设置
我们有.net web api休息服务，基本上将网页呈现为图像，并将该图像附加到电子邮件并将其发送到地址。我们将服务传递给JSON电子邮件对象，该对象包含要呈现的from，to，subject，body，mailServerName和html。这项服务将被大量调用，但它是第一次提出问题的电话。
问题
当天的第一个.net web api休息服务请求总是抛出以下异常：
消息：＆＃34;发生了错误。＆＃34;，ExceptionMessage：＆＃34; Object 引用未设置为对象的实例＆＃34;，ExceptionType： ＆＃34; System.NullReferenceException＆＃34;，Stacktrace：＆＃34;在 EmailService.Controllers.EmailController.Post（电子邮件） lambda_method（Closure，Object，Object []）at ...
注意：
在下面添加自定义例外后，现在出现以下异常消息：
ExceptionMessage：＆＃34;将图像保存到Jpeg流时出现异常＆＃34;
堆栈跟踪：
在EmailService.Controllers.EmailController.Post（电子邮件） lambda_method（Closure，Object，Object []）at System.Web.Http.Controllers.ReflectedHttpActionDescriptor.ActionExecutor＆LT;＆GT; c_DisplayClass10.b_9（对象 instance，Object [] methodParameters）at System.Web.Http.Controllers.ReflectedHttpActionDescriptor.ExecuteAsync（HttpControllerContext controllerContext，IDictionary`2参数，CancellationToken cancelToken） System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess（Tasktask） 在 System.RuntimeCompilerServices.TaskAwaiter.HandleNonSuccessAnsDebuggerNotification（任务 任务）......
与此行代码相关：
websiteImage.Save(websiteStream, System.Drawing.Imaging.ImageFormat.Jpeg);
在执行第一个请求后，服务正常工作。再次，只有经过漫长的等待，通常直到第二天，我们才能看到上述异常抛出。
问题
是什么导致服务抛出NullReferenceException以及如何解决它？
控制器类的C＃代码和执行实际请求的jQuery如下所示。
控制器类：
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);
}
}
}
服务请求代码：
$.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; }
}
}