Question

I have the following code

nodes = data.Descendants(XName.Get("{http://schemas.microsoft.com/LiveSearch/2008/04/XML/web}Results")).Nodes();
        System.Collections.Generic.IEnumerable<Result> res = new List<Result>();
        if (nodes.Count() > 0)
        {
            var results = from uris in nodes
                          select new Result
        {
            URL =
((XElement)uris).Element(XName.Get("{http://schemas.microsoft.com/LiveSearch/2008/04/XML/web}Url")).Value,
            Title =
((XElement)uris).Element(XName.Get("{http://schemas.microsoft.com/LiveSearch/2008/04/XML/web}Title")).Value,
            Description =
((XElement)uris).Element(XName.Get("{http://schemas.microsoft.com/LiveSearch/2008/04/XML/web}Description")).Value,
            DateTime =
((XElement)uris).Element(XName.Get("{http://schemas.microsoft.com/LiveSearch/2008/04/XML/web}DateTime")).Value,
        };
            res = results;
        }

Where Results is a object who has those URL, Title, Description, and DateTime variables defined.

This all works fine normally, but when a node in nodes doesnt contain a Description element (or at least I think thats whats throwing it) the program hits the "res = results;" line of code and throws a object reference not set to... error and highlights the whole section right after "select new Results"..

我如何确定这一点?

Answer 1

最为简单的办法是投到<条码>,其中载明,而不是使用<条码>。这样,你就用<代码>null查询<代码>代替>。

但是,你的法典也可成为>>>>nicer:

XNamespace ns = "http://schemas.microsoft.com/LiveSearch/2008/04/XML/web";

var results = data.Descendants(ns + "Results")
                  .Elements()
                  .Select(x => new Result 
                          {
                            URL = (string) x.Element(ns + "Url"),
                            Title = (string) x.Element(ns + "Title"),
                            Description = (string) x.Element(ns + "Description"),
                            DateTime = (string) x.Element(ns + "DateTime")
                          })
                  .ToList();

参看更简单些什么? 所用的技术:

Calling ToList() on an empty sequence gives you a list anyway
This way you ll only ever perform the query once; before you were calling Count() which would potentially have iterated over each node. In general, use Any() instead of Count() > 0) - but this time just making the list unconditional is simpler.
Use the Elements() method to get child elements, rather than casting multiple times. (Your previous code would have thrown an exception if it had encountered any non-element nodes)
Use the implicit conversion from string to XNamespace
Use the +(XNamespace, string) operator to get an XName

Answer 2

如果该说明内容没有列入,请加以测试。

((XElement)uris).Element(XName.Get("{http://schemas.microsoft.com/LiveSearch/2008/04/XML/web}Description"))

在使用价值之前,不作废。该法典:

var results = from uris in nodes let des = ((XElement)uris).Element(XName.Get("{http://schemas.microsoft.com/LiveSearch/2008/04/XML/web}Description"))
                      select new Result
    {
        URL = ((XElement)uris).Element(XName.Get("{http://schemas.microsoft.com/LiveSearch/2008/04/XML/web}Url")).Value,
        Title = ((XElement)uris).Element(XName.Get("{http://schemas.microsoft.com/LiveSearch/2008/04/XML/web}Title")).Value,
        Description = (des != null) ? des.Value : string.Empty,
        DateTime = ((XElement)uris).Element(XName.Get("{http://schemas.microsoft.com/LiveSearch/2008/04/XML/web}DateTime")).Value,
    };

友情链接