ServerPrerendered Blazor component in Razor pages doesn't serialize parameter correctly

[jerome-raffalli]

Visual Studio 2019 16.6.1
.Net Core 3.1.4

Using ServerPrerendered the .razor component is initialized twice. First time correctly, the second time incorrectly.

I have the following folder layout:
/Pages/Test/
./RazorShapeComponent.razor
./ShapeContainer.cs
./ShapeBase.cs
./ShapeTriangle.cs

/Pages/
./ShapeTest.cshtml
./ShapeTest.cshtml.cs

Where:
ShapeTriangle extends ShapeBase
ShapeContainer has a List of ShapeBase

RazorShapeComponent.razor takes a ShapeContainer Parameter
[Parameter]
public ShapeContainer ShapeContainer { get; set; }

ShapeTest.cshtml.cs has: ShapeContainer MyShapeContainer { get; set; }
MyShapeContainer is instantiated in the ctor

and:
ShapeTest.cshtml uses the RazorShapeComponent
<component type="typeof(RazorShapeComponent)" render-mode="ServerPrerendered" param-ShapeContainer='@Model.MyShapeContainer' />

Behaviour
Then RazorShapeComponent.razor's OnInitialized() is called twice.

The first time, the ShapeContainer parameter is initialized correctly

The second time, the List is initialized but contains instances of the base type

BugLog.zip

Regards,
Michael

Hello,
I encounter exactly the same problem as Michael. It was wrongly classified as a question, but this is a real issue.
Could you please reexamine it ?

This problem seams to be specific to ServerPrerendered mode, where components are initialized twice.
The first time, parameter is correctly deserialized, but second time the deserialization fails.

Workaround : serialize / deserialize complex types in a string parameter

Many thanks,
Jérôme

[javiercn]

@jerome-raffalli inheritance is not supported during prerendering.

Passing parameters during prerendering is subject to the limitations System.Text.Json imposes, among those, the lack of inheritance support.

As a result if you try to use inheritance the information in the derived type will be lost during deserialization.

This is not something that we plan to change; if you want to support this scenario you can do so by creating and applying your own converter following the docs here