I was recently involved in working on some middleware which recieves requests from an asset management system , tranform the data and send it over to an on premise deployment of business management solution.

This middleware has to be implemented as a C# function app. One key feature of this middleware, was to use the guaranteed delivery pattern.

Implementing this as C# function app is quite a trivial which involves.

1. Receive the request.
2. Validate request, throw a validation error if neccessary and send response.
3. Send the request to Service Bus and return an HTTP response

In C# function apps, it is possible to return a HTTP response and send the request to a Service Bus using a class

And this class can be used as follows

However when it comes to returning errors, I encountered a problem using

await response.WriteAsJsonAsync("Some Text");

Using this statement results in the response status code always being 200 OK.
Gone are the days when developers would return a 200 OK status code regardless of the fact that an error has occurred. It is now a must to return the right status code when building APIs.

Code Segments to Trigger Errors

Bad Request with 200 OK Status Code.

Internal ServerError with 200 OK Status Code

Solution

I found that the solution to this is to use

response.WriteString("Some String"); 

and to set the following in the Program.cs

services.Configure<KestrelServerOptions>(options =>{
            options.AllowSynchronousIO  = true;
        });

Updated Code

Status Codes Being Returned Correctly

This took me a while to find this solution, and I hope it saves you a lot of time and headache in trying to debug what the issue.