ASP.Net Core 2.0 - How to return custom json or xml response from middleware?

Question

In ASP.Net Core 2.0, I am trying to return a message formatted as json or xml with a status code. I have no problems returning a custom message from a controller, but I don't know how to deal with it in a middleware.

My middleware class looks like this so far:

public class HeaderValidation
{
    private readonly RequestDelegate _next;
    public HeaderValidation(RequestDelegate next)
    {
        _next = next;
    }

    public async Task Invoke(HttpContext httpContext)
    {
        // How to return a json or xml formatted custom message with a http status code?

        await _next.Invoke(httpContext);
    }
}

Answer 1

To fill response in middleware use httpContext.Response property that returns HttpResponse object for this request. The following code shows how to return 500 response with JSON content:

public async Task Invoke(HttpContext httpContext)
{
    if (<condition>)
    {
       context.Response.StatusCode = 500;  

       context.Response.ContentType = "application/json";

       string jsonString = JsonConvert.SerializeObject(<your DTO class>);

       await context.Response.WriteAsync(jsonString, Encoding.UTF8);

       // to stop futher pipeline execution 
       return;
    }

    await _next.Invoke(httpContext);
}