JsonSerializer ignore JsonPolymorphic attribute when own converter is used (Dotvvm 5.0 preview 7)

[Mitroz]

Dotvvm version 5.0 preview 7.
NET 9.0

In VM, I have an workflow graph represented something like this:

[JsonConverter(typeof(WorkflowGraphJsonConverter))]
public class WorkflowGraph : BidirectionalGraph<GraphNode, GraphEdge<GraphNode, string>>
{

Where GraphNode has x descendants:

[JsonPolymorphic(TypeDiscriminatorPropertyName = "$type")]
[JsonDerivedType(typeof(GraphNode), nameof(GraphNode))]
[JsonDerivedType(typeof(ConditionGraphNode), nameof(ConditionGraphNode))]
[JsonDerivedType(typeof(RetryHandlerNode), nameof(RetryHandlerNode))]
[JsonDerivedType(typeof(DelayStep), nameof(DelayStep))]
[JsonDerivedType(typeof(LoopIteratorNode), nameof(LoopIteratorNode))]
.....
public class GraphNode
{

Write method on converter using simple wrapper class where in Nodes list are instances of GraphNode :

    public override void Write(Utf8JsonWriter writer, WorkflowGraph value, JsonSerializerOptions options)
    {
        var wrapper = new WorkflowGraphData
        {
            Nodes = value.Vertices.ToList(),
            Edges = value.Edges.Select(e => new GraphEdgeData
            {
                Source = e.Source.Name,
                Target = e.Target.Name,
                Tag = e.Tag,
                Condition = e.Condition
            }).ToList()
        };

        JsonSerializer.Serialize(writer, wrapper, options);
    }

If I verify the functionality of the converter with a test, everything is serialized correctly.
Dotvvm also uses the converter, verified by breakpoint.

But on client side I get only properties from base class GraphNode:

{
  "$type": "RX2xRc+exIlXuIVJ",
  "Name": "start",
  "Type": "GraphNode"
},
{
  "$type": "RX2xRc+exIlXuIVJ",
  "Name": "copy-files-to-s3:01",
  "Type": "CopyFilesToS3Node"
},
{
  "$type": "RX2xRc+exIlXuIVJ",
  "Name": "register-retention-target:02",
  "Type": "RegisterRetentionTargetStep"
},
{
  "$type": "RX2xRc+exIlXuIVJ",
  "Name": "copy-files-to-s3:01:retry",
  "Type": "RetryHandlerNode"
}

Is polymorphic serialization supported, or is JsonSerializer only used internally?

[exyi]

In short, no, polymorphic serialization is currently not supported.

Longer answer is that you can use it, if you just need to transfer data via DotVVM and don't want to use it bindings. You can safely use such object in JsComponent or other custom JS, and on the server when it's deserialized again. Some things may work in DotVVM bindings, but no promises about that: for instance, there is no way to type checks like x is LoopIteratorNode.

You can probably just mark your polymorphic type with [DotvvmSerialization(DisableDotvvmConverter = true)]. We have a test for this if you need a more complete example https://github.com/riganti/dotvvm/blob/main/src/Tests/ViewModel/SerializerTests.cs#L1452.

Let us know if you any further issues, we want to fully support this solution

[tomasherceg]

I'd like to add that we'd eventually like to support polymorphism, but it is not an easy feature, and it requires at least the support for the is and as operators in bindings.

What we use currently is something like the Table-Per-Hierarchy approach used in relational databases:

public class Field
{
    public FieldType Type { get; set; }   // an enum that serves as discriminator

    // data specific for concrete field types (only one of them is not null)

    public TextValue? TextValue { get; set; }

    public ImageValue? TextValue { get; set; }

    public DateTimeValue? DateTimeValue { get; set; }
}

• It is a good idea to implement IValidatableObject to ensure Type matches the nullability of the concrete properties
• Also, I usually implement OnTypeChanged function that is called when the user changes Type - it initializes or deinitializes the concrete properties so the object is always consistent

You can then render templates for each concrete type by using DataContext="{value: TextValue}" - the template will be rendered only if the TextValue is not null.

We even have the PolymorphTemplateSelector control that renders different templates based on the discriminator.

[exyi]

I decided to add E2E tests for the DisableDotvvmConverter = true solution, and sorry, it doesn't work properly yet. I have a fix in #2007