Question

I have two XELEMENTS XR and XTemp.

MY XR Contains :

     <result xmlns="http://abc.com/test">
           <report>
            <unit unit_number="1" id="S1">
                 <classification>Subject</classification>
                 <claim_reported count="0" status="No claims reported" />
            </unit>
           <unit unit_number="2" id="S2">
                <classification>Subject</classification>
                <claim_reported count="0" status="No claims reported" />
           </unit>
        <unit unit_number="3" id="V1">
          <classification>Vehicle</classification>
          <claim_reported count="1" status="Claims reported" />
        </unit>
    ......
     </report>
</result>

My XTemp contains :

 <result xmlns="http://abc.com/test">
   <report>
        <unit unit_number="1" id="S3">
             <classification>Subject</classification>
             <claim_reported count="1" status="Claims reported" />
        </unit>
       <unit unit_number="2" id="S4">
            <classification>Subject</classification>
            <claim_reported count="4" status="Claims reported" />
       </unit>
      <unit unit_number="3" id="V2">
         <classification>Vehicle</classification>
         <claim_reported count="0" status="No claims reported" />
      </unit>
      <unit unit_number="3" id="V3">
         <classification>Vehicle</classification>
         <claim_reported count="2" status="Claims reported" />
      </unit>
         ......
    </report>
 </result>

Trying to extract all the <unit> elements from XTemp based on a certain condtion and add them to XR.

The condition is : get all the <unit> elements whose id begins with “V”.

Below code was the best I was able to come up with.

But, I get Object reference not set to an instance Error.

XR.Descendants(xmlns + "unit").LastOrDefault().AddAfterSelf(XTemp.Descendants(xmlns + "unit")
  .Where(x => x.Element(xmlns + "unit").Attribute("id").Value.ToUpper().Contains("V")));

Thanks in advance

BB

Answer 1

Your problem is in this bit of code:

.Where(x => x.Element(xmlns + "unit").Attribute("id")...

At that point in the query you’re already querying over unit elements. A unit doesn’t contain a nested unit, which is why you get a NullReferenceException.

Change it to: .Where(x => x.Attribute("id")...

Personally, I find your query hard to read. Firstly, LastOrDefault means you potentially expect a null value to exist, yet you’re forging ahead with the rest of the query without addressing it. Secondly, you don’t need to find the last unit element and AddAfterSelf. Instead you can directly Add to the report element and the elements will be appended to the end:

var query = xtemp.Descendants(xmlns + "unit")
                 .Where(e => e.Attribute("id").Value.StartsWith("V", StringComparison.InvariantCultureIgnoreCase));

xr.Element(xmlns + "report").Add(query);

Also, notice how I checked for “V” while ignoring the case. This is considered best practice as opposed to using ToUpper.

---

EDIT: based on the comments, here is a solution to sort all the unit nodes while preserving the existing order of non-unit nodes. This code takes into account whether node occurs before and after existing nodes, or no other nodes at all.

Some assumptions are:

• XR always has unit nodes
• IDs follow the existing letter followed by a number format.
• unit nodes are kept together and are not interspersed between all other types of nodes.

The first two assumptions are trivial issues to code for if they’re incorrect.

var query = xtemp.Descendants(xmlns + "unit")
                 .Where(e => e.Attribute("id").Value.StartsWith("V", StringComparison.InvariantCultureIgnoreCase));

var units = xr.Element(xmlns + "report").Elements(xmlns + "unit");
var beforeUnit = units.First().ElementsBeforeSelf().FirstOrDefault();
var afterUnit = units.Last().ElementsAfterSelf().FirstOrDefault();

// order based on ID starting letter, then integer value
var orderedUnits = units.Concat(query)
                        .OrderBy(e => e.Attribute("id").Value[0])
                        .ThenBy(e => int.Parse(e.Attribute("id").Value.Substring(1)))
                        .ToList();

// remove original unit nodes
units.Remove();

// add ordered units based on their original position
if (beforeUnit != null)
{
    beforeUnit.AddAfterSelf(orderedUnits);
}
else if (afterUnit != null)
{
    afterUnit.AddBeforeSelf(orderedUnits);
}
else
{
    xr.Element(xmlns + "report").Add(orderedUnits);
}