System.Text.Json - How to Validate Discriminator Field Presence for Derived Classes during Deserialization

[mstfgueclue]

Hello,

I am working on a project using System.Text.Json for handling polymorphic deserialization and I've encountered a challenge with validating the presence of the discriminator field ($type) before deserialization completes.

Context:
My application uses a base class NotificationDtowith several derived types (like EmailNotificationDto and SmsNotificationDto). The type of derived class to instantiate is determined by a $type discriminator field in the JSON payload. However, .NET resolves this field internally and by the time deserialization completes, if the$typefield is missing, the object automatically falls back to being deserialized as the base type. Consequently, I can't access the$type field for further validation (e.g., in FluentValidation), which prevents me from correctly identifying and handling the derived types post-deserialization.

Here's a JSON structure example:

    {
      "name": "client name",
      "number": "client number,
      "notifications": [
        {
          "$type": "EmailNotification", // derived type
          "email": "example@example.com",
          "subject": "Welcome to Our Service!",
          "body": "Thank you for registering with our service."
        },
        {
          "$type": "SMSNotification", // derived type
          "phoneNumber": "+1234567890",
          "message": "Your order has been shipped and is on its way to you."
        },
        {
          "$type": "Notification", // base type
          "message": "You have a new friend request."
        }
      ]
    }

In the above JSON, if the $type field is missing, the type falls back to the base type, which is not ideal for my use case. I'm aware that one solution could involve adding a dedicated property in the DTO to mirror the $type field and validate its presence, but I'm curious if there's a way to handle this entirely within System.Text.Json attributes or through configuration.

Question:
Is there a mechanism in System.Text.Json to enforce the presence of the $type discriminator in the JSON before falling back to a base type during deserialization? I'd prefer to handle this directly in the library's deserialization process rather than adding workaround properties in my DTOs. It would be really nice having an option like IgnoreMissingTypeDiscriminator

Any suggestions or best practices on how to achieve this with System.Text.Json?

Thanks for any insights you can provide!

microsoft docs about serialization of polymorphic types: here

[eiriktsarpalis]

Hi, the only mechanism available today is to mark the base type as abstract, if possible.