:
{
"$type": "Tests.WeatherForecastDerived, Tests",
"Date": "2019-08-01T00:00:00-07:00",
"TemperatureCelsius": 25,
"Summary": "Hot",
"WindSpeed": 35
}
Polymorphic serialization of whitelisted inherited types has been implemented in .NET 7, and is available in Preview 6.
From the documentation page What’s new in System.Text.Json in .NET 7: Type Hierarchies:
System.Text.Json now supports polymorphic serialization and deserialization of user-defined type hierarchies. This can be enabled by decorating the base class of a type hierarchy with the new JsonDerivedTypeAttribute.
First, let's consider serialization. Say you have the following type hierarchy:
public abstract class BaseType { } // Properties omitted
public class DerivedType1 : BaseType { public string Derived1 { get; set; } }
public class DerivedType2 : BaseType { public int Derived2 { get; set; } }
And you have a data model that includes a value whose declared type is BaseType, e.g.
var list = new List<BaseType> { new DerivedType1 { Derived1 = "value 1" } };
In previous versions, System.Text.Json would only serialize the properties of the declared type BaseType. Now you will be able to include the properties of DerivedType1 when serializing a value declared as BaseType by adding [JsonDerivedType(typeof(TDerivedType))] to BaseType for all derived types:
[JsonDerivedType(typeof(DerivedType1))]
[JsonDerivedType(typeof(DerivedType2))]
public abstract class BaseType { } // Properties omitted
Having whitelisted DerivedType1 in this manner, serialization of your model:
var json = JsonSerializer.Serialize(list);
Results in
[{"Derived1" : "value 1"}]
Demo fiddle #1 here.
Do note that only derived types whitelisted via attribute (or through setting JsonTypeInfo.PolymorphismOptions in runtime) can be serialized via this mechanism. If you have some other derived type which is not whitelisted, e.g.:
public class DerivedType3 : BaseType { public string Derived3 { get; set; } }
Then JsonSerializer.Serialize(new BaseType [] { new DerivedType3 { Derived3 = "value 3" } }) will throw a System.NotSupportedException: Runtime type 'DerivedType3' is not supported by polymorphic type 'BaseType' exception. Demo fiddle #2 here.
That covers serialization. If you need to round-trip your type hierarchy, you will need to supply a type discriminator property value to use for each derived type. This may be done providing a value for JsonDerivedTypeAttribute.TypeDiscriminator for each derived type:
[JsonDerivedType(typeof(DerivedType1), "DerivedType1")]
[JsonDerivedType(typeof(DerivedType2), "DerivedType2")]
public abstract class BaseType { } // Properties omitted
Now when you serialize your model
var json = JsonSerializer.Serialize(list);
System.Text.Json will add an artificial type discriminator property "$type" indicating the type that was serialized:
[{"$type" : "DerivedType1", "Derived1" : "value 1"}]
Having done so, you can now deserialize your data model like so:
var list2 = JsonSerializer.Deserialize<List<BaseType>>(json);
And the actual, concrete type(s) serialized will be preserved. Demo fiddle #3 here.
It is also possible to inform System.Text.Json of your type hierarchy in runtime via Contract Customization. You might need to do this when your type hierarchy cannot be modified, or when some derived types are in different assemblies and cannot be referenced at compile time, or you are trying to interoperate between multiple legacy serializers. The basic workflow here will be to instantiate an instance of DefaultJsonTypeInfoResolver and add a modifier which sets up the necessary PolymorphismOptions for the JsonTypeInfo for your base type.
For example, polymorphic serialization for the BaseType hierarchy can be enabled in runtime like so:
var resolver = new DefaultJsonTypeInfoResolver
{
Modifiers =
{
// Add an Action<JsonTypeInfo> modifier that sets up the polymorphism options for BaseType
static typeInfo =>
{
if (typeInfo.Type != typeof(BaseType))
return;
typeInfo.PolymorphismOptions = new()
{
DerivedTypes =
{
new JsonDerivedType(typeof(DerivedType1), "Derived1"),
new JsonDerivedType(typeof(DerivedType2), "Derived2")
}
};
},
// Add other modifiers as required.
}
};
var options = new JsonSerializerOptions
{
TypeInfoResolver = resolver,
// Add other options as required
};
var json = JsonSerializer.Serialize(list, options);
Demo fiddle #4 here.
Notes:
The whitelisting approach is consistent with the approach of the data contract serializers, which use the KnownTypeAttribute, and XmlSerializer, which uses XmlIncludeAttribute. It is inconsistent with Json.NET, whose TypeNameHandling serializes type information for all types unless explicitly filtered via a serialization binder.
Allowing only whitelisted types to be deserialized prevents Friday the 13th: JSON Attacks type injection attacks including those detailed in TypeNameHandling caution in Newtonsoft Json and External json vulnerable because of Json.Net TypeNameHandling auto?.
Integers as well as strings may be used for the type discriminator name. If you define your type hierarchy as follows:
[JsonDerivedType(typeof(DerivedType1), 1)]
[JsonDerivedType(typeof(DerivedType2), 2)]
public abstract class BaseType { } // Properties omitted
Then serializing the list above results in
[{"$type" : 1, "Derived1" : "value 1"}]
Numeric type discriminator values are not used by Newtonsoft however, so if you are interoperating with a legacy serializer you might want to avoid this.
The default type discriminator property name, "$type", is the same type discriminator name used by Json.NET. If you would prefer to use a different property name, such as the name "__type" used by DataContractJsonSerializer, apply JsonPolymorphicAttribute to the base type and set TypeDiscriminatorPropertyName like so:
[JsonPolymorphic(TypeDiscriminatorPropertyName = "__type")]
[JsonDerivedType(typeof(DerivedType1), "DerivedType1")]
[JsonDerivedType(typeof(DerivedType2), "DerivedType2")]
public abstract class BaseType { } // Properties omitted
If you are interoperating with Json.NET (or DataContractJsonSerializer), you may set the value of TypeDiscriminator equal to the type discriminator value used by the legacy serializer.
If the serializer encounters a derived type that has not been whitelisted, you can control its behavior by setting JsonPolymorphicAttribute.UnknownDerivedTypeHandling to one of the following values:
JsonUnknownDerivedTypeHandling
Value
Meaning
FailSerialization
0
An object of undeclared runtime type will fail polymorphic serialization.
