在第一個請求上拋出Web API Rest Service空引用異常

Question

設置

我們有一個.net Web api rest服務，該服務基本上將網頁呈現為圖像並將該圖像附加到電子郵件中並將其發送到地址。 我們向服務傳遞一個JSON電子郵件對象，該對象包含要呈現的from，to，subject，body，mailServerName和html。 該服務將被稱為很多，但是它是第一個帶來問題的調用。

問題

當天的第一個.net Web api REST服務請求始終會引發以下異常：

消息：“發生錯誤。”，ExceptionMessage：“對象引用未設置為對象的實例”，ExceptionType：“ System.NullReferenceException”，Stacktrace：“ at
EmailService.Controllers.EmailController.Post（電子郵件電子郵件）位於
lambda_method（Closure，Object，Object []）在...

注意：

在下面添加了自定義例外之后，現在出現以下例外消息：

ExceptionMessage：“將圖像保存到Jpeg流時發生異常”

堆棧跟蹤：

在EmailService.Controllers.EmailController.Post（電子郵件電子郵件）位於
System.Web.Http.Controllers.ReflectedHttpActionDescriptor.ActionExecutor。<> c_DisplayClass10.b_9（Object instance，Object [] methodParameters）在System.Web.Http.Controllers.ReflectedHttpActionDescriptor.ExecuteAsync（ HttpControllerContext controllerContext，IDictionary`2個參數，System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess（Tasktask）的System.RuntimeCompilerServices.TaskAwaiter.HandleNonSuccessAnsDebuggerNotification（Task任務）....

與這行代碼有關：

websiteImage.Save(websiteStream, System.Drawing.Imaging.ImageFormat.Jpeg);

在執行第一個請求后，該服務運行正常。 同樣，只有經過長時間的等待（通常直到第二天），我們才能看到上述異常。

問題

是什么導致服務引發NullReferenceException，我該如何解決？

控制器類的C＃代碼和執行實際要求的jQuery如下。

控制器類：

namespace EmailService.Controllers
{

    public class EmailController : ApiController
    {

        // POST api/email
        public HttpResponseMessage Post(Email email)
        {

            if (email == null)
            {
                throw new Exception("Email parameter is null");
            }

            Bitmap websiteImage;

            try
            {
                websiteImage = WebsiteToImage.Generate();
            }

            catch (Exception)
            {

                // 500 Internal Server Error
                response.StatusCode = HttpStatusCode.InternalServerError;

                throw new Exception("Exception happened while generating Image");
            }



            // create memory stream from bitmap and save it as a jpeg, this allows us to attach the image from memory, without having to store it on the server
            System.IO.Stream websiteStream = new System.IO.MemoryStream();
            try
            {
                websiteImage.Save(websiteStream, System.Drawing.Imaging.ImageFormat.Jpeg);
            }


            catch (Exception)
            {

                // 500 Internal Server Error
                response.StatusCode = HttpStatusCode.InternalServerError;

                throw new Exception("Exception happened while saving Image to Jpeg stream");
            }


            try
            {

                websiteStream.Position = 0;
            }


            catch (Exception)
            {

                // 500 Internal Server Error
                response.StatusCode = HttpStatusCode.InternalServerError;

                throw new Exception("Exception happened while setting stream posiiton=0");
            }


            // create response with status code 200 OK, since we aren't actually creating anything 
            var response = this.Request.CreateResponse(HttpStatusCode.OK);


            try
            {
                // MailMessage is used to represent the e-mail being sent
                using (MailMessage message = new MailMessage(email.from, email.to, email.subject, email.body))
                {

                    // attach jpeg from memory
                    message.Attachments.Add(new Attachment(websiteStream, "letter.jpg", "image/jpeg"));

                    // create mail client
                    SmtpClient mailClient = new SmtpClient(email.mailServerName);

                    // use the Windows credentials of the account (i.e. user account) 
                    mailClient.UseDefaultCredentials = true;

                    // send the message
                    mailClient.Send(message);
                }

            }
            catch (Exception)
            {

                // 500 Internal Server Error
                response.StatusCode = HttpStatusCode.InternalServerError;

                throw new Exception("Exception happened while creating and sending mail message");

            }

            //return new HttpResponseMessage() { Content = new StringContent(html) };
            return response;

        }

        // PUT api/email/5
        public HttpResponseMessage Put(int id, [FromBody]string value)
        {
            // return response status code of 501 Not Implemented
            return this.Request.CreateResponse(HttpStatusCode.NotImplemented);
        }

    }
}

服務請求代碼：

$.ajax ({
        type: "POST",
        url: "/api/email/",
        contentType: "application/json; charset=utf-8",
        data: JSON.stringify(email),
        success: finish,
        error: error

