how to read from xml

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I have an xml file :

XML

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

I have this vb net piece code (partly changed) :

VB

Dim m_xmlr As XmlTextReader
m_xmlr = New XmlTextReader("C:\Program Files\OpenVPN\config\vpnservers.xml")
m_xmlr.Read()
m_xmlr.Read()
While Not m_xmlr.EOF
m_xmlr.Read()
    If Not m_xmlr.IsStartElement() Then
      Exit While
       End If
      Dim serv = m_xmlr.GetAttribute("vpn")
        m_xmlr.Read()
Dim upd = m_xmlr.GetAttribute("update")
            m_xmlr.Read()           
 Dim updt = m_xmlr.GetAttribute("date").ElementAt(0)
            m_xmlr.Read()
            TextBox2.Text = updt.ToString
TextBox2.Text = dte.ToString
end while

I get error valie cannot be null, how to read the date element value?

Posted 30-Oct-15 7:50am

Member 3892343

Updated 30-Oct-15 7:51am

v2

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Sergey Alexandrovich Kryukov 30-Oct-15 13:55pm

In what line? The problem looks very easy; just use the debugger.
—SA

Member 3892343 30-Oct-15 15:03pm

Line 4

Member 3892343 30-Oct-15 15:19pm

Dim updt = m_xmlr.GetAttribute("date").ElementAt(0)

Member 3892343 30-Oct-15 15:45pm

sorry I thought in xml Line thank you sergey

Member 3892343 30-Oct-15 15:01pm

Line 4

3 solutions

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Sergey Alexandrovich Kryukov · Answer 1 · 2015-10-30T12:49:00

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

VB

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

Good luck,

—SA

Maciej Los · Answer 2 · 2015-10-31T08:06:00

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

VB

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)

Result:

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

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

VB

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

Result:

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

For further information, please see:
DateTime.ParseExact Method (String, String, IFormatProvider)[^]
Parsing Non-Standard Date and Time Formats [Ron Petrusha][^]

Patrice T · Answer 3 · 2015-10-31T09:32:00

Solution 3

There is another possibility of error in

VB

TextBox2.Text = dte.ToString

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

Posted 31-Oct-15 9:32am

Patrice T

how to read from xml

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