C# Add nodes in xml

Question

I am trying to add my code for already existing code. I have below two xmls:

Out.xml

<School>
  <Classes>
    <Class>
     <Students>
       <Student SequenceNumber="1">
        <ID>123</ID>
        <Name>AAA</Name>
     </Student>
    </Students>
  </Class>
</Classes>  
</School>

In.xml

<School>
 <Classes>
  <Class>
   <Students>
     <Student SequenceNumber="1">
      <ID>456</ID>
      <Name>BBB</Name>
    </Student>
    <Student SequenceNumber="2">
      <ID>123</ID>
      <Name>AAA</Name>
    </Student>
    <Student SequenceNumber="3">
      <ID>789</ID>
      <Name>CCC</Name>
    </Student>
    </Students>
    </Class>
   </Classes>  
  </School>

Now I need to check Out.xml and In.xml and my final Out.xml must be like below. The rule here is check StudentID in Out and In xmls. If Out xml doesnot have it and In xml has it add it to Out.xml at end of already existing elements.

Out.xml

<School>
<Classes>
<Class>
  <Students>
    <Student SequenceNumber="1">
      <ID>123</ID>
      <Name>AAA</Name>
    </Student>
    <Student SequenceNumber="2">
      <ID>456</ID>
      <Name>BBB</Name>
    </Student>       
    <Student SequenceNumber="3">
      <ID>789</ID>
      <Name>CCC</Name>
    </Student>
  </Students>
  </Class>
 </Classes>  
</School>

Already existing code is as below

string inFileName = @"C:\In.xml";
string inXml = System.IO.File.ReadAllText(inFileName);
var xmlReaderSource = XmlReader.Create(new StringReader(inXml));
var mgr = new XmlNamespaceManager(xmlReaderSource.NameTable); 
mgr.AddNamespace("m", "http://www.mismo.org/residential/2009/schemas");
XDocument sourceXmlDoc = XDocument.Load(xmlReaderSource);

string outFileName = @"C:\Out.xml";
string outXml = System.IO.File.ReadAllText(outFileName);
XmlDocument targetXmlDoc = new XmlDocument();           
targetXmlDoc.LoadXml(outXml);

I cannot change above code now I need add my logic.

I added like below

string xpath = @"/m:School/m:Classes/m:Class/m:Students";

XmlNodeList outStudentNodes = targetXmlDoc.SelectNodes(xpath + "/m:Student", namespaceManager);

if(outStudentNodes== null || outStudentNodes.Count <= 0)
 { 
    return;
 }

XElement root = sourceXmlDoc.Root;
IEnumerable<XElement> inStudentsColl = from item in root.Elements("Classes").Descendants("Class")
                                             .Descendants("Students").Descendants("Student")
                             select item;

Now I have XmlNodeList and IEnumerble, trying to see whether I can use LINQ statement and make code simple for my comparison.

Node: I am not asking how to add nodes/elements using C#. I am looking for how to compare two xmls and then add nodes/elements into the one which is missing those nodes/elements. My issue here is one xml is read like XDocument and other using XmlDocument.

UPDATE

Thank you very much @TheAnatheme. I really appreciate it.

I followed what TheAnatheme suggested me and it worked. I marked TheAnatheme's answer as real solution. Please see below what I did in foreach block so that if anyone wants to use they can refer to this post.

string xpath = @"/m:School/m:Classes/m:Class/m:Students
XmlNode studentsNode = targetXmlDoc.SelectSingleNode(xpath, namespaceManager);  

foreach (var element in elementsToAdd)
{
   //Add Microsoft.CSharp.dll (if needed ) to your project for below statement to work
   dynamic studentElement = element as dynamic;
   if (studentElement != null)
    {
         XmlElement studentXmlElement = targetXmlDoc.CreateElement("Student");                               

          XmlElement studentIDXmlElement = targetXmlDoc.CreateElement("ID");

          studentIDXmlElement.InnerText = studentElement.ID;

          XmlElement studentNameXmlElement = targetXmlDoc.CreateElement("Name");
          studentNameXmlElement .InnerText = studentElement.Name;

          studentXmlElement.AppendChild(studentIDXmlElement);
          studentXmlElement.AppendChild(studentNameXmlElement);

          studentsNode.AppendChild(childElement);

    }
}

Answer 1

This projects both sets into an anonymous object List, makes comparisons, and gives you a set of anonymous objects that don't yet exist by which you can add to the out XML.

public static List<object> GetInStudents(XDocument sourceXmlDoc)
{
    IEnumerable<XElement> inStudentsElements = 
    sourceXmlDoc.Root.Elements("Classes").Descendants("Class")
                     .Descendants("Students").Descendants("Student");

    return inStudentsElements.Select(i => 
        new { Id = i.Elements().First().Value, 
            Name = i.Elements().Last().Value }).Cast<object>().ToList();
}

public static List<object> GetOutStudents(XmlDocument targetXmlDoc)
{
    XmlNodeList outStudentsElements = targetXmlDoc.GetElementsByTagName("Students")[0].ChildNodes;

    var outStudentsList = new List<object>();

    for (int i = 0; i < outStudentsElements.Count; i++)
    {
        outStudentsList.Add(new { Id = outStudentsElements[i].ChildNodes[0].InnerText, 
                                  Name = outStudentsElements[i].ChildNodes[1].InnerText });
    }

    return outStudentsList;
}

And you compare them as such:

var inStudents = GetInStudents(sourceXmlDoc);
var outStudents = GetOutStudents(targetXmlDoc);

if (inStudents.SequenceEqual(outStudents))
{
    return;
}
else
{
    var elementsToAdd = inStudents.Except(outStudents);

    foreach (var element in elementsToAdd)
    {
        // create xmlNode with element properties, add element to xml
    }
}