Dynamic Payloads in ASP.NET Core

It has always been possible (but a tad problematic) to submit dynamic contents to an ASP.NET Core controller action. In the dark, pre-Core days, we had to implement a custom model binder for this. Now it is no longer the case.

All we need is to declare an action method as having a dynamic parameter, obtained from the request body, and accepting the request through POST:

[HttpPost]
public IActionResult Post([FromBody] dynamic payload)
{
    return Json(new { Ok = true });
}

This will accept any content that we POST, if we use the Content Type application/json, and it will by default be turned into a System.Text.Json.JsonElement, a type of the new JSON API, which is the default serializer of ASP.NET Core 3.

If, for some reason, we want to go back to the previous serializer, JSON.NET (Newtonsoft.Json), we need to do a couple of things:

1. Install the Microsoft.AspNetCore.Mvc.NewtonsoftJson Nuget package (it will bring all of the necessary dependencies)
2. Configure it as the default serializer (AddMvc or AddControllers or AddRazorPages or AddControllersWithViews all allow this):

   services
       .AddMvc()
       .AddNewtonsoftJson();
   

If you do, the payload will instead by a Newtonsoft.Json.Linq.JObject instance. The main difference between the two serializer is outlined here. In the end, System.Text.Json supports most of the usual cases and is already included.

In general, I prefer using strongly-typed models, but there may be use cases for this, particularly when implementing routers, API gateways or similar functionality.

As always, hope this is useful!