在第一个请求上抛出Web API Rest Service空引用异常

Question

The Setup

We have a .net web api rest service that basically will render a web page as an image and attach that image to an email and send it to an address. We pass the service a JSON email object that contains the from, to, subject, body, mailServerName, and html to render. This service will be called aa lot, however, its the first call that gives the problem.

The problem

The first .net web api rest service request of the day always throws the following exception:

Message: "An error has occurred.", ExceptionMessage: "Object Reference not set to an instance of an object", ExceptionType: "System.NullReferenceException", Stacktrace: "at
EmailService.Controllers.EmailController.Post(Email email) at
lambda_method(Closure, Object, Object[]) at ...

Note:

After adding the custom exceptions below, the exception message is now the following:

ExceptionMessage: "Exception happened while saving Image to Jpeg stream"

Stacktrace:

at EmailService.Controllers.EmailController.Post(Email email) at
lambda_method(Closure, Object, Object[]) at System.Web.Http.Controllers.ReflectedHttpActionDescriptor.ActionExecutor.<>c_DisplayClass10.b_9(Object instance,Object[] methodParameters) at System.Web.Http.Controllers.ReflectedHttpActionDescriptor.ExecuteAsync(HttpControllerContext controllerContext,IDictionary`2 arguments, CancellationToken cancellationToken) at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Tasktask) at System.RuntimeCompilerServices.TaskAwaiter.HandleNonSuccessAnsDebuggerNotification(Task task)....

which is related to this line of code:

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

The service works fine after the first request in performed. Again, its only after a long wait, typically until the next day, do we see the above exception thrown.

The Question

What is causing the service to throw the NullReferenceException and how can I fix it?

The C# code for the controller class and the jQuery that performs the actual requst are below.

Controller class:

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);
        }

    }
}

Service request code:

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

    });

Global.asax

   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

From everything I have been reading it looks like there are some issues using the Single Threaded ApartmentState (STA) in an ASP.NET application. Take a look at the article here which describes how the author was able to work around these issues.

I have not been able to determine why exactly this behavior is only happening for you on the initial request of the day.

Additional resources for this can be found here and here . I hope this helps!

Try changing your generate method to (assuming that m_Bitmap has been constructed elsewhere in that class):

public Bitmap Generate()
{
    return m_Bitmap;
}