FallBackToBaseType
1
An object of undeclared runtime type will fall back to the serialization contract of the base type.
FallBackToNearestAncestor
2
An object of undeclared runtime type will revert to the serialization contract of the nearest declared ancestor type. Certain interface hierarchies are not supported due to diamond ambiguity constraints.
Thats my JsonConverter for all abstract types:
private class AbstractClassConverter : JsonConverter<object>
{
public override object Read(ref Utf8JsonReader reader, Type typeToConvert,
JsonSerializerOptions options)
{
if (reader.TokenType == JsonTokenType.Null) return null;
if (reader.TokenType != JsonTokenType.StartObject)
throw new JsonException("JsonTokenType.StartObject not found.");
if (!reader.Read() || reader.TokenType != JsonTokenType.PropertyName
|| reader.GetString() != "$type")
throw new JsonException("Property $type not found.");
if (!reader.Read() || reader.TokenType != JsonTokenType.String)
throw new JsonException("Value at $type is invalid.");
string assemblyQualifiedName = reader.GetString();
var type = Type.GetType(assemblyQualifiedName);
using (var output = new MemoryStream())
{
ReadObject(ref reader, output, options);
return JsonSerializer.Deserialize(output.ToArray(), type, options);
}
}
private void ReadObject(ref Utf8JsonReader reader, Stream output, JsonSerializerOptions options)
{
using (var writer = new Utf8JsonWriter(output, new JsonWriterOptions
{
Encoder = options.Encoder,
Indented = options.WriteIndented
}))
{
writer.WriteStartObject();
var objectIntend = 0;
while (reader.Read())
{
switch (reader.TokenType)
{
case JsonTokenType.None:
case JsonTokenType.Null:
writer.WriteNullValue();
break;
case JsonTokenType.StartObject:
writer.WriteStartObject();
objectIntend++;
break;
case JsonTokenType.EndObject:
writer.WriteEndObject();
if(objectIntend == 0)
{
writer.Flush();
return;
}
objectIntend--;
break;
case JsonTokenType.StartArray:
writer.WriteStartArray();
break;
case JsonTokenType.EndArray:
writer.WriteEndArray();
break;
case JsonTokenType.PropertyName:
writer.WritePropertyName(reader.GetString());
break;
case JsonTokenType.Comment:
writer.WriteCommentValue(reader.GetComment());
break;
case JsonTokenType.String:
writer.WriteStringValue(reader.GetString());
break;
case JsonTokenType.Number:
writer.WriteNumberValue(reader.GetInt32());
break;
case JsonTokenType.True:
case JsonTokenType.False:
writer.WriteBooleanValue(reader.GetBoolean());
break;
default:
throw new ArgumentOutOfRangeException();
}
}
}
}
public override void Write(Utf8JsonWriter writer, object value, JsonSerializerOptions options)
{
writer.WriteStartObject();
var valueType = value.GetType();
var valueAssemblyName = valueType.Assembly.GetName();
writer.WriteString("$type", $"{valueType.FullName}, {valueAssemblyName.Name}");
var json = JsonSerializer.Serialize(value, value.GetType(), options);
using (var document = JsonDocument.Parse(json, new JsonDocumentOptions
{
AllowTrailingCommas = options.AllowTrailingCommas,
MaxDepth = options.MaxDepth
}))
{
foreach (var jsonProperty in document.RootElement.EnumerateObject())
jsonProperty.WriteTo(writer);
}
writer.WriteEndObject();
}
public override bool CanConvert(Type typeToConvert) =>
typeToConvert.IsAbstract && !EnumerableInterfaceType.IsAssignableFrom(typeToConvert);
}
I really liked the answer of Demetrius, but I think you can go even further in terms of re-usability. I came up with the following solution:
The JsonConverterFactory:
/// <summary>
/// Represents the <see cref="JsonConverterFactory"/> used to create <see cref="AbstractClassConverter{T}"/>
/// </summary>
public class AbstractClassConverterFactory
: JsonConverterFactory
{
/// <summary>
/// Gets a <see cref="Dictionary{TKey, TValue}"/> containing the mappings of types to their respective <see cref="JsonConverter"/>
/// </summary>
protected static Dictionary<Type, JsonConverter> Converters = new Dictionary<Type, JsonConverter>();
/// <summary>
/// Initializes a new <see cref="AbstractClassConverterFactory"/>
/// </summary>
/// <param name="namingPolicy">The current <see cref="JsonNamingPolicy"/></param>
public AbstractClassConverterFactory(JsonNamingPolicy namingPolicy)
{
this.NamingPolicy = namingPolicy;
}
/// <summary>
/// Gets the current <see cref="JsonNamingPolicy"/>
/// </summary>
protected JsonNamingPolicy NamingPolicy { get; }
/// <inheritdoc/>
public override bool CanConvert(Type typeToConvert)
{
return typeToConvert.IsClass && typeToConvert.IsAbstract && typeToConvert.IsDefined(typeof(DiscriminatorAttribute));
}
/// <inheritdoc/>
public override JsonConverter CreateConverter(Type typeToConvert, JsonSerializerOptions options)
{
if(!Converters.TryGetValue(typeToConvert, out JsonConverter converter))
{
Type converterType = typeof(AbstractClassConverter<>).MakeGenericType(typeToConvert);
converter = (JsonConverter)Activator.CreateInstance(converterType, this.NamingPolicy);
Converters.Add(typeToConvert, converter);
}
return converter;
}
}
The JsonConverter:
/// <summary>
/// Represents the <see cref="JsonConverter"/> used to convert to/from an abstract class
/// </summary>
/// <typeparam name="T">The type of the abstract class to convert to/from</typeparam>
public class AbstractClassConverter<T>
: JsonConverter<T>
{
/// <summary>
/// Initializes a new <see cref="AbstractClassConverter{T}"/>
/// </summary>
/// <param name="namingPolicy">The current <see cref="JsonNamingPolicy"/></param>
public AbstractClassConverter(JsonNamingPolicy namingPolicy)
{
this.NamingPolicy = namingPolicy;
DiscriminatorAttribute discriminatorAttribute = typeof(T).GetCustomAttribute<DiscriminatorAttribute>();
if (discriminatorAttribute == null)
throw new NullReferenceException($"Failed to find the required '{nameof(DiscriminatorAttribute)}'");
this.DiscriminatorProperty = typeof(T).GetProperty(discriminatorAttribute.Property, BindingFlags.Default | BindingFlags.Public | BindingFlags.Instance);
if (this.DiscriminatorProperty == null)
