webmethod 回傳錯誤

var httpContext=System.Web.HttpContext.Current;
httpContext.Response.Write(錯誤訊息);
httpContext.Response.StatusCode = (int)System.Net.HttpStatusCode.InternalServerError;
httpContext.Response.Flush();
httpContext.Response.SuppressContent = true;
httpContext.ApplicationInstance.CompleteRequest();

※若為 async 函數，await 後就無法再取得 System.Web.HttpContext.Current，必須先用變數記錄起來，await 之後再填回

var httpcontext = HttpContext.Current;

await xxxAsync().ConfigureAwait(false);

HttpContext.Current = httpcontext;