Deserialize interfaces using System.Text.Json.JsonSerializer

[douglasg14b]

#30956 seems to have been the original request for this, however it's closed as completed, but there is still no support for JSON Serialization of interfaces as of .Net 7...?

Following through this chain of issues #30956 -> #30083 -> #63747 -> #71346

---

It's suggested in the implementation noted in #63747 that the following would allow for interface de-serialization:

[JsonPolymorphic(UnknownDerivedTypeHandling = JsonUnknownDerivedTypeHandling.FallBackToNearestAncestor)]
[JsonDerivedType(typeof(Node))]
interface INode 
{
    string Value { get; set; }
}

class Node : INode
{
    public string Value { get; set; }
}

// Should be able to de-serialize to this
class MyClass 
{
    public INode Value { get; set; }
}

Hhowever doing something similar to this results in a pretty clear exception:

System.NotSupportedException: Deserialization of interface types is not supported

All threads I could find related to this feature are closed as completed. So here I am!

Is this a feature already tracked?

Tagging subscribers to this area: @dotnet/area-system-text-json, @gregsdennis
See info in area-owners.md if you want to be subscribed.

Issue Details

---

#30956 seems to have been the original request for this, however it's closed as completed, but there is still no support for JSON Serialization of interfaces as of .Net 7...?

It seems as the chain if linked issues gets longer, the problems get lost along the way? #30956 -> #30083 -> #63747 -> #71346

---

It's suggested in the implementation noted in #63747 that the following would allow for interface de-serialization:

[JsonPolymorphic(UnknownDerivedTypeHandling = JsonUnknownDerivedTypeHandling.FallBackToNearestAncestor)]
[JsonDerivedType(typeof(Node))]
interface INode 
{
    string Value { get; set; }
}

class Node : INode
{
    public string Value { get; set; }
}

This however results in a pretty clear exception:

System.NotSupportedException: Deserialization of interface types is not supported

All threads I could find related to this feature are closed as completed. So here I am!

Is this a feature already tracked?

Author:	douglasg14b
Assignees:	-
Labels:	area-System.Text.Json
Milestone:	-

[eiriktsarpalis]

Deserialization via JsonDerivedType is only supported if you additionally specify a type discriminator on the attribute like so:

string json = JsonSerializer.Serialize(new Node { Value = "value" }); // { "$type" : "node", "Value" : "value" }
JsonSerializer.Deserialize<INode>(json) is Node; // true

[JsonDerivedType(typeof(Node), "node")]
interface INode 
{
    string Value { get; set; }
}

Please see the relevant article for more details.

If polymorphism is not what you were specifically looking for, and you just want the serializer to always fill in INode values with instances of type Node, you can configure this via the contract customization feature:

var options = new JsonSerializerOptions
{
     TypeInfoResolver = new DefaultJsonTypeInfoResolver
     {
           Modifiers =
           {
                 static typeInfo =>
                 {
                       if (typeInfo.Type == typeof(INode))
                       {
                             typeInfo.CreateObject = () => new Node();
                       }
                 }
           }
     }
};

JsonSerializer.Deserialize<INode>("{}", options) is Node; // true

Hope this helps.