C＃使用linq查找匹配两个元素级别的xml元素

Question

I am trying to select an XElement from an xml document and match on two "levels" of the xml document. my file structure is:

<app>
  <Library Name="Main" Path="C:\somefile.Xml">
    <ReadingList Name="Test1">
      <Book>...
    <ReadingList Name="Test2">
      <Book>...
  <Library Name="Backup" Path="C:\somefile.Xml">

And I want to find the reading list name "test2" in library "Main" so I can copy this element and all child elements to another Library node.

I would prefer a solution using linq as I am trying to learn this.

Thanks in advance for any help

When I add a new "reading list" I do it this way:

public void AddReadingList(string fullyQualifiedPath, Library lib, string name)
{
    XDocument xdoc = XDocument.Load(fullyQualifiedPath);

    XElement library = xdoc.Element("eStack").Elements("Library")
                .Single(x => x.Attribute("Name").Value == lib.Name);

    library.Add(new XElement("ReadingList", new XAttribute("Name", name)));

    xdoc.Save(fullyQualifiedPath);                        
}

but the operation I want to perform is a copy of this element and sub elements. The problem is, there may be more than one "library" element with the same name so I need to check the library name and reading list name. does that make sense?

Answer 1

Using a combination of .Descendants and Where s will do the trick:

var result = XDocument.Load(fullyQualifiedPath)
     .Descendants("Library")
         //Can replace with `FirstOrDefault` if you know there is only one
     .Where(element => element.Attribute("Name")?.Value == "Main") 
     .Descendants("ReadingList")
     .Where(element => element.Attribute("Name")?.Value == "Test2").ToList();

You can also use .Elements instead of Descendants I just preferred to use it so to not specify also the level of app or any other along the way.

---

For prior c# 6.0 you can do:

var result = XDocument.Load("data.xml")
     .Descendants("Library")
     .Where(element =>
     {
        var att = element.Attribute("Name");
        return att != null ? (att.Value == "Main" ? true : false) : false;
     })
     .Descendants("ReadingList")
         //Make sure to do above change here too
     .Where(element => element.Attribute("Name")?.Value == "Test2").ToList();

Or if creating a method to help with it instead of repeating the code:

 Func<XElement, string, string> tryGetAttributeValue = (element, attributeName) =>
 {
    var attribute = element.Attribute(attributeName);
    return attribute == null ? string.Empty : attribute.Value;
 };

 var result = XDocument.Load("data.xml")
     .Descendants("Library")
     .Where(element => tryGetAttributeValue(element,"Name") == "Main")
     .Descendants("ReadingList")
     .Where(element => tryGetAttributeValue(element, "Name") == "Test2").ToList();

Answer 2

Are you looking for something like this?

        var table = XElement.Parse(@"<app>
                                        <Library Name=""Main"" Path=""C:\somefile.Xml"">
                                            <ReadingList Name=""Test1"">
                                                <Book>Book1</Book>
                                            </ReadingList>
                                            <ReadingList Name=""Test2"">
                                                <Book>Book2</Book>
                                            </ReadingList>
                                        </Library>
                                        <Library Name=""Backup"" Path=""C:\somefile.Xml""></Library>
                                    </app>");


        var readingList = table
            .Elements("Library")
            .FirstOrDefault(x => x.Attribute("Name")?.Value == "Main")
            .Elements("ReadingList")
            .FirstOrDefault(x => x.Attribute("Name")?.Value == "Test2");

This would essentially solve your query of getting a ReadingList that is named Test2 and under a ReadingList named Test 1 .

You can opt for Where instead of FirstOrDefault but that would end up with a list of all possible matches.

        var readingList = table
            .Elements("Library")
            .Where(x => x.Attribute("Name").Value == "Main")
            .Elements("ReadingList")
            .Where(x => x.Attribute("Name").Value == "Test2")
            .ToList(); 

Opting for a descendant variant would end up in lesser lookup of course.

        var readingList = table
            .Descendants("Library")
            .Where(x => x.Attribute("Name").Value == "Main")
            .Descendants("ReadingList")
            .Where(x => x.Attribute("Name").Value == "Test2")
            .ToList();

Answer 3

You can also do this query:

var query=  xdoc.Descendants("Library")
                .Where(e=>e.Attribute("Name").Value=="Main")
                .SelectMany(e=>e.Elements("ReadingList").Where(t=>t.Attribute("Name").Value=="Test2"));

If there are just one Main element you can also use FirstOrDefault extension method:

var query=  xdoc.Descendants("Library")
                .FirstOrDefault(e=>e.Attribute("Name").Value=="Main")
                .Descendants("ReadingList").Where(t=>t.Attribute("Name").Value=="Test2"));