带有一项的C＃Xml.Linq集合使用.ToList（）返回“对象引用未设置为对象的实例”

Question

I have a List<XElement> with a number of XML elements. When I use the Where() method, I manage to find the one item successfully. Using First() returns that item successfully, and if I use Any() it returns true. However if I use Count() or ToList() it returns Object reference not set to an instance of an object.

Many thanks in advance.

    //Elements:
    <meta name="ncc:sidebars" content="0" xmlns="http://www.w3.org/1999/xhtml" />
    <meta name="ncc:setInfo" content="1 of 1" xmlns="http://www.w3.org/1999/xhtml" />
    <meta name="ncc:tocItems" content="12" xmlns="http://www.w3.org/1999/xhtml" />
    <meta name="ncc:totalTime" content="8:02:54" xmlns="http://www.w3.org/1999/xhtml" />
    <!-- another 30 other elements... -->
    <meta http-equiv="Content-type" content="text/html; charset=ISO-8859-1" xmlns="http://www.w3.org/1999/xhtml" />

public static List<XElement> GetElements(this List<XElement> els, String nameTag)
{
    var elementsFound = els.Where(e => e.Attribute("name").Value.ToLower() == "ncc:totaltime");
    if (elementsFound.Any())
        return elementsFound.ToList();
    else
        throw new Exception("Some text");
}

Answer 1

this should work:

var elementsFound = els.Where(e => e.Attribute("name") != null && e.Attribute("name").Value.ToLower() == nameTag);
if (elementsFound.Any())
    return elementsFound.ToList();
else
    throw new Exception("Some text");

Answer 2

I think one or more of your XElement objects do not have a attribute named "name". All your LINQ queries ("where" in this case) will be only executed if you actually use the result.

Answer 3

There exists an item in your list which has an attribute which returns null . It is not the first one. .Any() and First() will both loop just over the Enumerable until the first element is found which fulfills the condition.

ToList() will loop over all elements --> one Attribute returns null and the call of the .ToLower instance method will result in a NullReferenceException