throw new NullReferenceException($"Failed to find the specified discriminator property '{discriminatorAttribute.Property}' in type '{typeof(T).Name}'");
this.TypeMappings = new Dictionary<string, Type>();
foreach (Type derivedType in TypeCacheUtil.FindFilteredTypes($"nposm:json-polymorph:{typeof(T).Name}",
(t) => t.IsClass && !t.IsAbstract && t.BaseType == typeof(T)))
{
DiscriminatorValueAttribute discriminatorValueAttribute = derivedType.GetCustomAttribute<DiscriminatorValueAttribute>();
if (discriminatorValueAttribute == null)
continue;
string discriminatorValue = null;
if (discriminatorValueAttribute.Value.GetType().IsEnum)
discriminatorValue = EnumHelper.Stringify(discriminatorValueAttribute.Value, this.DiscriminatorProperty.PropertyType);
else
discriminatorValue = discriminatorValueAttribute.Value.ToString();
this.TypeMappings.Add(discriminatorValue, derivedType);
}
}
/// <summary>
/// Gets the current <see cref="JsonNamingPolicy"/>
/// </summary>
protected JsonNamingPolicy NamingPolicy { get; }
/// <summary>
/// Gets the discriminator <see cref="PropertyInfo"/> of the abstract type to convert
/// </summary>
protected PropertyInfo DiscriminatorProperty { get; }
/// <summary>
/// Gets an <see cref="Dictionary{TKey, TValue}"/> containing the mappings of the converted type's derived types
/// </summary>
protected Dictionary<string, Type> TypeMappings { get; }
/// <inheritdoc/>
public override T Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
{
if (reader.TokenType != JsonTokenType.StartObject)
throw new JsonException("Start object token type expected");
using (JsonDocument jsonDocument = JsonDocument.ParseValue(ref reader))
{
string discriminatorPropertyName = this.NamingPolicy?.ConvertName(this.DiscriminatorProperty.Name);
if (!jsonDocument.RootElement.TryGetProperty(discriminatorPropertyName, out JsonElement discriminatorProperty))
throw new JsonException($"Failed to find the required '{this.DiscriminatorProperty.Name}' discriminator property");
string discriminatorValue = discriminatorProperty.GetString();
if (!this.TypeMappings.TryGetValue(discriminatorValue, out Type derivedType))
throw new JsonException($"Failed to find the derived type with the specified discriminator value '{discriminatorValue}'");
string json = jsonDocument.RootElement.GetRawText();
return (T)JsonSerializer.Deserialize(json, derivedType);
}
}
/// <inheritdoc/>
public override void Write(Utf8JsonWriter writer, T value, JsonSerializerOptions options)
{
JsonSerializer.Serialize(writer, (object)value, options);
}
}
The DiscriminatorAttribute:
/// <summary>
/// Represents the <see cref="Attribute"/> used to indicate the property used to discriminate derived types of the marked class
/// </summary>
[AttributeUsage(AttributeTargets.Class, AllowMultiple = false)]
public class DiscriminatorAttribute
: Attribute
{
/// <summary>
/// Initializes a new <see cref="DiscriminatorAttribute"/>
/// </summary>
/// <param name="property">The name of the property used to discriminate derived types of the class marked by the <see cref="DiscriminatorAttribute"/></param>
public DiscriminatorAttribute(string property)
{
this.Property = property;
}
/// <summary>
/// Gets the name of the property used to discriminate derived types of the class marked by the <see cref="DiscriminatorAttribute"/>
/// </summary>
public string Property { get; }
}
The DiscriminatorValueAttribute:
/// <summary>
/// Represents the <see cref="Attribute"/> used to indicate the discriminator value of a derived type
/// </summary>
[AttributeUsage(AttributeTargets.Class, AllowMultiple = false)]
public class DiscriminatorValueAttribute
: Attribute
{
/// <summary>
/// Initializes a new <see cref="DiscriminatorValueAttribute"/>
/// </summary>
/// <param name="value">The value used to discriminate the derived type marked by the <see cref="DiscriminatorValueAttribute"/></param>
public DiscriminatorValueAttribute(object value)
{
this.Value = value;
}
/// <summary>
/// Gets the value used to discriminate the derived type marked by the <see cref="DiscriminatorValueAttribute"/>
/// </summary>
public object Value { get; }
}
And finally, an example of how to use it on classes:
[Discriminator(nameof(Type))]
public abstract class Identity
{
public virtual IdentityType Type { get; protected set; }
}
[DiscriminatorValue(IdentityType.Person)]
public class Person
: Identity
{
}
And... Voilà!
All that is left to do is to register the factory:
this.Services.AddControllersWithViews()
.AddJsonOptions(options =>
{
options.JsonSerializerOptions.Converters.Add(new AbstractClassConverterFactory(options.JsonSerializerOptions.PropertyNamingPolicy));
});
Throwing this option out there: Using a source code generator to generate a JsonConverter automatically for objects with a property marked with a special attribute
You can try it with this package, but it requires .net5
https://github.com/wivuu/Wivuu.JsonPolymorphism
The generator looks at the type of the property marked with a discriminator attribute, and then looks for types inheriting from the type holding the discriminator to match up with each case of the enum
Source here: https://github.com/wivuu/Wivuu.JsonPolymorphism/blob/master/Wivuu.JsonPolymorphism/JsonConverterGenerator.cs
enum AnimalType
{
Insect,
Mammal,
Reptile,
Bird // <- This causes an easy to understand build error if it's missing a corresponding inherited type!
}
// My base type is 'Animal'
abstract partial record Animal( [JsonDiscriminator] AnimalType type, string Name );
// Animals with type = 'Insect' will automatically deserialize as `Insect`
record Insect(int NumLegs = 6, int NumEyes=4) : Animal(AnimalType.Insect, "Insectoid");
record Mammal(int NumNipples = 2) : Animal(AnimalType.Mammal, "Mammalian");
record Reptile(bool ColdBlooded = true) : Animal(AnimalType.Reptile, "Reptilian");
I want to throw in another implementation suitable for hierarchical, secure, bi-directional, generic usage.
