Answer by CountZero for ASP.NET Core Web API exception handling

Firstly, thanks to Andrei as I've based my solution on his example.

I'm including mine as it's a more complete sample and might save readers some time.

The limitation of Andrei's approach is that doesn't handle logging, capturing potentially useful request variables and content negotiation (it will always return JSON no matter what the client has requested - XML / plain text etc).

My approach is to use an ObjectResult which allows us to use the functionality baked into MVC.

This code also prevents caching of the response.

The error response has been decorated in such a way that it can be serialized by the XML serializer.

public class ExceptionHandlerMiddleware
{
    private readonly RequestDelegate next;
    private readonly IActionResultExecutor<ObjectResult> executor;
    private readonly ILogger logger;
    private static readonly ActionDescriptor EmptyActionDescriptor = new ActionDescriptor();

    public ExceptionHandlerMiddleware(RequestDelegate next, IActionResultExecutor<ObjectResult> executor, ILoggerFactory loggerFactory)
    {
        this.next = next;
        this.executor = executor;
        logger = loggerFactory.CreateLogger<ExceptionHandlerMiddleware>();
    }

    public async Task Invoke(HttpContext context)
    {
        try
        {
            await next(context);
        }
        catch (Exception ex)
        {
            logger.LogError(ex, $"An unhandled exception has occurred while executing the request. Url: {context.Request.GetDisplayUrl()}. Request Data: " + GetRequestData(context));

            if (context.Response.HasStarted)
            {
                throw;
            }

            var routeData = context.GetRouteData() ?? new RouteData();

            ClearCacheHeaders(context.Response);

            var actionContext = new ActionContext(context, routeData, EmptyActionDescriptor);

            var result = new ObjectResult(new ErrorResponse("Error processing request. Server error."))
            {
                StatusCode = (int) HttpStatusCode.InternalServerError,
            };

            await executor.ExecuteAsync(actionContext, result);
        }
    }

    private static string GetRequestData(HttpContext context)
    {
        var sb = new StringBuilder();

        if (context.Request.HasFormContentType && context.Request.Form.Any())
        {
            sb.Append("Form variables:");
            foreach (var x in context.Request.Form)
            {
                sb.AppendFormat("Key={0}, Value={1}<br/>", x.Key, x.Value);
            }
        }

        sb.AppendLine("Method: " + context.Request.Method);

        return sb.ToString();
    }

    private static void ClearCacheHeaders(HttpResponse response)
    {
        response.Headers[HeaderNames.CacheControl] = "no-cache";
        response.Headers[HeaderNames.Pragma] = "no-cache";
        response.Headers[HeaderNames.Expires] = "-1";
        response.Headers.Remove(HeaderNames.ETag);
    }

    [DataContract(Name= "ErrorResponse")]
    public class ErrorResponse
    {
        [DataMember(Name = "Message")]
        public string Message { get; set; }

        public ErrorResponse(string message)
        {
            Message = message;
        }
    }
}