如何从xml中读取

谢谢澄清。没有属性 date ，有一个元素。因此， m_xmlr.GetAttribute（date）返回null。然后你尝试通过获取一些属性来解除它，并抛出异常。你真的需要学习如何自己解决这些问题。



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。但是如果下一个记录功能的条件对象引用未设置为对象的实例则会出错。



有时候，你不能这样做在调试器下，由一个或另一个原因。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 。



现在，关于 XmlReader 的使用，我建议你在调试器下运行它一个循环

Thank your for the clarification. There is no attribute date, there is an element. So, m_xmlr.GetAttribute("date") returns null. Then you try to derefence it by taking some property, and it throws the exception. You really need to learn how to sort out such things all by yourself.

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.

Now, as to the use of XmlReader, I would advise you to run it all under the debugger in a loop

while (reader.Read)
  ' check up reader.NodeType
  ' and other properties, to see where you are
end while

祝你好运，

最简单的方法是使用 Linq To Xml [ ^ ]。

The easiest way is to use Linq To Xml[^].

Dim s As String ="<?xml version='1.0'?>
<vpn>
    <update>
        <date>Tue Oct 27 08:42:22 PDT 2015</date>
    </update>
</vpn>"

'Replace
'Parse(s)
'with
'Load("C:\Program Files\OpenVPN\config\vpnservers.xml")
Dim xdoc As XDocument = XDocument.Parse(s)
'get single date
Dim stringDate = xdoc.Descendants("update").Select(Function(x) x.Element("date").Value).SingleOrDefault()
'display
Console.WriteLine("String date is: {0}", stringDate)

结果：

Result:

String date is: Tue Oct 27 08:42:22 PDT 2015

如果您想将上述字符串转换为 DateTime ，您可能有兴趣了解如何实现这一目标：

If you would like to convert above string into DateTime, you might be interested to see how to achieve that:

Dim provider As System.Globalization.CultureInfo = System.Globalization.CultureInfo.InvariantCulture
Dim format As String = "ddd MMM dd HH:mm:ss PDT yyyy"
Dim result As DateTime 
Try
	result = DateTime.ParseExact(stringDate, format, provider)
	Console.WriteLine("Result date is: {0}", result)

	Catch ex As System.FormatException
	     Console.WriteLine("{0} is not in the correct format.", stringDate)
End Try

结果：

Result:

Result date is: 2015-10-27 08:42:22

如需了解更多信息，请参阅：

DateTime.ParseExact Method（String，String，IFormatProvider） [ ^ ]

解析非标准日期和时间格式[Ron Petrusha] [ ^ ]

There is another possibility of error in

TextBox2.Text = dte.ToString

变量 dte 无处可定。

，这一行会杀死你在上一行中所做的事。

The variable dte is defined nowhere.
and this line kills what you do in previous line.