ASP.NET Core MVC : How to get raw JSON bound to a string without a type?

The cleanest option I've found is adding your own simple InputFormatter:

public class RawJsonBodyInputFormatter : InputFormatter
{
    public RawJsonBodyInputFormatter()
    {
        this.SupportedMediaTypes.Add("application/json");
    }

    public override async Task<InputFormatterResult> ReadRequestBodyAsync(InputFormatterContext context)
    {
        var request = context.HttpContext.Request;
        using (var reader = new StreamReader(request.Body))
        {
            var content = await reader.ReadToEndAsync();
            return await InputFormatterResult.SuccessAsync(content);
        }
    }

    protected override bool CanReadType(Type type)
    {
        return type == typeof(string);
    }
}

And in your Startup.cs inside ConfigureServices:

services
    .AddMvc(options =>
    {
        options.InputFormatters.Insert(0, new RawJsonBodyInputFormatter());
    });

That will let you get at the raw JSON payload in your controllers:

[HttpPost]
public IActionResult Post([FromBody]string value)
{
    // value will be the request json payload
}

The following works in .net core 1.x, but not in .net core 2.x.

As I commented, the solution is to use [FromBody]dynamic data as my parameter list, using dynamic instead of string, and I will receive a JObject.

Caution: If your architecture calls for a single WebApi server to be equally fluent in producing XML and JSON, depending on content-type header entries, this kind of direct-JSON-consumption strategy can backfire on you. (Supporting both XML and JSON on the same service is possible with sufficient work, but then you're taking stuff that was further UP the MVC asset pipeline and moving it down into your controller methods, which turns out to be against the spirit of MVC, where models come to you as POCOs already parsed.)

Once you convert to a string inside the method, converting the incoming JObject (Newtonsoft.JSON in memory data type for JSON) to a string.

Found at other answer here.

Sample code, thanks to Jeson Martajaya:

With dynamic:

[HttpPost]
public System.Net.Http.HttpResponseMessage Post([FromBody]dynamic value)
{
   //...
}

Sample code with JObject:

[HttpPost]
public System.Net.Http.HttpResponseMessage Post([FromBody]Newtonsoft.Json.Linq.JObject value)
{
   //...
}

Found a solution for ASP.NET Core 3.1 Web API.

Looks like following:

public async Task<IActionResult> PutAsync([FromBody] System.Text.Json.JsonElement entity)
{ 
    // your code here
}