解析XML：变量元素的NullReferenceException(Parsing XML: NullReferenceException for Variable Elements)

 我从网站获取文本字符串并将其解析为XDocument。 我希望将某些元素的值提供给一个非常简单的对象（名为NWSevent）。 我的问题是原始字符串更改，XML树变化; 有时会有很多事件，最多40个，有时只有一个，有时只有一个不具备所有特征。 如果没有警报，则“event”元素具有标题，但没有areaDesc，摘要或严重性。 
 
 我有两个NWSevent构造函数，一个接受一个字符串，另一个接受四个字符串参数。 我无法绕过NullReferenceException。 下面的if语句不能这样做，因为没有可比较的值。 我很感激任何帮助。 
 
    public static void ParseWeatherData(String xmlString)
    {
        String ticker = string.Empty;
        XDocument root = XDocument.Parse(xmlString);

        XNamespace ns = XNamespace.Get("http://www.w3.org/2005/Atom");
        XNamespace nsCap = XNamespace.Get("urn:oasis:names:tc:emergency:cap:1.1");

        //get list of entry elements, set conditions for title, areaDesc, etc
        var xlist = root.Descendants(ns + "entry").Select(elem => new
        { //use first or default to deal with possiblity of null return
            Title = elem.Descendants(ns + "title").FirstOrDefault(),
            AreaDesc = elem.Descendants(nsCap + "areaDesc").FirstOrDefault(),
            Severity = elem.Descendants(nsCap + "severity").FirstOrDefault(),
            Summary = elem.Descendants(ns + "summary").FirstOrDefault()
        });

        foreach (var el in xlist)       //need to address null values when no alerts
        {
            if (el.AreaDesc.Value != null)      //causes yellow null ERROR; no value exists for el.areaDesc.value
            {
                String titleIn = el.Title.Value;
                String areaIn = el.AreaDesc.Value;
                String severityIn = el.Severity.Value;
                String summaryIn = el.Summary.Value;
                new Models.NWSevent(titleIn, areaIn, severityIn, summaryIn);
            }
            else
            {
                String titleIn = el.Title.Value;
                new Models.NWSevent(titleIn);
            }
        }

I'm getting a text string from a website and parsing it into an XDocument. I'm looking to feed the value of certain elements into a very simple object (named NWSevent). My problem is that the original string changes and the XML tree varies; sometimes there are numerous events, up to 40, sometimes there is only one, and sometimes there is only one that does not have all the characteristics. If there are no alerts, then the "event" element has a title, but no areaDesc, summary, or severity.
 
I have two constructors for NWSevent, one takes in a single string, the other takes in four string arguments. I'm having trouble getting around a NullReferenceException. The if statement below can't do it because there is no value to compare. I'd appreciate any help.
 
    public static void ParseWeatherData(String xmlString)
    {
        String ticker = string.Empty;
        XDocument root = XDocument.Parse(xmlString);

        XNamespace ns = XNamespace.Get("http://www.w3.org/2005/Atom");
        XNamespace nsCap = XNamespace.Get("urn:oasis:names:tc:emergency:cap:1.1");

        //get list of entry elements, set conditions for title, areaDesc, etc
        var xlist = root.Descendants(ns + "entry").Select(elem => new
        { //use first or default to deal with possiblity of null return
            Title = elem.Descendants(ns + "title").FirstOrDefault(),
            AreaDesc = elem.Descendants(nsCap + "areaDesc").FirstOrDefault(),
            Severity = elem.Descendants(nsCap + "severity").FirstOrDefault(),
            Summary = elem.Descendants(ns + "summary").FirstOrDefault()
        });

        foreach (var el in xlist)       //need to address null values when no alerts
        {
            if (el.AreaDesc.Value != null)      //causes yellow null ERROR; no value exists for el.areaDesc.value
            {
                String titleIn = el.Title.Value;
                String areaIn = el.AreaDesc.Value;
                String severityIn = el.Severity.Value;
                String summaryIn = el.Summary.Value;
                new Models.NWSevent(titleIn, areaIn, severityIn, summaryIn);
            }
            else
            {
                String titleIn = el.Title.Value;
                new Models.NWSevent(titleIn);
            }
        }

原文：https://stackoverflow.com/questions/19897438