Deserializing JSON to abstract class

C# Problem Overview

I am trying to deserialize a JSON string to a concrete class, which inherits from an abstract class, but I just can't get it working. I have googled and tried some solutions but they don't seem to work either.

This is what I have now:

abstract class AbstractClass { }

class ConcreteClass { }

public AbstractClass Decode(string jsonString)
{
    JsonSerializerSettings jss = new JsonSerializerSettings();
    jss.TypeNameHandling = TypeNameHandling.All;
    return (AbstractClass)JsonConvert.DeserializeObject(jsonString, null, jss);
}

However, if I try to cast the resulting object, it just doesn't work.

The reason why I don't use DeserializeObject is that I have many concrete classes.

Any suggestions?

I am using Newtonsoft.Json

C# Solutions

Solution 1 - C#

One may not want to use TypeNameHandling (because one wants more compact json or wants to use a specific name for the type variable other than "$type"). Meanwhile, the customCreationConverter approach will not work if one wants to deserialize the base class into any of multiple derived classes without knowing which one to use in advance.

An alternative is to use an int or other type in the base class and define a JsonConverter.

[JsonConverter(typeof(BaseConverter))]
abstract class Base
{
    public int ObjType { get; set; }
    public int Id { get; set; }
}

class DerivedType1 : Base
{
    public string Foo { get; set; }
}

class DerivedType2 : Base
{
    public string Bar { get; set; }
}

The JsonConverter for the base class can then deserialize the object based on its type. The complication is that to avoid a stack overflow (where the JsonConverter repeatedly calls itself), a custom contract resolver must be used during this deserialization.

public class BaseSpecifiedConcreteClassConverter : DefaultContractResolver
{
    protected override JsonConverter ResolveContractConverter(Type objectType)
    {
        if (typeof(Base).IsAssignableFrom(objectType) && !objectType.IsAbstract)
            return null; // pretend TableSortRuleConvert is not specified (thus avoiding a stack overflow)
        return base.ResolveContractConverter(objectType);
    }
}

public class BaseConverter : JsonConverter
{
    static JsonSerializerSettings SpecifiedSubclassConversion = new JsonSerializerSettings() { ContractResolver = new BaseSpecifiedConcreteClassConverter() };

    public override bool CanConvert(Type objectType)
    {
        return (objectType == typeof(Base));
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        JObject jo = JObject.Load(reader);
        switch (jo["ObjType"].Value<int>())
        {
            case 1:
                return JsonConvert.DeserializeObject<DerivedType1>(jo.ToString(), SpecifiedSubclassConversion);
            case 2:
                return JsonConvert.DeserializeObject<DerivedType2>(jo.ToString(), SpecifiedSubclassConversion);
            default:
                throw new Exception();
        }
        throw new NotImplementedException();
    }

    public override bool CanWrite
    {
        get { return false; }
    }

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        throw new NotImplementedException(); // won't be called because CanWrite returns false
    }
}

That's it. Now you can use serialize/deserialize any derived class. You can also use the base class in other classes and serialize/deserialize those without any additional work:

class Holder
    {
        public List<Base> Objects { get; set; }
    }
string json = @"
        [
            {
                ""Objects"" : 
                [
                    { ""ObjType"": 1, ""Id"" : 1, ""Foo"" : ""One"" },
                    { ""ObjType"": 1, ""Id"" : 2, ""Foo"" : ""Two"" },
                ]
            },
            {
                ""Objects"" : 
                [
                    { ""ObjType"": 2, ""Id"" : 3, ""Bar"" : ""Three"" },
                    { ""ObjType"": 2, ""Id"" : 4, ""Bar"" : ""Four"" },
                ]
            },
        ]";

            List<Holder> list = JsonConvert.DeserializeObject<List<Holder>>(json);
            string serializedAgain = JsonConvert.SerializeObject(list);
            Debug.WriteLine(serializedAgain);

