如何解决NullReferenceException [英] How to resolve NullReferenceException

查看：102
本文介绍了如何解决NullReferenceException的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！

问题描述

我正在做一个简单的邮件应用程序，我想在其中设置一个静态配置文件pic或公司的任何徽标，我编写了以下代码，但我不能正常工作，NullReferenceException即将到来，而我正在调试我的代码。请尝试解决此问题，并提前致谢。



以下是我在按钮点击时写的代码。







I am doing one simple mail Application in which I want to set one static profile pic or any logo of the company I wrote the following code but I its not working and NullReferenceException is coming while I am debugging my code. Please try to solve this and Thanks in Advance.

Below is my code which I wrote in button click.



using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Net;
using System.Net.Mail;
using System.IO;
using System.Net.Mime;

public partial class _Default : System.Web.UI.Page
{
    MailMessage msgobj;
    protected void Page_Load(object sender, EventArgs e)
    {

    }
    protected void btnSend_Click(object sender, EventArgs e)
    {
        string strMailContent = "Welcome new user";
        string ContentId = "image1";
        SmtpClient serverobj = new SmtpClient();

        serverobj.Credentials = new NetworkCredential(txtFrom.Text, txtPassword.Text);
        serverobj.Port = 587;//25
        serverobj.Host = "smtp.gmail.com";
        serverobj.EnableSsl = true;
        String FileName = fileUploadControl.PostedFile.FileName;
        string path = Server.MapPath("~/download.jpg");
        LinkedResource logo = new LinkedResource(path);
        logo.ContentId = "download.jpg";
        //AlternateView av1 = new AlternateView();
        AlternateView av1 = AlternateView.CreateAlternateViewFromString("<html><body><img src=cid:http://localhost:58146/WebMail/download.jpg /><br></body></html>" + strMailContent, null, MediaTypeNames.Text.Html);
        av1.LinkedResources.Add(logo);

        msgobj.AlternateViews.Add(av1);
        msgobj = new MailMessage();
        msgobj.From = new MailAddress(txtFrom.Text, "Mukesh", System.Text.Encoding.UTF8);
        string s = @"<br /> <img src=""http://localhost:58146/WebMail/download.jpg"" alt=""Logo"" /><br />";

        msgobj.Body = s + txtMessageBox.Text;
        msgobj.To.Add(txtTO.Text);
        msgobj.Subject = txtSubjcet.Text;
       
        msgobj.IsBodyHtml = true;
        msgobj.DeliveryNotificationOptions = DeliveryNotificationOptions.OnFailure;
        serverobj.Send(msgobj);
        lblMessageStatus.Text = "Mail Sent Successfully";
    }
}

推荐答案

您没有显示带有对象引用未设置为消息的异常一个对象的实例被抛出。



不用担心。这是检测和修复的最简单的案例之一。它只是意味着某些引用类型的某个成员/变量通过使用和它的实例（非静态）成员解除引用，这要求此成员/变量为非null，但实际上它似乎为null。只需在调试器下执行它，它将停止抛出异常的执行。在该行上设置一个断点，重新启动应用程序并再次到达这一点。评估下一行中涉及的所有引用，并查看哪一个为null，而不需要为null。解决这个问题之后，修复代码：要么确保将成员/变量正确初始化为非空引用，要么将其检查为null，如果为null，则执行其他操作。



另请参阅：想要在按钮点击时显示下一条记录。但是如果下一条记录功能的条件对象引用没有设置为对象的实例 [ ^ ]。



有时候，你不能在调试器下，由于一个或另一个原因做到这一点。一个非常讨厌的情况是，只有在调试信息不​​可用时构建软件时才会出现问题。在这种情况下，你必须使用更难的方式。首先，你需要确保你永远不会通过静默处理异常来阻止异常的传播（这是开发人员对自己的犯罪，但很常见）。您需要在每个线程的最顶层堆栈帧上捕获绝对所有异常。如果处理类型 System.Exception 的异常，则可以执行此操作。在处理程序中，您需要记录所有异常信息，尤其是 System.Exception.StackTrace ：

http://msdn.microsoft.com/en-us/library/system.exception.aspx [ ^ ]，

http://msdn.microsoft.com/en-us/library/system.exception。 stacktrace.aspx [ ^ ]。



堆栈跟踪只是一个字符串，显示从throw语句到处理程序的异常传播的完整路径。通过阅读，您总能找到目的。对于日志记录，使用类 System.Diagnostics.EventLog ：

http://msdn.microsoft.com/en-us/library/system.diagnostics.eventlog.aspx [ ^ ]。
-SA
You did not show where the exception with the message "Object reference not set to an instance of an object" is thrown.