The following caveats
It is a performance and memory "nightmare" but good enough for most scenarios (why: because you need to read ahead $type and then would need to go back on the reader).
It works only if the polymorphic base is abstract / never serialized as instance itself (why: because otherwise the regular converter cannot work on the derived classes as it goes into stack overflow).
Works under .NET 6 ... will not in 3.1.
Example
public abstract record QueryClause(); // the abstract is kind of important
public record AndClause(QueryClause[] SubClauses) : QueryClause();
public record OrClause(QueryClause[] SubClauses) : QueryClause();
// ...
JsonSerializerOptions options = new JsonSerializerOptions();
options.Converters.Add(new BaseClassConverter<QueryClause>(
typeof(AndClause),
typeof(OrClause)));
// ...
Converter
public class BaseClassConverter<TBaseType> : JsonConverter<TBaseType>
where TBaseType : class
{
private readonly Type[] _types;
private const string TypeProperty = "$type";
public BaseClassConverter(params Type[] types)
{
_types = types;
}
public override bool CanConvert(Type type)
=> typeof(TBaseType) == type; // only responsible for the abstract base
public override TBaseType Read(
ref Utf8JsonReader reader,
Type typeToConvert,
JsonSerializerOptions options)
{
TBaseType result;
if (JsonDocument.TryParseValue(ref reader, out var doc))
{
if (doc.RootElement.TryGetProperty(TypeProperty, out var typeProperty))
{
var typeName = typeProperty.GetString();
var type = _types.FirstOrDefault(t => t.Name == typeName) ?? throw new JsonException($"{TypeProperty} specifies an invalid type");
var rootElement = doc.RootElement.GetRawText();
result = JsonSerializer.Deserialize(rootElement, type, options) as TBaseType ?? throw new JsonException("target type could not be serialized");
}
else
{
throw new JsonException($"{TypeProperty} missing");
}
}
else
{
throw new JsonException("Failed to parse JsonDocument");
}
return result;
}
public override void Write(
Utf8JsonWriter writer,
TBaseType value,
JsonSerializerOptions options)
{
var type = value.GetType();
if (_types.Any(t => type.Name == t.Name))
{
var jsonElement = JsonSerializer.SerializeToElement(value, type, options);
var jsonObject = JsonObject.Create(jsonElement) ?? throw new JsonException();
jsonObject[TypeProperty] = type.Name;
jsonObject.WriteTo(writer, options);
}
else
{
throw new JsonException($"{type.Name} with matching base type {typeof(TBaseType).Name} is not registered.");
}
}
}
If you find something, shoot me a comment.
Some kudos to 1.
currently with new feature of .NET 7 we can do this without write handy codes to implement this.
see here: https://devblogs.microsoft.com/dotnet/announcing-dotnet-7-preview-5/
[JsonDerivedType(typeof(Derived1), 0)]
[JsonDerivedType(typeof(Derived2), 1)]
[JsonDerivedType(typeof(Derived3), 2)]
public class Base { }
JsonSerializer.Serialize<Base>(new Derived2()); // { "$type" : 1, ... }
i hope this can help you
I changed a couple things based on ahsonkhan's answer.
Personally I like this way since the client can just give their object to the server.
However, the 'Type' property must be first in the object.
Base class and derived classes:
public interface IBaseClass
{
public DerivedType Type { get; set; }
}
public class DerivedA : IBaseClass
{
public DerivedType Type => DerivedType.DerivedA;
public string Str { get; set; }
}
public class DerivedB : IBaseClass
{
public DerivedType Type => DerivedType.DerivedB;
public bool Bool { get; set; }
}
private enum DerivedType
{
DerivedA = 0,
DerivedB = 1
}
You can create JsonConverter<IBaseClass> that reads and checks the 'Type' property while serializing. It will use that to figure out which type to deserialize.
The reader has to be copied since we read the first property as the type. And then we have to read the full object again (pass it to the Deserialize method).
public class BaseClassConverter : JsonConverter<IBaseClass>
{
public override IBaseClass Read(
ref Utf8JsonReader reader,
Type typeToConvert,
JsonSerializerOptions options)
{
// Creating a copy of the reader (The derived deserialisation has to be done from the start)
Utf8JsonReader typeReader = reader;
if (reader.TokenType != JsonTokenType.StartObject)
{
throw new JsonException();
}
if (!reader.Read() || reader.TokenType != JsonTokenType.PropertyName)
{
throw new JsonException();
}
if (!reader.Read() || reader.TokenType != JsonTokenType.Number)
{
throw new JsonException();
}
IBaseClass baseClass = default;
DerivedType type= (DerivedType)reader.GetInt32();
switch (type)
{
case DerivedType.DerivedA:
baseClass = (DerivedA)JsonSerializer.Deserialize(ref reader, typeof(DerivedA));
break;
case DerivedType.DerivedB:
baseClass = (DerivedB)JsonSerializer.Deserialize(ref reader, typeof(DerivedB));
break;
default:
throw new NotSupportedException();
}
return baseClass;
}
public override void Write(
Utf8JsonWriter writer,
IBaseClass value,
JsonSerializerOptions options)
{
switch(value)
{
case DerivedA derivedA:
JsonSerializer.Serialize(writer, derivedA, options);
break;
case DerivedB derivedB:
JsonSerializer.Serialize(writer, derivedB, options);
break;
default:
throw new NotSupportedException();
}
}
}
The client is now able to send objects as follows:
// DerivedA
{
"Type": 0,
"Str": "Hello world!"
}
// DerivedB
{
"Type": 1,
"Bool": false
}
EDIT:
Edited the Read method to be able to deal with the property name not being in the first order. Now it just reads through the json and stops until it finds the 'Type' property name
public override IBaseClass Read(
ref Utf8JsonReader reader,
Type typeToConvert,
JsonSerializerOptions options)
{
Utf8JsonReader typeReader = reader;
if (typeReader.TokenType != JsonTokenType.StartObject)
{
throw new JsonException();
}
while (typeReader.Read())
{
if (typeReader.TokenType != JsonTokenType.PropertyName)
{
throw new JsonException();
}
string propertyName = typeReader.GetString();
if (propertyName.Equals(nameof(IBaseClass.Type)))
{
break;
}
typeReader.Skip();
}
if (!typeReader.Read() || typeReader.TokenType != JsonTokenType.Number)
{
throw new JsonException();
}
IGraphOptions baseClass = default;
GraphType type = (GraphType)typeReader.GetInt32();
....
