View a XML file in xdocument and linq

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This gives me a "Null Reference". The XML file exists. Added reference "using System.Xml.Linq".

C#

var xmldoc1 = XDocument.Load(Server.MapPath("~/xml/VSOP87-J2000.xml"));
var elements1 = from data in xmldoc1.Descendants("VSOP87-J2000")
    select new
    {
        ID = Convert.ToDouble(data.Element("ID").Value),
        A5sun = Convert.ToDouble(data.Element("A5sun").Value),
        B5sun = Convert.ToDouble(data.Element("B5sun").Value),
        C5sun = Convert.ToDouble(data.Element("C5sun").Value)
     };

foreach (var element in elements1){
    ov = ov + 1;
    vpi[ov] = element.ID;
    vpi[ov] = element.A5sun;
    vpi[ov] = element.B5sun;
    vpi[ov] = element.C5sun;
}

XML

<<?xml version="1.0" encoding="UTF-8"?>
<dataroot>
   <VSOP87-J2000>
        <ID>1</ID>
        <A5sun>0.00000006867</A5sun>
        <B5sun>4.39735582882</B5sun>
        <C5sun>117.3198682202</C5sun><pre>
   </VSOP87-J2000>
   <VSOP87-J2000>
         <ID>2</ID>
         <A5sun>0.00000007226</A5sun>
         <B5sun>4.04267257839</B5sun>
         <C5sun>1265.5674786264</C5sun><
    </VSOP87-J2000>
</dataroot>

What I have tried:

Tried:

C#

XmlDocument xmlDoc= new XmlDocument();
xmlDoc.Load("VSOP87-J2000.xml");

Could not figure out how to use the Descendant.

Posted 13-Jan-23 9:20am

John Ertle Jr

Updated 13-Jan-23 11:25am

v2

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Comments

Richard Deeming 16-Jan-23 6:30am

Ignoring the errors in your XML markup, the code you've posted used with the document you've posted produces the expected results:

[
    { "ID": 1, "A5sun": 0.00000006867, "B5sun": 4.39735582882, "C5sun": 117.3198682202 },
    { "ID": 2, "A5sun": 0.00000007226, "B5sun": 4.04267257839, "C5sun": 1265.5674786264 }
]

So the only possibility left in the code provided is that vpi is null. (Although setting vpi[ov] to four different values seems like a mistake.)

2 solutions

Solution 1

This is one of the most common problems we get asked, and it's also the one we are least equipped to answer, but you are most equipped to answer yourself.

Let me just explain what the error means: You have tried to use a variable, property, or a method return value but it contains null - which means that there is no instance of a class in the variable.
It's a bit like a pocket: you have a pocket in your shirt, which you use to hold a pen. If you reach into the pocket and find there isn't a pen there, you can't sign your name on a piece of paper - and you will get very funny looks if you try! The empty pocket is giving you a null value (no pen here!) so you can't do anything that you would normally do once you retrieved your pen. Why is it empty? That's the question - it may be that you forgot to pick up your pen when you left the house this morning, or possibly you left the pen in the pocket of yesterday's shirt when you took it off last night.

We can't tell, because we weren't there, and even more importantly, we can't even see your shirt, much less what is in the pocket!

Back to computers, and you have done the same thing, somehow - and we can't see your code, much less run it and find out what contains null when it shouldn't.
But you can - and Visual Studio will help you here. Run your program in the debugger and when it fails, it will show you the line it found the problem on. You can then start looking at the various parts of it to see what value is null and start looking back through your code to find out why. So put a breakpoint at the beginning of the method containing the error line, and run your program from the start again. This time, the debugger will stop before the error, and let you examine what is going on by stepping through the code looking at your values.

But we can't do that - we don't have your code, we don't know how to use it if we did have it, we don't have your data. So try it - and see how much information you can find out!

Posted 13-Jan-23 9:52am

OriginalGriff

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Graeme_Grant · Accepted Answer · 2023-01-13T11:25:00

You're not using the System.Xml.Linq namespace. Here is the official documentation: How to find descendant elements - LINQ to XML | Microsoft Learn[^]

This is an example based on the documentation link above:

C#

using System.Xml.Linq;

XDocument xDoc =  XDocument.Load(Path.Combine(Environment.CurrentDirectory, @".\xml\VSOP87-J2000.xml"));

var elements = xDoc.Root.Descendants("VSOP87-J2000");

foreach (XElement el in elements)
{
    Console.WriteLine(el.Name);

    if (el.HasElements)
    {
        foreach (var subEl in el.Descendants())
        {
            Console.WriteLine($"    {subEl.Name} = {subEl.Value}");
        } 
    }
}

And here is the output:

VSOP87-J2000
    ID = 1
    A5sun = 0.00000006867
    B5sun = 4.39735582882
    C5sun = 117.3198682202
VSOP87-J2000
    ID = 2
    A5sun = 0.00000007226
    B5sun = 4.04267257839
    C5sun = 1265.5674786264

This should be enough to point you in the right direction.