Exception parsing json with System.Text.Json.Serialization

In it's current state, JSON Support in .NET Core 3.0 is still not finished, and it seems only a parameterless constructor is supported. It might be, that that feature will be added in future.

One workaround option would be to make a parameterless constructor for your serialized model, when you want to use the new Json API from the .net framework. Probably we shouldn't use constructors for plain datatransfer objects at all, hence I see it as option, not as a workaround.

If you search for a way, on how to migrate from an older version to .net core 3.0, or use Newtonsoft.Json anyway, this is documented here:

MVC:

Install Microsoft.AspNetCore.Mvc.NewtonsoftJson package, and register it to your services:

services.AddMvc().AddNewtonsoftJson();

SignalR:

Install Microsoft.AspNetCore.SignalR.Protocols.NewtonsoftJson package

//Client
new HubConnectionBuilder()
.WithUrl("/chatHub")
.AddNewtonsoftJsonProtocol(...)
.Build();

//Server
services.AddSignalR().AddNewtonsoftJsonProtocol(...);

That way you should* be able to use Json.NET Features in .Net Core 3.0

_{*I don't have installed it, so I can not test it}