// The switch..
....
To be honest, I think the way this custom System.Text JsonConverter is set up is unneccesary complex and I prefer the Newtonsoft JsonConverter.
Basing on the accepted answer, but using KnownTypeAttribute to discover the types (often enumerating all types can lead to unwanted type load exceptions) , and adding the discriminator property in the converter instead of having the class implement it itself:
public class TypeDiscriminatorConverter<T> : JsonConverter<T>
{
private readonly IEnumerable<Type> _types;
public TypeDiscriminatorConverter()
{
var type = typeof(T);
var knownTypes = type.GetCustomAttributes(typeof(KnownTypeAttribute), false).OfType<KnownTypeAttribute>();
_types = knownTypes.Select(x => x.Type).ToArray();
}
public override T Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
{
if (reader.TokenType != JsonTokenType.StartObject)
{
throw new JsonException();
}
using (var jsonDocument = JsonDocument.ParseValue(ref reader))
{
if (!jsonDocument.RootElement.TryGetProperty("discriminator",
out var typeProperty))
{
throw new JsonException();
}
var type = _types.FirstOrDefault(x => x.FullName == typeProperty.GetString());
if (type == null)
{
throw new JsonException();
}
var jsonObject = jsonDocument.RootElement.GetRawText();
var result = (T)JsonSerializer.Deserialize(jsonObject, type, options);
return result;
}
}
public override void Write(Utf8JsonWriter writer, T value, JsonSerializerOptions options)
{
writer.WriteStartObject();
using (JsonDocument document = JsonDocument.Parse(JsonSerializer.Serialize(value)))
{
writer.WritePropertyName("discriminator");
writer.WriteStringValue(value.GetType().FullName);
foreach (var property in document.RootElement.EnumerateObject())
{
property.WriteTo(writer);
}
}
writer.WriteEndObject();
}
}
which you can use like this:
[JsonConverter(typeof(JsonInheritanceConverter))]
[KnownType(typeof(DerivedA))]
[KnownType(typeof(DerivedB))]
public abstract class BaseClass
{
//..
}
Don't write like this
public override bool CanConvert(Type type)
{
return typeof(BaseClass).IsAssignableFrom(type);
}
If you class contain baseClass property then you deserialize him like baseClass.
If you baseClass is abstract and contain baseClass property then you got Exception.
It's safer to write like this:
public class BaseClass
{
public int Int { get; set; }
}
public class DerivedA : BaseClass
{
public string Str { get; set; }
public BaseClass derived { get; set; }
}
public class DerivedB : BaseClass
{
public bool Bool { get; set; }
public BaseClass derived { get; set; }
}
public class BaseClassConverter : JsonConverter<BaseClass>
{
private enum TypeDiscriminator
{
BaseClass = 0,
DerivedA = 1,
DerivedB = 2
}
public override bool CanConvert(Type type)
{
return typeof(BaseClass) == type;
}
public override BaseClass Read(
ref Utf8JsonReader reader,
Type typeToConvert,
JsonSerializerOptions options)
{
if (reader.TokenType != JsonTokenType.StartObject)
{
throw new JsonException();
}
if (!reader.Read()
|| reader.TokenType != JsonTokenType.PropertyName
|| reader.GetString() != "TypeDiscriminator")
{
throw new JsonException();
}
if (!reader.Read() || reader.TokenType != JsonTokenType.Number)
{
throw new JsonException();
}
BaseClass baseClass;
TypeDiscriminator typeDiscriminator = (TypeDiscriminator)reader.GetInt32();
switch (typeDiscriminator)
{
case TypeDiscriminator.DerivedA:
if (!reader.Read() || reader.GetString() != "TypeValue")
{
throw new JsonException();
}
if (!reader.Read() || reader.TokenType != JsonTokenType.StartObject)
{
throw new JsonException();
}
baseClass = (DerivedA)JsonSerializer.Deserialize(ref reader, typeof(DerivedA), options);
break;
case TypeDiscriminator.DerivedB:
if (!reader.Read() || reader.GetString() != "TypeValue")
{
throw new JsonException();
}
if (!reader.Read() || reader.TokenType != JsonTokenType.StartObject)
{
throw new JsonException();
}
baseClass = (DerivedB)JsonSerializer.Deserialize(ref reader, typeof(DerivedB), options);
break;
case TypeDiscriminator.BaseClass:
if (!reader.Read() || reader.GetString() != "TypeValue")
{
throw new JsonException();
}
if (!reader.Read() || reader.TokenType != JsonTokenType.StartObject)
{
throw new JsonException();
}
baseClass = (BaseClass)JsonSerializer.Deserialize(ref reader, typeof(BaseClass));
break;
default:
throw new NotSupportedException();
}
if (!reader.Read() || reader.TokenType != JsonTokenType.EndObject)
{
throw new JsonException();
}
return baseClass;
}
public override void Write(
Utf8JsonWriter writer,
BaseClass value,
JsonSerializerOptions options)
{
writer.WriteStartObject();
if (value is DerivedA derivedA)
{
writer.WriteNumber("TypeDiscriminator", (int)TypeDiscriminator.DerivedA);
writer.WritePropertyName("TypeValue");
JsonSerializer.Serialize(writer, derivedA, options);
}
else if (value is DerivedB derivedB)
{
writer.WriteNumber("TypeDiscriminator", (int)TypeDiscriminator.DerivedB);
writer.WritePropertyName("TypeValue");
JsonSerializer.Serialize(writer, derivedB, options);
}
else if (value is BaseClass baseClass)
{
writer.WriteNumber("TypeDiscriminator", (int)TypeDiscriminator.BaseClass);
writer.WritePropertyName("TypeValue");
JsonSerializer.Serialize(writer, baseClass);
}
else
{
throw new NotSupportedException();
}
writer.WriteEndObject();
}
}
But you BaseClass don't must contain property with type BaseClass or inheritor.
