如何读取具有特定属性名称的xmlelement [英] How to read the xmlelement with specific attribute name

问题描述

您好我的xml类似于以下

且每个xml可以在IRP节点下拥有多个IRO节点。



< irp> ;



< irn n =name1>

< ts = selected1>

< ts =selected13>

Hi i have the xml similar to below
and each xml can have multiple IRO nodes under IRP node.

<irp>

<irn n="name1">

<t s="selected1">
<t s="selected13">

< ts =selected6>

< ts =selected8>

<t s="selected6">
<t s="selected8">

<pre lang="XML"><?xml version="1.0" encoding="UTF-8"?>

-<IR>


-<IRP comprised="True" n="EFPLANT-SC-2">


-<IRO n="U1-ABV -13820ACT" cd="Schema document version">


-<P i="True" dn="Construction status 2" pd="ConstructionStatus2">

<T v="Recalibrate" s="LastIn"/>

<T v="Recalibrate" s="ELE"/>

<T v="New" s="IIC"/>

<T v="New" s="PID"/>

</P>


-<P i="True" dn="Construction status" pd="ConstructionStatus">

<T v="Existing" s="LastIn"/>

<T v="Existing" s="ELE"/>

<T v="New" s="IIC"/>

<T v="New" s="PID"/>

</P>


-<P i="False" dn="Instrument sequence in loop" pd="InstSequenceInLoop">

<T v="0" s="LastIn"/>

<T v="0" s="ELE"/>

<T v="0" s="IIC"/>

</P>


-<P i="False" dn="X-coincidence factor" pd="XCoincidenceFactor">

<T v="0" s="LastIn"/>

<T v="0" s="ELE"/>

</P>


-<P i="False" dn="Y-coincidence factor" pd="YCoincidenceFactor">

<T v="0" s="LastIn"/>

<T v="0" s="ELE"/>

</P>

我尝试过：



私有函数ProcessFileToKnowToolsWhichHavePropPublishedInDomain（ByVal pstrPath As String ，ByVal pstrname As String）As List（Of String）



Dim lcolDomainwhichhaveatleastoneproperty As New List（Of String）

Dim lvalidateDOMReply As New XmlDocument

lvalidateDOMReply.Load（ps trPath）

Dim xml As XElement = XElement.Load（pstrPath）

'Dim userNodes As XmlNodeList = lvalidateDOMReply.SelectNodes（IR / IRP / IRO [@n = + pstrname +]）

Dim myNodes As IEnumerable（Of XElement）= xml.Descendants（IRP）。Descendants（IRO）。Where（Function（x）x.Attribute（ n）。值= pstrname）

'lvalidateDOMReply。



每个lobjPropertyNode作为myEodes中的XElement

Dim工具As String =（lobjPropertyNode.Elements（s）。Value）.ToString（）

如果不是lcolDomainwhichhaveatleastoneproperty.Contains（工具）那么lcolDomainwhichhaveatleastoneproperty.Add（工具）

下一页



返回lcolDomainwhichhaveatleastoneproperty

结束功能





i已经尝试过上面的代码，但得到了对象引用异常。无法找到它们的价值什么都没有请建议

What I have tried:

Private Function ProcessFileToKnowToolsWhichHavePropPublishedInDomain(ByVal pstrPath As String, ByVal pstrname As String) As List(Of String)

Dim lcolDomainwhichhaveatleastoneproperty As New List(Of String)
Dim lvalidateDOMReply As New XmlDocument
lvalidateDOMReply.Load(pstrPath)
Dim xml As XElement = XElement.Load(pstrPath)
'Dim userNodes As XmlNodeList = lvalidateDOMReply.SelectNodes("IR/IRP/IRO[@n=" + pstrname + "]")
Dim myNodes As IEnumerable(Of XElement) = xml.Descendants("IRP").Descendants("IRO").Where(Function(x) x.Attribute("n").Value = pstrname)
'lvalidateDOMReply.

For Each lobjPropertyNode As XElement In myNodes
Dim tool As String = (lobjPropertyNode.Elements("s").Value).ToString()
If Not lcolDomainwhichhaveatleastoneproperty.Contains(tool) Then lcolDomainwhichhaveatleastoneproperty.Add(tool)
Next

Return lcolDomainwhichhaveatleastoneproperty
End Function


i HAVE TRIED ABOVE CODE but getting the object refernce exception.not able to find wher ethe value is nothing please suggest

推荐答案

请看我对这个问题的评论。更好地使用 System.Xml.Linq.XDocument 更适合支持LINQ to XML的事情：

XDocument Class（System.Xml.Linq），

LINQ to XML 。

Please see my comment to the question. Better use System.Xml.Linq.XDocument more suitable for such things as it supports LINQ to XML:
XDocument Class (System.Xml.Linq),
LINQ to XML.

成员12573064写道：

Member 12573064 wrote:

它已在每个循环中抛出异常
Dim Tool As string

It has thrown the exception in the for each loop at
Dim Tool As string

至于你的异常，看起来你在这里有一个真正的塞子。看看这一行：

As to your exception, it looks like you have a real stopper here. Look at this line:

Dim tool As String = (lobjPropertyNode.Elements("s").Value).ToString()

如果您确定这是引发异常的行，则只有 lobjPropertyNode.Elements（s）返回null，然后你尝试取消引用这个对象，假设它不是null，取其成员 .Value 。此时，抛出异常。



然后你可能做错了。您不必检查调试器下的表达式，而是开始猜测并提出您应该回答的问题，而不是其他任何人。面对这种情况时你不能这样做;学会自己处理它们非常重要。



不用担心。这是最容易检测和修复的案例之一。它只是意味着某些引用类型的某个成员/变量通过使用及其实例（非静态）成员进行解引用，这需要此成员/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 。



祝你好运，

-SA

If you are sure that this is the line where the exception is thrown, it can be only if lobjPropertyNode.Elements("s") returns null, and then you try to dereference this object, assuming it is not null, by taking its member .Value. At this point, the exception is thrown.

And then you probably do wrong thing. Instead of inspecting the expressions under the debugger, you start guessing and asking questions which you should answer, not anyone else. You cannot do it when you face such situations; it's very important to learn dealing with them all by yourself.

Not to worry. This is one of the easiest cases to detect and fix. It simply means that some member/variable of some reference type is dereferences 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

您正在选择匹配 IRO 节点，然后尝试从那些读取元素 s 节点。根据您发布的XML， IRO 节点没有名为 s 的子元素。 br />


如果要从 T 元素中读取属性，则需要选择这些元素，并阅读属性：

You are selecting the matching IRO nodes, and then trying to read the element "s" from those nodes. Based on the XML you've posted, the IRO nodes do not have a child element called "s".

If you want to read the attribute from the T elements, you need to select those elements, and read the attribute:

Dim myNodes As IEnumerable(Of XElement) = xml
    .Elements("IRP")
    .Elements("IRO")
    .Where(Function(x) x.Attribute("n").Value = pstrname)
    .Elements("P")
    .Elements("T")

For Each lobjPropertyNode As XElement In myNodes
    Dim tool As String = lobjPropertyNode.Attribute("s").Value
    ...
Next

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