需要使用linq C＃将xml元素添加到最后一个父级

Question

I'm really new to Linq and C# and I'm stuck on what is probably an obvious problem.

I have an existing XML file

<?xml version="1.0" encoding="utf-8" standalone="yes"?>
<books>
  <book>
    <title>This is Title 1</title>
    <author>John Doe</author>
    <categories>
      <category>How to</category>
      <category>Technical</category>       
  </book>
  <book>
    <title>This is Title 2</title>
    <author>Jane Brown</author>
    <categories>
      <category>Fantasy</category>
    </categories>
  </book>
</books>

I want to add a 2nd category to the second book in this file.

I've gotten this far:

var thiscat = doc.Root
  .Element("book")
  .Element("categories");

thiscat.Add(new XElement("category", "novel"));

But this adds a 3rd category to the first book. I need to learn how to point 'thiscat' at the last categories element rather than the first one. I've been sniffing around LastNode but haven't managed to get the syntax right.

This is my first question here. Please let me know if I'm not being clear or if I'm doing anything wrong.

Answer 1

Pete,

Here is an example that will search for the book by title This is Title 2 and add another category.

var elem = doc.Root.Elements("book").FirstOrDefault(x => x.Element("title").Value.Equals("This is Title 2"));
if (elem != null)
{
    var category = elem.Element("categories");
    category.Add(new XElement("category", "novel"));
}

Edit: More explanatoin.

First of we search the documents book elements for the matching title of This is Title 2 (effectively your second entry). By executing the FirstOrDefault extension method we either the get the first matching element (as XElement ) or null .

Because we 'could' get a null value we must check if the value is null if not we move into the next step of locating the categories element. This can be done simply calling the elem.Element() method as we only expect one element.

Finally we add a new XElement to the category element.

Hope this helps.

Cheers.

Answer 2

To answer your question quite literally, you could modify the statement as follows:

var thiscat = doc.Root
.Elements("book")
.Skip(1)
.First()
.Element("categories");

The "Element" function returns the first element of that type found. In this case, we used "Elements" instead to return an IEnumerable containing all of the elements named "book", and then we used the LINQ "skip" function to skip the first (returning another IEnumerable of all the remaining elements), and then we took just the first element in the IEnumerable (back to a single XElement).

Another way you could have gotten to the answer is as follows:

var thiscat = doc.Root
.Element("book")
.ElementsAfterSelf()
.First()
.Element("categories");

ElementsAfterSelf returns an IEnumerable of all the sibling elements after the calling object.

LINQ is a really critical part of programming in C# and it's good to see you're trying to learn it from the beginning. Although your methodology here in adding a specific element to a specific place programmatically is questionable (obviously it is a contrived example), in playing around like this you will probably learn a bit about LINQ and that is always good.

Answer 3

First you should get your second book element.According to your code:

var thiscat = doc.Root
                 .Element("book")
                 .Element("categories");

This statement returns just one categories element which belongs to your first book.Because you are using Element instead of Elements . Let's go step by step.

A proper way to get second element is using Descendants like this:

var secondBook = doc.Descendants("book")[1];

Descendants returning a collection of your books.And we are getting second element with indexer.Now we need to select your categories element under the book element.

var categories = secondBook.Element("categories");

Now we have our categories element and we can add our new category and save Xml Document :

categories.Add(new XElement("category", "novel"));
doc.Save(path);

And that's all.If you understand that logic you can modify your html file however you like.Besides you can make all of these in one line:

doc.Descendants("book")[1]
            .Element("categories")
            .Add(new XElement("category", "novel"));

Answer 4

This should work( slightly lengthy solution as it helps understand the fundamentals better):

            XmlElement rootNode = xd.DocumentElement;  //gives <books> the root node
            XmlNodeList cnodes= rootNode.ChildNodes; //gets the childnodes of <books>
            XmlNode secondBook= cnodes.Item(1); //second child of <books> i.e., the <book> you want
            XmlNodeList bnodes= secondBook.ChildNodes; //gets the childnodes of that <book>
            XmlNode categories= bnodes.Item(2); //gets the third child i.e.,<categories>
//making the new <category> node
            string xmlContent = "<category>novel</category>";
            XmlDocument doc = new XmlDocument();
            doc.LoadXml(xmlContent);
            XmlNode newNode = doc.DocumentElement;
//making the new node completes

            categories.AppendChild(newNode);  //append the new node to <categories> as a child