Linq to XML - 使用linq query.n时出现Null引用异常

Question

I have a simple XML file:

<?xml version="1.0" encoding="utf-8"?>
<ConvenioValidacao>
    <convenio ven_codigo="1" tipoValidacao="CPF"></convenio>
    <convenio ven_codigo="1" tipoValidacao="MATRICULA"></convenio>
    <convenio ven_codigo="3" tipoValidacao="CPF"></convenio>
    <convenio ven_codigo="4" tipoValidacao="CPF"></convenio>
</ConvenioValidacao>

I'm trying to do a simple query against this xml file using Linq to XML , here is what i'm doing:

var myXmlDoc = XElement.Load(filePath);
var result =  from convenio in myXmlDoc.Element("ConvenioValidacao").Elements("convenio")
                 where (string)convenio.Attribute("ven_codigo") == "1" &&
                 (string)convenio.Attribute("tipoValidacao") == "CPF"
                 select convenio;

It is not working, I'm getting null reference exception.

What I'm doing wrong?

Answer 1

Use this instead:

var result = from convenio in myXmlDoc.Elements("convenio")
                 where (string)convenio.Attribute("ven_codigo") == "1" &&
                 (string)convenio.Attribute("tipoValidacao") == "CPF"
                 select convenio;

Since myXmlDoc is of type XElement there is no "document element" and as such the root of the element is the root node ( <ConveioValidacao> ). Since this is the root node you don't need to specify it in an Elements method since that is current position in document.

As a side note, I recommend that you rename myXmlDoc to myXmlElement to reduce confusion.

Answer 2

.Element方法获取给定元素的第一个子元素，在这里ConveioValidacao不是子元素，它是父元素，当你通过XEelemnt.Load（）方法加载时，它获取ConveioValidacao及其子元素，所以你应该使用Andrew的码。

Answer 3

Try Descendants instead of Elements

var result =  from convenio in myXmlDoc.Descendants("ConveioValidacao").Descendants("convenio")
                 where (string)convenio.Attribute("ven_codigo") == "1" &&
                 (string)convenio.Attribute("tipoValidacao") == "CPF"
                 select convenio;