C#•3y ago

Custom 400 return type (application/fhir+xml)

Currently working on an API that has to be FHIR compliant. FHIR has it's own conventions for dealing with for example a 400.

I wrote a middleware to handle with these. It works fine for 401,403 and 404 errors. However as soon as I attempt this on a 400 I get this exception:

System.InvalidOperationException: Headers are read-only, response has already started.

System.InvalidOperationException: Headers are read-only, response has already started.

The code used to write the custom error to the response:

if (context.Request.Headers.Accept.Contains("application/fhir+xml"))
            {
                var serializer = new BaseFhirXmlPocoSerializer(FhirRelease.STU3);
                var xml = serializer.SerializeToString(operationOutcome);
                context.Response.ContentType = "application/fhir+xml";
                await context.Response.WriteAsync(xml, Encoding.UTF8);
                return;
            }

if (context.Request.Headers.Accept.Contains("application/fhir+xml"))
            {
                var serializer = new BaseFhirXmlPocoSerializer(FhirRelease.STU3);
                var xml = serializer.SerializeToString(operationOutcome);
                context.Response.ContentType = "application/fhir+xml";
                await context.Response.WriteAsync(xml, Encoding.UTF8);
                return;
            }

This is the same code in the same loccation that also handles the 404 errors. And for those it works perfect. It seems ASP starts writing the instant a controller method returns a "BadRequest". How do I stop that behaviour?