Not to worry. This is one of the very easiest cases to detect and fix. It simply means that some member/variable of some reference type is dereferenced by using and of its instance (non-static) members, which requires this member/variable to be non-null, but in fact it appears to be null. Simply execute it under debugger, it will stop the execution where the exception is thrown. Put a break point on that line, restart the application and come to this point again. Evaluate all references involved in next line and see which one is null while it needs to be not null. After you figure this out, fix the code: either make sure the member/variable is properly initialized to a non-null reference, or check it for null and, in case of null, do something else.

Please see also: want to display next record on button click. but got an error in if condition of next record function "object reference not set to an instance of an object"[^].

Sometimes, you cannot do it under debugger, by one or another reason. One really nasty case is when the problem is only manifested if software is built when debug information is not available. In this case, you have to use the harder way. First, you need to make sure that you never block propagation of exceptions by handling them silently (this is a crime of developers against themselves, yet very usual). The you need to catch absolutely all exceptions on the very top stack frame of each thread. You can do it if you handle the exceptions of the type System.Exception. In the handler, you need to log all the exception information, especially the System.Exception.StackTrace:
http://msdn.microsoft.com/en-us/library/system.exception.aspx[^],
http://msdn.microsoft.com/en-us/library/system.exception.stacktrace.aspx[^].

The stack trace is just a string showing the full path of exception propagation from the throw statement to the handler. By reading it, you can always find ends. For logging, it's the best (in most cases) to use the class System.Diagnostics.EventLog:
http://msdn.microsoft.com/en-us/library/system.diagnostics.eventlog.aspx[^].

Good luck,
—SA


NullReference意味着什么应该已经初始化或设置值不是。你所要做的就是找到错误所在的代码行，然后修复它。



既然你没有告诉我们错误发生的地方我要去猜测它是在这一行，fileUploadControl.PostedFile.FileName;



上传控件不保留发布的文件，我不相信。当它上传时它会进行回发，所以你需要将图像存储在某处，然后点击发送电子邮件的按钮后，你将不得不再次检索该图像。
NullReference means that something that should have been initialized or set with a value wasn't. All you have to do is find the line of code where the error is and then fix it.

Since you didn't tell us where the error occurred I am going to guess it was on this line, fileUploadControl.PostedFile.FileName;

The upload control does not retain the posted file, I do not believe. When it uploads it does a postback so you'll need to store the image somewhere and then after the button to send email is clicked then you'll have to retrieve that image again.


那里对我来说这不是一个适合所有人的解决方案。你得到这个的原因是因为代码试图访问一个null变量的成员：

例如

foo.Bar

或

foo.Bar（）

甚至

foo.Bar。 SomethingElse



如果 foo 为null您将获得异常。此外，如果您尝试将 foo 传递给方法并在那里操作 - 您将获得它。



在实践中，这意味着异常可能出现在您的代码中的任何位置 - 甚至在您传递空值的框架代码中。





一般情况下：



  • 该异常应该会为您提供代码中出现问题的位置 - 使用它来查找问题。查看堆栈跟踪以缩小问题范围。
  • 从IE以调试模式运行代码。 IDE应该将您移动到发生错误的位置
  • 如果其他所有操作都失败，请在调用的方法的顶部放置一个断点，在调试和步骤下运行，直到遇到问题为止。这可能需要几个时间。
There isn't a one size fits all solution for this I'm afraid. The reason you get this is because the code tries to access a member of a variable that is null:
e.g.
foo.Bar
or
foo.Bar()
or even
foo.Bar.SomethingElse

If foo is null you'll get the exception. Also if you try an pass foo into a method and manipulate it there - you'll get it.

In practise this means the exception could come anywhere in your code - or even in framework code where you've passed a null in.


In general:

  • The exception should give you where the problem occurred in code - use this to find the problem. Take a look at the stack trace to narrow your problem down.
  • Run the code in debug mode from IE. The IDE should move you to where the error happens
  • If all else fails put a breakpoint at the top of the method called, run under debug and step though until you get the problem. This might take a couple of goes.


这篇关于如何解决NullReferenceException的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！

查看全文
相关文章
其他开发语言最新文章
热门教程
热门工具
登录 关闭
扫码关注1秒登录
发送“验证码”获取 | 15天全站免登陆