Znajdź XElement z pewną nazwą atrybutu i wartości z LINQ

Question

XDocument xDocument = XDocument.Load("..."); 
IEnumerable<XElement> elements = xDocument 
    .Element("lfm") 
    .Element("events") 
    .Elements("event"); 

try 
{    
    foreach (XElement elm in elements) 
    { 
     comm.Parameters.AddWithValue("extID", elm.Element("id").Value ?? ""); 
     comm.Parameters.AddWithValue("Title", elm.Element("title").Value ?? ""); 
     comm.Parameters.AddWithValue("HeadlineArtist", 
     elm.Element("artists").Element("headliner").Value ?? ""); 

, ale chcę mieć wartość elementu "image" z atrybutem "size = large", szukałem całą noc, a to jest najbliższe przyszedłem:

comm.Parameters.AddWithValue("LargeImage", 
    elm.Descendants("image") 
     .FirstOrDefault(i => (string)i.Attribute("size") == "large").Value); 

próbki z części odpowiedzi XML:

<lfm status="ok"> 
    <events xmlns:geo="http://www.w3.org/2003/01/geo/wgs84_pos#" 
      location="Chicago, United States" page="1" perPage="1" 
      totalPages="341" total="341" festivalsonly="0" tag=""> 
     <event xmlns:geo="http://www.w3.org/2003/01/geo/wgs84_pos#"> 
      <id>3264699</id> 
      <title>Iron And Wine</title> 
      <artists> 
       <artist>Iron And Wine</artist> 
       <artist>Dr. John</artist> 
       <headliner>Iron And Wine</headliner> 
      </artists> 
      <venue> 
       <id>8915382</id> 
       <name>Ravinia Festival</name> 
       <location> 
        <city>Highland Park</city> 
        <country>United States</country> 
        <street>200 Ravinia Park Rd</street> 
        <postalcode>60035</postalcode> 
        <geo:point> 
         <geo:lat>42.15831</geo:lat> 
         <geo:long>-87.778409</geo:long> 
        </geo:point> 
       </location> 
       <url>http://www.last.fm/venue/8915382+Ravinia+Festival</url> 
       <website>http://www.ravinia.org/</website> 
       <phonenumber>847.266.5100</phonenumber> 
       <image size="small">http://userserve-ak.last.fm/serve/34/63026487.jpg</image> 
       <image size="medium">http://userserve-ak.last.fm/serve/64/63026487.jpg</image> 
       <image size="large">http://userserve-ak.last.fm/serve/126/63026487.jpg</image> 
       <image size="extralarge">http://userserve-ak.last.fm/serve/252/63026487.jpg</image> 

Answer 1

Spróbuj

XElement result = elm.Descendants("image") 
    .FirstOrDefault(el => el.Attribute("size") != null && 
         el.Attribute("size").Value == "large"); 
if (result != null) { 
    process result.Value ... 
} 

Począwszy od C# 6.0 (VS 2015), można napisać:

XElement result = elm.Descendants("image") 
    .FirstOrDefault(el => el.Attribute("size")?.Value == "large"); 
if (result != null) { 
    process result.Value ... 
} 

A nieoczywiste alternatywę (jak @RandRandom zauważył) jest rzucić atrybutu string:

XElement result = elm.Descendants("image") 
    .FirstOrDefault(el => (string)el.Attribute("size") == "large"); 
if (result != null) { 
    process result.Value ... 
} 

Działa to, ponieważ z powodu XAttribute Explicit Conversion (XAttribute to String).

Answer 2

można użyć XPathSelectElement extension method

var node = elm.XPathSelectElement("descendant::image[@size='large']"); 
if (node!=null) 
{ 
    var path = node.Value; 
}