Dynamic Payloads in ASP.NET Core

It has always been possible (but atad 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!