For interface property deserialization I've created a simple StaticTypeMapConverter
public class StaticTypeMapConverter<SourceType, TargetType> : JsonConverter<SourceType>
where SourceType : class
where TargetType : class, new()
{
public override SourceType Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
{
if (reader.TokenType != JsonTokenType.StartObject)
{
throw new JsonException();
}
using (var jsonDocument = JsonDocument.ParseValue(ref reader))
{
var jsonObject = jsonDocument.RootElement.GetRawText();
var result = (SourceType)JsonSerializer.Deserialize(jsonObject, typeof(TargetType), options);
return result;
}
}
public override void Write(Utf8JsonWriter writer, SourceType value, JsonSerializerOptions options)
{
JsonSerializer.Serialize(writer, (object)value, options);
}
}
You can use it like this:
var jsonSerializerOptions = new JsonSerializerOptions()
{
Converters = {
new StaticTypeMapConverter<IMyInterface, MyImplementation>(),
new StaticTypeMapConverter<IMyInterface2, MyInterface2Class>(),
},
WriteIndented = true
};
var config = JsonSerializer.Deserialize<Config>(configContentJson, jsonSerializerOptions);
Not very elegant or efficient, but quick to code for a small number of child types:
List<Dictionary<string, object>> generics = JsonSerializer.Deserialize<List<Dictionary<string, object>>>(json);
List<InputOutputInstanceDto> result = new List<ParentType>();
foreach (Dictionary<string, object> item in generics)
{
switch(item["dataType"]) // use whatever field is in your parent/interface
{
case "Type1":
result.Add(JsonSerializer.Deserialize<Type1>(
JsonSerializer.Serialize(item)));
break
// add cases for each child type supported
default:
result.Add(JsonSerializer.Deserialize<ParentType>(
JsonSerializer.Serialize(item)));
break;
}
}
I like to share with you an issue I found using System.Text.Json. I followed the approach TypeDiscriminatorConverter that Demetrius Axenowski. It works very well.
My problems started when I added some annotations for the JSON. For example:
[JsonPropertyName("name")]
I have lost all day to understand why the code didn't work. I created some dummy code to understand where the problem was. All the source code is now on GitHub.
So, the problem was in the JsonPropertyName for the property I check in the converter. For example, this is a class
public class Radiobutton : ElementBase
{
[JsonPropertyName("type")]
public string Type => "Radiobutton";
public ElementType ElementType = ElementType.Radiobutton;
public List<string>? Choices { get; set; }
}
As you can see, I set the JsonPropertyName because I like to see type in lower case. Now, if I convert the class with this converter:
public class ElementTypeConverter<T> : JsonConverter<T> where T : IElementType
{
private readonly IEnumerable<Type> _types;
public ElementTypeConverter()
{
var type = typeof(T);
_types = AppDomain.CurrentDomain.GetAssemblies()
.SelectMany(s => s.GetTypes())
.Where(p => type.IsAssignableFrom(p) && p.IsClass && !p.IsAbstract)
.ToList();
}
public override T Read(ref Utf8JsonReader reader,
Type typeToConvert, JsonSerializerOptions options)
{
if (reader.TokenType != JsonTokenType.StartObject)
{
throw new JsonException();
}
using (var jsonDocument = JsonDocument.ParseValue(ref reader))
{
if (!jsonDocument.RootElement.TryGetProperty(
nameof(IElementType.Type), out var typeProperty))
{
throw new JsonException();
}
var type = _types.FirstOrDefault(x => x.Name ==
typeProperty.GetString());
if (type == null)
{
throw new JsonException();
}
var jsonObject = jsonDocument.RootElement.GetRawText();
var result = (T)JsonSerializer.Deserialize(jsonObject, type, options);
return result;
}
}
public override void Write(Utf8JsonWriter writer, T value,
JsonSerializerOptions options)
{
JsonSerializer.Serialize(writer, (object)value, options);
}
}
I get the following error:
Test method SurveyExampleNetStardard21.Tests.UnitTest1.TestConversionJson_SystemTextJson_3Textbox_1radiobutton threw exception:
System.Text.Json.JsonException: The JSON value could not be converted to System.Collections.Generic.List`1[SurveyExampleNetStardard21.Interfaces.IElement]. Path: $.Elements[3] | LineNumber: 42 | BytePositionInLine: 5.
I removed the JsonPropertyName and it works fine. I tried to set
[JsonPropertyName("Type")]
(basically, the same as the variable) and it works fine. So, don't change the name. The converter is working both ways (object to Json and Json to object). This is the test code:
var jsonSerializerOptions = new JsonSerializerOptions()
{
Converters = { new ElementTypeConverter<IElement>() },
WriteIndented = true
};
var json = JsonSerializer.Serialize(form, jsonSerializerOptions);
var back = JsonSerializer.Deserialize<Form>(json, jsonSerializerOptions);
var json2 = JsonSerializer.Serialize(back, jsonSerializerOptions);
Another annotation is related to Newtonsoft.Json: I converted the object to Json and it was good without any particular configuration. When I tried to convert the result Json in the object, I got issues in the conversion.
Polymorphism support is released as preview versions(v7).
https://github.com/dotnet/runtime/issues/63747
I have a generic type that wraps a single primitive type to give it value equality semantics
public class ValueObject<T>
{
public T Value { get; }
public ValueObject(T value) => Value = value;
// various other equality members etc...
