How to return error response?

Hi, ActionFilterAttribute is able to achieve this.

You could refence this sample.

    public class MyfilterAttribute : ActionFilterAttribute
    {
        private readonly string[] _acceptTypes;

        public MyfilterAttribute(params string[] acceptTypes)
        {           
            _acceptTypes = acceptTypes;
        }
        public override void OnActionExecuting(ActionExecutingContext filterContext)
        {
            var headers = filterContext.HttpContext.Request.Headers;
            var acceptHeaderValues = headers.ContainsKey("Accept") ? headers["Accept"] : new StringValues();
            if (!(acceptHeaderValues.Any() && _acceptTypes.Any(d => acceptHeaderValues.Contains(d))))
            {
                filterContext.Result = new JsonResult(new { Error = "An error message here" }) { StatusCode = 415 };
            }
        }
    }

Then add [Myfilter("text/csv")] to your action, it will return your custom response.

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