Solution 2 - C#

I would suggest to use CustomCreationConverter in the following way:

public enum ClassDiscriminatorEnum
    {
        ChildClass1,
        ChildClass2
    }

    public abstract class BaseClass
    {
        public abstract ClassDiscriminatorEnum Type { get; }
    }

    public class Child1 : BaseClass
    {
        public override ClassDiscriminatorEnum Type => ClassDiscriminatorEnum.ChildClass1;
        public int ExtraProperty1 { get; set; }
    }

    public class Child2 : BaseClass
    {
        public override ClassDiscriminatorEnum Type => ClassDiscriminatorEnum.ChildClass2;
    }

    public class BaseClassConverter : CustomCreationConverter<BaseClass>
    {
        private ClassDiscriminatorEnum _currentObjectType;

        public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
        {
            var jobj = JObject.ReadFrom(reader);
            _currentObjectType = jobj["Type"].ToObject<ClassDiscriminatorEnum>();
            return base.ReadJson(jobj.CreateReader(), objectType, existingValue, serializer);
        }

        public override BaseClass Create(Type objectType)
        {
            switch (_currentObjectType)
            {
                case ClassDiscriminatorEnum.ChildClass1:
                    return new Child1();
                case ClassDiscriminatorEnum.ChildClass2:
                    return new Child2();
                default:
                    throw new NotImplementedException();
            }
        }
    }

Solution 3 - C#

try something like this

public AbstractClass Decode(string jsonString)
{
    var jss = new JavaScriptSerializer();
    return jss.Deserialize<ConcreteClass>(jsonString);
}

UPDATE
for this scenario methinks all work as you want

public abstract class Base
{
    public abstract int GetInt();
}
public class Der:Base
{
    int g = 5;
    public override int GetInt()
    {
        return g+2;
    }
}
public class Der2 : Base
{
    int i = 10;
    public override int GetInt()
    {
        return i+17;
    }
}

....

var jset = new JsonSerializerSettings() { TypeNameHandling = TypeNameHandling.All };
Base b = new Der()
string json = JsonConvert.SerializeObject(b, jset);
....

Base c = (Base)JsonConvert.DeserializeObject(json, jset);

where c type is test.Base {test.Der}

UPDATE

@Gusman suggest use TypeNameHandling.Objects instead of TypeNameHandling.All. It is enough and it will produce a less verbose serialization.

Solution 4 - C#

Actually, as it has been stated in an update, the simplest way (in 2019) is to use a simple custom pre-defined JsonSerializerSettings, as explained here

        string jsonTypeNameAll = JsonConvert.SerializeObject(priceModels, Formatting.Indented,new JsonSerializerSettings
        {
            TypeNameHandling = TypeNameHandling.All
        });

And for deserializing :

TDSPriceModels models = JsonConvert.DeserializeObject<TDSPriceModels>(File.ReadAllText(jsonPath), new JsonSerializerSettings
        {
            TypeNameHandling = TypeNameHandling.All
        });

Solution 5 - C#

 public class CustomConverter : JsonConverter
{
    private static readonly JsonSerializer Serializer = new JsonSerializer();

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        var jObject = JObject.Load(reader);
        var typeString = jObject.Value<string>("Kind"); //Kind is a property in json , from which we know type of child classes
        var requiredType = RecoverType(typeString);

        return Serializer.Deserialize(jObject.CreateReader(), requiredType);
    }

    private Type RecoverType(string typeString)
    {
        if (typeString.Equals(type of child class1, StringComparison.OrdinalIgnoreCase))
            return typeof(childclass1);
        if (typeString.Equals(type of child class2, StringComparison.OrdinalIgnoreCase))
            return typeof(childclass2);            
        
        throw new ArgumentException("Unrecognized type");
    }

    public override bool CanConvert(Type objectType)
    {
        return typeof(Base class).IsAssignableFrom(objectType) || typeof((Base class) == objectType;
    }

    public override bool CanWrite { get { return false; } }
}

Now add this converter in JsonSerializerSettings as below

   var jsonSerializerSettings = new JsonSerializerSettings();
        jsonSerializerSettings.Converters.Add(new Newtonsoft.Json.Converters.StringEnumConverter());
        jsonSerializerSettings.Converters.Add(new CustomConverter());