}
It is used like:
public class CustomerId : ValueObject<Guid>
{
public CustomerId(Guid value) : base(value) { }
}
public class EmailAddress : ValueObject<string>
{
public EmailAddress(string value) : base(value) { }
}
The issue is when serializing a type like:
public class Customer
{
public CustomerId Id { get; }
public EmailAddress Email { get; }
public Customer(CustomerId id, EmailAddress email)
{
Id = id;
Email = email;
}
}
Each object the inherits from ValueObject<T> is wrapped in a Value property (as expected). For example
var customerId = new CustomerId(Guid.NewGuid());
var emailAddress = new EmailAddress("some#email.com");
var customer = new Customer(customerId, emailAddress);
var customerAsJson = JsonConvert.SerializeObject(customer, Formatting.Indented, new JsonSerializerSettings
{
ContractResolver = new CamelCasePropertyNamesContractResolver()
})
Results in
{
"id": {
"value": "f5ce21a5-a0d1-4888-8d22-6f484794ac7c"
},
"email": {
"value": "some#email.com"
}
}
Is there a way to write a custom JsonConverter so the the Value property is excluded for types subclassing ValueObject<T> so that the above example would output
{
"id": "f5ce21a5-a0d1-4888-8d22-6f484794ac7c",
"email": "some#email.com"
}
I would prefer to have a single JsonConverter that can handle all ValueObject<T> rather than having to define a separate JsonConverter for each ValueObject<T> subclass
My first attempt was
public class ValueObjectOfTConverter : JsonConverter
{
private static readonly Type ValueObjectGenericType = typeof(ValueObject<>);
private static readonly string ValuePropertyName = nameof(ValueObject<object>.Value);
public override bool CanConvert(Type objectType) =>
IsSubclassOfGenericType(objectType, ValueObjectGenericType);
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
// converts "f5ce21a5-a0d1-4888-8d22-6f484794ac7c" => "value": "f5ce21a5-a0d1-4888-8d22-6f484794ac7c"
var existingJsonWrappedInValueProperty = new JObject(new JProperty(ValuePropertyName, JToken.Load(reader)));
return existingJsonWrappedInValueProperty.ToObject(objectType, serializer);
}
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
// to implement
}
private static bool IsSubclassOfGenericType(Type typeToCheck, Type openGenericType)
{
while (typeToCheck != null && typeToCheck != typeof(object))
{
var cur = typeToCheck.IsGenericType ? typeToCheck.GetGenericTypeDefinition() : typeToCheck;
if (openGenericType == cur) return true;
typeToCheck = typeToCheck.BaseType;
}
return false;
}
}
You can do this with a custom JsonConverter similar to the ones shown in Json.Net: Serialize/Deserialize property as a value, not as an object. However, since ValueObject<T> does not have a non-generic method to get and set the Value as an object, you will need to use reflection.
Here's one approach:
class ValueConverter : JsonConverter
{
static Type GetValueType(Type objectType)
{
return objectType
.BaseTypesAndSelf()
.Where(t => t.IsGenericType && t.GetGenericTypeDefinition() == typeof(ValueObject<>))
.Select(t => t.GetGenericArguments()[0])
.FirstOrDefault();
}
public override bool CanConvert(Type objectType)
{
return GetValueType(objectType) != null;
}
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
// You need to decide whether a null JSON token results in a null ValueObject<T> or
// an allocated ValueObject<T> with a null Value.
if (reader.SkipComments().TokenType == JsonToken.Null)
return null;
var valueType = GetValueType(objectType);
var value = serializer.Deserialize(reader, valueType);
// Here we assume that every subclass of ValueObject<T> has a constructor with a single argument, of type T.
return Activator.CreateInstance(objectType, value);
}
const string ValuePropertyName = nameof(ValueObject<object>.Value);
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
var contract = (JsonObjectContract)serializer.ContractResolver.ResolveContract(value.GetType());
var valueProperty = contract.Properties.Where(p => p.UnderlyingName == ValuePropertyName).Single();
// You can simplify this to .Single() if ValueObject<T> has no other properties:
// var valueProperty = contract.Properties.Single();
serializer.Serialize(writer, valueProperty.ValueProvider.GetValue(value));
}
}
public static partial class JsonExtensions
{
public static JsonReader SkipComments(this JsonReader reader)
{
while (reader.TokenType == JsonToken.Comment && reader.Read())
;
return reader;
}
}
public static class TypeExtensions
{
public static IEnumerable<Type> BaseTypesAndSelf(this Type type)
{
while (type != null)
{
yield return type;
type = type.BaseType;
}
}
}
You could then apply the converter directly to ValueType<T> like so:
[JsonConverter(typeof(ValueConverter))]
public class ValueObject<T>
{
// Remainder unchanged
}
Or apply it in settings instead:
var settings = new JsonSerializerSettings
{
Converters = { new ValueConverter() },
ContractResolver = new CamelCasePropertyNamesContractResolver()
};
var customerAsJson = JsonConvert.SerializeObject(customer, Formatting.Indented, settings);
Working sample .Net fiddle #1 here.
Alternatively, you might consider adding a non-generic method to access the value as an object, e.g. like so:
public interface IHasValue
{
object GetValue(); // A method rather than a property to ensure the non-generic value is never serialized directly.
}
public class ValueObject<T> : IHasValue
{
public T Value { get; }
public ValueObject(T value) => Value = value;
// various other equality members etc...
#region IHasValue Members
object IHasValue.GetValue() => Value;
#endregion
}
With this addition, WriteJson() becomes much simpler:
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
serializer.Serialize(writer, ((IHasValue)value).GetValue());
}
Working sample .Net fiddle #2 here.
Notes:
ReadJson() assumes that every subclass of Value<T> has a public constructor taking a single argument of type T.
Applying the converter directly to ValueType<T> using [JsonConverter(typeof(ValueConverter))] will have slightly better performance, since CanConvert need never get called. See Performance Tips: JsonConverters for details.
You need to decide how to handle a null JSON token. Should it result in a null ValueType<T>, or an allocated ValueType<T> with a null Value?
In the second version of ValueType<T> I implemented IHasValue.GetValue() explicitly to discourage its use in cases where an instance of ValueType<T> is used in statically typed code.
If you really only want to apply the converter to types subclassing ValueObject<T> and not ValueObject<T> itself, in GetValueType(Type objectType) add a call to .Skip(1):
static Type GetValueType(Type objectType)
{
return objectType
.BaseTypesAndSelf()
.Skip(1) // Do not apply the converter to ValueObject<T> when not subclassed
.Where(t => t.IsGenericType && t.GetGenericTypeDefinition() == typeof(ValueObject<>))
.Select(t => t.GetGenericArguments()[0])
.FirstOrDefault();
}
And then apply the converter in JsonSerializerSettings.Converters rather than directly to ValueObject<T>.
Json.Net uses the default .Net Dictionary for Serialization of JSON Dictionaries when i just use the IDictionary interface. I want to replace it against my Custom Version of a Dictionary globally.
Do I need to write a JsonConverter in order to do this or do i just miss a Setting like:
config.DefaultDictType = typeof(MyDict);
somewhere
There isn't a simple setting for this. Properties declared as IDictionary<TKey,TValue> are allocated as Dictionary<TKey, TValue> if the current value is null.
