Linq使用where子句过滤将xml读入类

Question

I have xml in this format. I am trying to load this data into classes, based on some selective clauses

<Data

<CST>

<CA Type="type" Idea="Idea">
  <Var Name="Test" Value="some value" />
</CA>


<CA Type="type2" Idea="Idea2">
  <Var Name="Test2" Value="some value" />
</CA>


 <CA Type="type3" Idea="Idea3">
    <Var Name="Test" Value="some value" />
 </CA>

 </CST>

 </Data>

My code in linq to read xml is like this.

        IEnumerable<CA> casValues = (from s in doc.Root.Element("CST").Elements("CA")
                                 select new CA
                                 {
                                     Type = s.Attribute("Type").Value,
                                     Idea = s.Attribute("Idea").Value,
                                     Variable = (from k in s.Elements("Var")
                                                 select new Variable
                                                 {
                                                     Name = k.Attribute("Name").Value,
                                                     Value = k.Attribute("Value").Value
                                                 }).FirstOrDefault()
                                 });

This returns all the CA and their var values loaded properly. However when I try to filter it further, because I am only interested in values with var name="test", I get an empty collection with error

"{"Object reference not set to an instance of an object."}"

I am using simple filtering like this.

 IEnumerable<CA> tt = casValues.Where(x => x.Variable.Name == "Test");

I am assuming this is because Variable is a class and I cannot directly use the value. The same works when I load each element with foreach checking for my condition. What I am missing with above code?

Thanks for looking.

Answer 1

Your code will throw such exception in some cases, among them, in case when Type or Idea attribute not found in the current <CA> node, or when <Var> child node is not found.

One possible way to avoid exception in above mentioned occasions, instead of accessing the Value property directly, try to cast XAttribute or XElement to the expected type* :

var casValues = (from s in doc.Root.Element("CST").Elements("CA")
                 select new CA
                            {
                                Type = (string)s.Attribute("Type"),
                                Idea = (string)s.Attribute("Idea"),
                                Variable = (from k in s.Elements("Var")
                                            select new Variable
                                                       {
                                                           Name = (string)k.Attribute("Name"),
                                                           Value = (string)k.Attribute("Value")
                                                       }).FirstOrDefault()
                            });

_* ) List of available types for explicit conversion : XElement Explicit Conversion Operators , XAttribute Explicit Conversion Operators