JsonBody

Compile and run the project (using .NET Core 3.0 preview 8). The simplest way is to start Visual Studio Code in the project folder, then press F5.

Following routes are handled:

• GET → http://localhost:5000/test/hello.
  This should return Hello, world..
• POST → http://localhost:5000/test/demo.
  This should accept a JSON payload in the body of the POST message and display the formatted JSON in the console.

POST with Content-Type header

Using curl to post an empty JSON payload works only if the Content-Type HTTP header is properly set to application/json:

curl --data "{}" --header "Content-Type: application/json" -X POST http://localhost:5000/test/demo

POST without Content-Type header

Without the Content-Type header, this would not work:

curl --data "{}" --header "Content-Type:" -X POST http://localhost:5000/test/demo

By default, AspNetCore returns this error:

{
  "type": "https://tools.ietf.org/html/rfc7231#section-6.5.13",
  "title": "Unsupported Media Type",
  "status": 415,
  "traceId": "|c74b9116-4d2a19eef496c2f3."
}

Injecting Content-Type with a middleware

It is straightforward with ASP.NET Core to add a dedicated middleware:

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
  // ...
  app.UseMiddleware<AddDefaultContentTypeMiddleware> ();
  // ...
}

The middleware itself is implemented in Middlewares/AddDefaultContentTypeMiddleware.cs.

Basically, all you have to do is implement a constructor and a method named InvokeAsync():

public async Task InvokeAsync(HttpContext context)
{
    if ((context.Request.Method == "POST") &&
        (context.Request.Headers.ContainsKey ("Content-Type") == false))
    {
        //  No Content-Type found for this request...
        context.Request.Headers.Add ("Content-Type", new Microsoft.Extensions.Primitives.StringValues ("application/json"));
    }

    await this.next (context);
}

Injecting Content-Type with a resource filter

Another solution to inject a default content type is the resource filter; it can be provided as an implemention of IAsyncResourceFilter in an attribute class, which can then be used to decorate the action methods where the specific behavior is required:

[AttributeUsage (AttributeTargets.Method | AttributeTargets.Class)]
public class AddDefaultContentTypeAttribute : System.Attribute, IAsyncResourceFilter
{
  public AddDefaultContentTypeAttribute(string contentType) { ... }
  public async Task OnResourceExecutionAsync(ResourceExecutingContext context, ResourceExecutionDelegate next) { ... }
}

The action method (or the whole controller class) can then be decorated with an [AddDefaultContentType] attribute:

[HttpPost]
[AddDefaultContentType ("application/json")]
public IActionResult Demo([FromBody] JObject x)
{
  // ...
}

This provides more flexibility than the middleware and does not impact every request coming in through the pipeline, as would be the case with the first solution (middleware).