Preserved references should avoid calling object converter

[jozkee]

As of now, if the JSON object that is passed to the internal converter for an object type is a reference { "$ref": "#" } we create a JsonElement for such object/property.

Instead we could search into the map of references and take the value from there.

Repro:

private class Node
{
    public Node Child { get; set; }
    public object Sibling { get; set; }
}

[Fact]
public static void PreserveReferenceOnTypesWithObjectConverter()
{
    Node root = new Node();
    root.Child = new Node();
    root.Sibling = root.Child;
    // Ok.
    Assert.Same(root.Child, root.Sibling);

    JsonSerializerOptions opts = new JsonSerializerOptions
    {
        ReferenceHandling = ReferenceHandling.Preserve
    };

    string json = JsonSerializer.Serialize(root, opts);
    /* serialized object:
    {
        "$id": "1",
        "Child": {
            "$id": "2"
        },
        "Sibling": {
            "$ref": "2"
        }
    }
    */

    Node rootCopy = JsonSerializer.Deserialize<Node>(json, opts);
    // Fails, rootCopy.Sibling is a JsonElement.
    Assert.Same(rootCopy.Child, rootCopy.Sibling);
}

cc @ahsonkhan, @steveharter, @layomia

[ahsonkhan]

Marked is as 5.0, since we should resolve this issue before shipping the feature (including the decision whether to do it or not). Otherwise, this behavior change might be considered breaking, if we do it in the future.

[jozkee]

Should we enforce the same semantic rules that we use for non-object types?
I would suggest to avoid throwing and instead fallback on the object converter and return a JsonElement.

Also, is it ok to assume that the fix for this would be to only support $ref on object?

$id is not going to have any special effect and is going to be a property in the resulting JsonElement. One of the reasons for this is that JsonElement is a struct which is not a reference type.