    });

Global.asax中

   public class MvcApplication : System.Web.HttpApplication
    {
        protected void Application_Start()
        {

            // add details to server errors 
            GlobalConfiguration.Configuration.IncludeErrorDetailPolicy = IncludeErrorDetailPolicy.Always;

            AreaRegistration.RegisterAllAreas();

            WebApiConfig.Register(GlobalConfiguration.Configuration);
            FilterConfig.RegisterGlobalFilters(GlobalFilters.Filters);
            RouteConfig.RegisterRoutes(RouteTable.Routes);


        }
    }

WebApiConfig.cs

 public static class WebApiConfig
    {
        public static void Register(HttpConfiguration config)
        {

            config.Routes.MapHttpRoute(
                name: "DefaultApi",
                routeTemplate: "api/{controller}/{id}",
                defaults: new { id = RouteParameter.Optional }
            );
        }
    }

WebsiteToImage.cs

public class WebsiteToImage
{
    private Bitmap m_Bitmap;
    private string m_Url;
    private string m_FileName = string.Empty;

    private string m_html;

    public WebsiteToImage(string html)
    {
        // Without file 
        //m_Url = url;

        m_html = html;

    }

    public WebsiteToImage(string url, string fileName)
    {
        // With file 
        m_Url = url;
        m_FileName = fileName;
    }

    public Bitmap Generate()
    {
        // Thread 
        var m_thread = new Thread(_Generate);
        m_thread.SetApartmentState(ApartmentState.STA);
        m_thread.Start();
        m_thread.Join();
        return m_Bitmap;
    }

    private void _Generate()
    {
        WebBrowser browser = new WebBrowser { ScrollBarsEnabled = false };
        //browser.Navigate(m_Url);
        browser.DocumentText = m_html;

        browser.DocumentCompleted += WebBrowser_DocumentCompleted;

        while (browser.ReadyState != WebBrowserReadyState.Complete)
        {
            Application.DoEvents();
        }

        browser.Dispose();
    }

    private void WebBrowser_DocumentCompleted(object sender, WebBrowserDocumentCompletedEventArgs e)
    {
        // Capture 
        var browser = (WebBrowser)sender;
        browser.ClientSize = new Size(browser.Document.Body.ScrollRectangle.Width, browser.Document.Body.ScrollRectangle.Bottom);
        browser.ScrollBarsEnabled = false;
        m_Bitmap = new Bitmap(browser.Document.Body.ScrollRectangle.Width, browser.Document.Body.ScrollRectangle.Bottom);
        browser.BringToFront();
        browser.DrawToBitmap(m_Bitmap, browser.Bounds);

        // Save as file? 
        if (m_FileName.Length > 0)
        {
            // Save 
            m_Bitmap.SaveJPG100(m_FileName);
        }
    }
}

public static class BitmapExtensions
{
    public static void SaveJPG100(this Bitmap bmp, string filename)
    {
        var encoderParameters = new EncoderParameters(1);
        encoderParameters.Param[0] = new EncoderParameter(System.Drawing.Imaging.Encoder.Quality, 100L);
        bmp.Save(filename, GetEncoder(ImageFormat.Jpeg), encoderParameters);
    }

    public static void SaveJPG100(this Bitmap bmp, Stream stream)
    {
        var encoderParameters = new EncoderParameters(1);
        encoderParameters.Param[0] = new EncoderParameter(System.Drawing.Imaging.Encoder.Quality, 100L);
        bmp.Save(stream, GetEncoder(ImageFormat.Jpeg), encoderParameters);
    }

    public static ImageCodecInfo GetEncoder(ImageFormat format)
    {
        var codecs = ImageCodecInfo.GetImageDecoders();

        foreach (var codec in codecs)
        {
            if (codec.FormatID == format.Guid)
            {
                return codec;
            }
        }

        // Return 
        return null;
    }
}

Email.cs

namespace EmailService.Models
{
    public class Email
    {
        // email properties
        public string mailServerName {get; set;}
        public string from {get; set;}
        public string to {get; set;}
        public string subject {get; set;}
        public string body {get; set;}
        public string content { get; set; }

    }
}

Answer 1

從我閱讀的所有內容來看，在ASP.NET應用程序中使用單線程ApartmentState（STA）似乎存在一些問題。 在這里看一下這篇文章，它描述了作者如何解決這些問題。

我無法確定為什么這種行為僅在當天的最初請求下才為您發生。

可以在此處和此處找到其他資源。 我希望這有幫助！

嘗試將您的generate方法更改為（假設m_Bitmap已在該類的其他地方構造）：

public Bitmap Generate()
{
    return m_Bitmap;
}