After adding serialize or deserialize base class object as below

 JsonConvert.DeserializeObject<Type>("json string", jsonSerializerSettings );

Solution 6 - C#

I had a similar issue, and I solved it with another way, maybe this would help someone: I have json that contains in it several fields that are always the same, except for one field called "data" that can be a different type of class every time. I would like to de-serialize it without analayzing every filed specific. My solution is: To define the main class (with 'Data' field) with , the field Data is type T. Whenever that I de-serialize, I specify the type:

MainClass:

public class MainClass<T>
{
    [JsonProperty("status")]
    public Statuses Status { get; set; }

    [JsonProperty("description")]
    public string Description { get; set; }

    [JsonProperty("data")]
    public T Data { get; set; }
    
    public static MainClass<T> Parse(string mainClsTxt)
    {
        var response = JsonConvert.DeserializeObject<MainClass<T>>(mainClsTxt);
        return response;
        
    }
}

User

public class User
{

    [JsonProperty("id")]
    public int UserId { get; set; }

    [JsonProperty("first_name")]
    public string FirstName { get; set; }

    [JsonProperty("last_name")]
    public string LastName { get; set; }

}

Product

public class Product
{

    [JsonProperty("product_id")]
    public int ProductId { get; set; }

    [JsonProperty("product_name")]
    public string ProductName { get; set; }

    [JsonProperty("stock")]
    public int Stock { get; set; }

}

Using

var v = MainClass<User>.Parse(userJson);
var v2 = MainClass<Product>.Parse(productJson);

json example

userJson: "{"status":1,"description":"my description","data":{"id":12161347,"first_name":"my fname","last_name":"my lname"}}"

productJson: "{"status":1,"description":"my description","data":{"product_id":5,"product_name":"my product","stock":1000}}"

Solution 7 - C#

public abstract class JsonCreationConverter<T> : JsonConverter
{
    protected abstract T Create(Type objectType, JObject jObject);

    public override bool CanConvert(Type objectType)
    {
        return typeof(T) == objectType;
    }

    public override object ReadJson(JsonReader reader,Type objectType,
        object existingValue, JsonSerializer serializer)
    {
        try
        {
            var jObject = JObject.Load(reader);
            var target = Create(objectType, jObject);
            serializer.Populate(jObject.CreateReader(), target);
            return target;
        }
        catch (JsonReaderException)
        {
            return null;
        }
    }

    public override void WriteJson(JsonWriter writer, object value,
        JsonSerializer serializer)
    {
        throw new NotImplementedException();
    }
}

Now implement this interface

public class SportActivityConverter : JsonCreationConverter<BaseSportActivity>
{
    protected override BaseSportActivity Create(Type objectType, JObject jObject)
    {
        BaseSportActivity result = null;
        try
        {
            switch ((ESportActivityType)jObject["activityType"].Value<int>())
            {
                case ESportActivityType.Football:
                    result = jObject.ToObject<FootballActivity>();
                    break;

                case ESportActivityType.Basketball:
                    result = jObject.ToObject<BasketballActivity>();
                    break;
            }
            
        }
        catch(Exception ex)
        {
            Debug.WriteLine(ex);
        }

        return result;
        
    }
}

Content Type	Original Author	Original Content on Stackoverflow
Question	aochagavia	View Question on Stackoverflow
Solution 1 - C#	mbabramo	View Answer on Stackoverflow
Solution 2 - C#	Denis	View Answer on Stackoverflow
Solution 3 - C#	Grundy	View Answer on Stackoverflow
Solution 4 - C#	XavierAM	View Answer on Stackoverflow
Solution 5 - C#	Dalip Choudhary	View Answer on Stackoverflow
Solution 6 - C#	Hadassa Itzkowitz	View Answer on Stackoverflow
Solution 7 - C#	Amir Touitou	View Answer on Stackoverflow