However, if you pre-allocate the custom dictionaries in the default constructors for your classes, Json.NET will use the pre-existing allocated dictionary rather than allocating a new one. (This is why JsonConverter.ReadJson has an existingValue argument.)
For instance:
public class MyDictionary<TKey, TValue> : Dictionary<TKey, TValue>
{
}
public class DictionaryContainer
{
public DictionaryContainer()
{
this.Values = new MyDictionary<string, string>();
}
public IDictionary<string, string> Values { get; set; }
}
public static class TestClass
{
public static void Test()
{
var container = new DictionaryContainer();
container.Values["one"] = "first";
container.Values["two"] = "second";
container.Values["three"] = "third";
Debug.Assert(container.Values.GetType() == typeof(MyDictionary<string, string>)); // No assert
var json = JsonConvert.SerializeObject(container, Formatting.Indented);
var container2 = JsonConvert.DeserializeObject<DictionaryContainer>(json);
Debug.Assert(container.Values.GetType() == container2.Values.GetType());// No assert
}
}
If this is impractical (because of, e.g., issues with legacy code), the easiest way to achieve what you want is probably with a single global converter that, when reading, allocates dictionaries for all properties defined to return type IDictionary<TKey,TValue> for any key and value:
public abstract class CustomDictionaryTypeReaderBase : JsonConverter
{
protected abstract Type CreateDictionaryType(Type tKey, Type tValue);
bool GetIDictionaryGenericParameters(Type objectType, out Type tKey, out Type tValue)
{
tKey = tValue = null;
if (!objectType.IsGenericType)
return false;
var genericType = objectType.GetGenericTypeDefinition();
if (genericType != typeof(IDictionary<,>))
return false;
var args = objectType.GetGenericArguments();
if (args.Length != 2)
tKey = tValue = null;
tKey = args[0];
tValue = args[1];
return true;
}
public override bool CanConvert(Type objectType)
{
Type tKey, tValue;
return GetIDictionaryGenericParameters(objectType, out tKey, out tValue);
}
public override bool CanWrite { get { return false; } }
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
if (existingValue != null)
{
serializer.Populate(reader, existingValue);
return existingValue;
}
else
{
Type tKey, tValue;
bool ok = GetIDictionaryGenericParameters(objectType, out tKey, out tValue);
if (!ok)
{
return serializer.Deserialize(reader, objectType);
}
else
{
var realType = CreateDictionaryType(tKey, tValue);
Debug.Assert(objectType.IsAssignableFrom(realType));
return serializer.Deserialize(reader, realType);
}
}
}
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
throw new NotImplementedException();
}
}
public sealed class MyCustomDictionaryTypeReader : CustomDictionaryTypeReaderBase
{
protected override Type CreateDictionaryType(Type tKey, Type tValue)
{
var dictType = typeof(MyDictionary<,>).MakeGenericType(new[] { tKey, tValue });
return dictType;
}
}
And then use it like:
var settings = new JsonSerializerSettings();
settings.Converters = new[] { new MyCustomDictionaryTypeReader() };
var multiContainer2 = JsonConvert.DeserializeObject<MultipleDictionaryContainer>(json2, settings);
Another way to achieve what you wanted is by modifying the DefaultContractResolver, override its CreateDictionaryContract() method, and then set it as the serializer's default ContractResolver.
Assuming your custom dictionary is defined as:
public class CustomDictionary<TKey, TValue> : Dictionary<TKey, TValue>
{
public string Custom { get { return "I'm Custom"; } }
}
You could implement the custom contract resolver as:
public class CustomDictionaryContractResolver : DefaultContractResolver
{
public CustomDictionaryContractResolver() : base(true) { }
protected override JsonDictionaryContract CreateDictionaryContract(Type objectType)
{
// let the base class do the heavy lifting
var contract = base.CreateDictionaryContract(objectType);
// nothing todo if the created type is already our custom dictionary type
if (IsGenericDefinition(contract.CreatedType, typeof (CustomDictionary<,>)))
return contract;
if (IsGenericDefinition(contract.UnderlyingType, typeof(IDictionary<,>)) || (typeof(IDictionary).IsAssignableFrom(contract.UnderlyingType)))
{
contract.CreatedType = typeof(CustomDictionary<,>)
.MakeGenericType(contract.DictionaryKeyType ?? typeof(object), contract.DictionaryValueType ?? typeof(object));
// Set our object instantiation using the default constructor;
// You need to modify this, if your custom dictionary does not
// have a default constructor.
contract.DefaultCreator = () => Activator.CreateInstance(contract.CreatedType);
}
return contract;
}
static bool IsGenericDefinition(Type type, Type genericDefinition)
{
return type.IsGenericType && type.GetGenericTypeDefinition() == genericDefinition;
}
}
And then use it like:
JsonConvert.DefaultSettings = () => new JsonSerializerSettings
{
ContractResolver = new CustomDictionaryContractResolver()
};
For example:
public class MyObject
{
public string FirstName { get; set; }
public string LastName { get; set; }
public IDictionary<string, object> Properties { get; set; }
}
public static class TestClass
{
public static void Test()
{
const string json = #"{""FirstName"": ""John"",""LastName"": ""Smith"",""Properties"": {""Email"": ""john.smith#example.com"",""Level"": 42,""Admin"": true}}";
JsonConvert.DefaultSettings = () => new JsonSerializerSettings
{
ContractResolver = new CustomDictionaryContractResolver()
};
var myObject = JsonConvert.DeserializeObject<MyObject>(json);
Debug.WriteLineIf(myObject.FirstName == "John", "FirstName is John");
Debug.WriteLineIf(myObject.LastName == "Smith", "LastName is Smith");
Debug.WriteLineIf(myObject.Properties != null, "Properties is not NULL");
Debug.WriteLineIf(myObject.Properties.Count == 3, "Properties has 3 items");
Debug.WriteLineIf(myObject.Properties.GetType() == typeof(CustomDictionary<string, object>), "Properties is CustomDictionary<,>");
var customDictionary = myObject.Properties as CustomDictionary<string, object>;
Debug.WriteLineIf(customDictionary != null, "Properties say " + customDictionary.Custom);
}
}
If you run the TestClass.Test() method, you should see the following in the Output:
FirstName is John
LastName is Smith
Properties is not NULL
Properties has 3 items
Properties is CustomDictionary
Properties say I'm Custom