System.Text.Json: How to get already mapped members when deserializing?

[albyho]

public class Song {
  public int Id {get; set;}
  public string? Title {get; set;}
  public string? Foo {get; set;}
  public string? Bar {get; set;}
}

var song = JsonSerializer.Deserialize<Song>("""{"id": 1, "title": "Test", "foo": "a"}""");

How to know song contains only id, title and foo, not bar?

[ghost]

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Issue Details

---

public class Song {
  public int Id {get; set;}
  public string? Title {get; set;}
  public string? Foo {get; set;}
  public string? Bar {get; set;}
}

var song = JsonSerializer.Deserialize<Song>("""{"id": 1, "title": "Test", "foo": "a"}""");

How to know song contains only id, title and foo, not bar?

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

[gregsdennis]

I thhink you have a few options.

Just deserialize it

Deserializing as you have in your code sample will result in song.Bar being null. The downside here is that you can't tell the difference between bar missing from the JSON or bar being present with a JSON-null value.

Parse first

You can parse the JSON text into JsonNode or JsonDocument first. These are JSON DOM types that allow you to examine the structure of the JSON without deserializing it. Then you can simply check the DOM to see if a bar property exists.

I find JsonNode easier to work with, but JsonDocument will give better performance because it's not allocating anything.

JSON Schema

This builds on the previous option since you still have to parse the JSON.

Use something like JsonSchema.Net (disclaimer: it's mine) to define constraints that you want for the JSON with JSON Schema. Then you can evaluate the JSON with the schema, and you'll get output that indicates whether the JSON meets the required constraints.

[eiriktsarpalis]

Deserializing as you have in your code sample will result in song.Bar being null. The downside here is that you can't tell the difference between bar missing from the JSON or bar being present with a JSON-null value.

That's probably what I would do as well. There shouldn't be a compelling semantic difference between bar being explicitly set to null in the JSON or missing since it's not representible in the model but FWIW you can exclude the latter possibility by marking the property as required (or using the JsonRequiredAttribute).

[TonyValenti]

I have a custom type I've created called optional which lets me distinguish between null and not-set values.