How do I remove the charset from Content-Type in a ASP.NET Core MVC response? - asp.net

Question

No matter what I try I cannot seem to remove the ; charset=utf-8 part from my response's Content-Type header.
[HttpGet("~/appid")]
// Doesn't work
//[Produces("application/fido.trusted-apps+json")]
public string GetAppId()
{
// Doesn't work
Response.ContentType = "application/fido.trusted-apps+json";
// Doesn't work
//Response.ContentType = null;
//Response.Headers.Add("Content-Type", "application/fido.trusted-apps+json");
return JsonConvert.SerializeObject(new
{
foo = true
});
}
I always get application/fido.trusted-apps+json; charset=utf-8 when I only want application/fido.trusted-apps+json.
Note: This is to conform with the FIDO AppID and Facet Specification v1.0 for U2F which states:
The response must set a MIME Content-Type of "application/fido.trusted-apps+json".

Answer 1

I went with the following approach, using middleware to replace the header on the way out. Seems kinda hacky to have to use middleware like this:
Middleware
public class AdjustHeadersMiddleware
{
private readonly RequestDelegate _next;
public AdjustHeadersMiddleware(RequestDelegate next)
{
_next = next;
}
public async Task Invoke(HttpContext httpContext, CurrentContext currentContext)
{
httpContext.Response.OnStarting((state) =>
{
if(httpContext.Response.Headers.Count > 0 && httpContext.Response.Headers.ContainsKey("Content-Type"))
{
var contentType = httpContext.Response.Headers["Content-Type"].ToString();
if(contentType.StartsWith("application/fido.trusted-apps+json"))
{
httpContext.Response.Headers.Remove("Content-Type");
httpContext.Response.Headers.Append("Content-Type", "application/fido.trusted-apps+json");
}
}
return Task.FromResult(0);
}, null);
await _next.Invoke(httpContext);
}
}
Startup.cs Configure()
app.UseMiddleware<AdjustHeadersMiddleware>();

Answer 2

I have found that you can use ContentResult to override this in your controller. So you could achieve what you want by doing the following for example
string bodyJson = JsonConvert.SerializeObject(new
{
foo = true
})
var response = new ContentResult()
{
Content = bodyJson,
ContentType = "application/fido.trusted-apps+json",
StatusCode = (int)System.Net.HttpStatusCode.OK,
};
return response;

Answer 3

If the system requesting your MVC endpoint sends a proper Accept: application/fido.trusted-apps+json, then I believe a custom formatter is what you're looking for.
See:
ASP.Net Core Custom Formatters (sample code)
Write Your Own ASP.NET Core MVC Formatters
It would look something like this (borrowed from the second link):
public class FidoTrustedAppOutputFormatter : IOutputFormatter
{
public FidoTrustedAppOutputFormatter
{
SupportedMediaTypes.Add(MediaTypeHeaderValue.Parse("application/fido.trusted-apps+json"));
}
public bool CanWriteResult(OutputFormatterCanWriteContext context)
{
if (context == null) throw new ArgumentNullException(nameof(context));
if (context.ContentType == null || context.ContentType.ToString() == "application/fido.trusted-apps+json")
return true;
return false;
}
public async Task WriteAsync(OutputFormatterWriteContext context)
{
if (context == null) throw new ArgumentNullException(nameof(context));
var response = context.HttpContext.Response; response.ContentType = "application/fido.trusted-apps+json";
using (var writer = context.WriterFactory(response.Body, Encoding.UTF8))
{
// replace with Json.net implementation
Jil.JSON.Serialize(context.Object, writer);
await writer.FlushAsync();
}
}
}
public class FidoTrustedAppInputFormatter : IInputFormatter
{
public FidoTrustedAppInputFormatter
{
SupportedMediaTypes.Add(MediaTypeHeaderValue.Parse("application/fido.trusted-apps+json"));
}
public bool CanRead(OutputFormatterCanWriteContext context)
{
if (context == null) throw new ArgumentNullException(nameof(context));
if (context.ContentType == null || context.ContentType.ToString() == "application/fido.trusted-apps+json")
return true;
return false;
}
public Task<InputFormatterResult> ReadAsync(InputFormatterContext context)
{
if (context == null) throw new ArgumentNullException(nameof(context));
var request = context.HttpContext.Request; if (request.ContentLength == 0)
{
if (context.ModelType.GetTypeInfo().IsValueType)
return InputFormatterResult.SuccessAsync(Activator.CreateInstance(context.ModelType));
else return InputFormatterResult.SuccessAsync(null);
}
var encoding = Encoding.UTF8;//do we need to get this from the request im not sure yet
using (var reader = new StreamReader(context.HttpContext.Request.Body))
{
var model = Jil.JSON.Deserialize(reader, context.ModelType);
return InputFormatterResult.SuccessAsync(model);
}
}
}
Then register it in your startup:
services.AddMvcCore(options =>
{
options.InputFormatters.Insert(0, new FidoTrustedAppInputFormatter ());
options.OutputFormatters.Insert(0, new FidoTrustedAppOutputFormatter ());
});