I have a multi-level polymorphic type hierarchy that I previously serialized using the data contract serializers. I would like to convert that to System.Text.Json using the new type hierarchy support in .NET 7. Where should I apply the [JsonDerivedType] attributes so that "grandchild" and other deeply derived subtypes of subtypes can be serialized correctly?

My original type hierarchy looked like this:

[KnownType(typeof(DerivedType))]
public abstract class BaseType { } // Properties omitted

[KnownType(typeof(DerivedOfDerivedType))]
public class DerivedType : BaseType { public string DerivedValue { get; set; } } 

public class DerivedOfDerivedType : DerivedType { public string DerivedOfDerivedValue { get; set; } }

I replaced the [KnownType] attributes with [JsonDerivedType] attributes as follows:

[JsonDerivedType(typeof(DerivedType), "DerivedType:#MyNamespace")]
public abstract class BaseType { } // Properties omitted

[JsonDerivedType(typeof(DerivedOfDerivedType), "DerivedOfDerivedType:#MyNamespace")]
public class DerivedType : BaseType { public string DerivedValue { get; set; } } 

public class DerivedOfDerivedType : DerivedType { public string DerivedOfDerivedValue { get; set; } }

However when I serialize as List<BaseType> as follows:

var list = new List<BaseType> { new DerivedOfDerivedType { DerivedValue = "value 1", DerivedOfDerivedValue = "value of DerivedOfDerived" } };
var json = JsonSerializer.Serialize(list);

I get the following exception:

System.NotSupportedException: Runtime type 'MyNamespace.DerivedOfDerivedType' is not supported by polymorphic type 'MyNamespace.BaseType'. Path: $.
 ---> System.NotSupportedException: Runtime type 'MyNamespace.DerivedOfDerivedType' is not supported by polymorphic type 'MyNamespace.BaseType'.

Where should the JsonDerivedType attributes be applied to make this work?

CodePudding user response：

The [JsonDerivedType] attribute must be applied to every base type (other than System.Object) that might be declared for serialization.

Thus [JsonDerivedType(typeof(DerivedOfDerivedType), "DerivedOfDerivedType:#MyNamespace")] must be duplicated on BaseType and DerivedType like so:

// Derived types of BaseType
[JsonDerivedType(typeof(DerivedType), "DerivedType:#MyNamespace")]
// Derived types of DerivedType copied from DerivedType
[JsonDerivedType(typeof(DerivedOfDerivedType), "DerivedOfDerivedType:#MyNamespace")] 
public abstract class BaseType { } // Properties omitted

[JsonDerivedType(typeof(DerivedOfDerivedType), "DerivedOfDerivedType:#MyNamespace")]
public class DerivedType : BaseType { public string DerivedValue { get; set; } } 

public class DerivedOfDerivedType : DerivedType { public string DerivedOfDerivedValue { get; set; } }

Notes:

• The [JsonDerivedType] attribute must be applied to the intermediate type DerivedType in order to serialize values that are declared to be DerivedType, e.g.:

   var list = new List<DerivedType> { new DerivedOfDerivedType { DerivedValue = "value 1", DerivedOfDerivedValue = "value of DerivedOfDerived" } };
   var json = JsonSerializer.Serialize(list);
  

  If intermediate types in the polymorphic type hierarchy are never serialized independently, [JsonDerivedType] need only be applied only to the root base type.

• While the data contract serializers and XmlSerializer will automatically discover known types of known types recursively during serialization of a base type, it seems that this feature was omitted from System.Text.Json. Thus the application must do this manually by copying [JsonDerivedType] attributes onto all relevant base types, or alternatively writing some custom contract resolver that propagates the derived types to base types automatically.

Demo fiddle here.

CodePudding user response：

I've dabbled with the same task and wrote some POC custom contact resolver which applies all JsonDerivedTypeAttribute's from the hierarchy to the root:

private static void AddNestedDerivedTypes(JsonTypeInfo jsonTypeInfo)
{
    if (jsonTypeInfo.PolymorphismOptions is null)
    {
        return;
    }

    var derivedTypes = jsonTypeInfo.PolymorphismOptions.DerivedTypes
        .Where(t => t.DerivedType.GetCustomAttribute<JsonDerivedTypeAttribute>() != null)
        .Select(t => t.DerivedType)
        .ToList();

    var hashset = new HashSet<Type>(derivedTypes);
    var queue = new Queue<Type>(derivedTypes);
    while (queue.TryDequeue(out var derived))
    {
        if (!hashset.Contains(derived))
        {
            // Todo: handle discriminators
            jsonTypeInfo.PolymorphismOptions.DerivedTypes.Add(new JsonDerivedType(derived, derived.FullName));   
        }


        var attribute = derived.GetCustomAttributes<JsonDerivedTypeAttribute>();
        foreach (var jsonDerivedTypeAttribute in attribute)
        {
            queue.Enqueue(jsonDerivedTypeAttribute.DerivedType);
        }
    }
}

Which can be set up in the options:

var options = new JsonSerializerOptions
{
    TypeInfoResolver = new DefaultJsonTypeInfoResolver
    {
        Modifiers = { AddNestedDerivedTypes }
    }
};
SomeRootType container = ...;
var json = JsonSerializer.Serialize(container, options);
var typedToBase = JsonSerializer.Deserialize<SomeRootType>(json, options);

Obviously implementation is far from perfect and requires a lot of refining both feature- and performance-wise (supporting discriminators from the attributes, possibly caching type infos, maybe even using source generators).

Demo fiddle