LINQ To XML 入门(1)

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Some Common Operations using LINQ To XML - Part I

In this article, we will explore some common ‘How Do I’ kind of examples using LINQ to XML. This article is the Part I of the 3-part LINQ to XML series. I assume you are familiar with LINQ. If not, you can start off with LINQ to XML by checking some tutorials at Getting Ready for .NET 3.5 and LINQ – Exploring C# 3.0 – Part I and over here.

LINQ to XML程序集(System.Xml.Linq.dll)在3个不同的命名空间System.Xml.Linq、System.Xml.Schema和System.Xml.XPath。后两者只定义了少数类型。
System.Xml.Linq中有一套可控制的类型，如下:

成员含义XDocument整个XML文档XElementXML文档中的某个给定元素XDeclarationXML文档开头的声明XCommentXML注释XAttribute代表一个特定XML元素的XML属性XName、XNamespace提供了一个简单的方式来定义和引用XML命名空间

We will be using a sample file called ‘Employees.xml’ for our demonstrations. The mark up will be as follows:

<?xml version="1.0" encoding="utf-8" ?>

<Street>7A Cox Street</Street>

<City>Acampo</City>

</Address>

</Employee>

<Sex>Female</Sex>

</Address>

</Employee>

<Sex>Female</Sex>

<Street>23 Boxen Street</Street>

<City>Milford</City>

</Address>

</Employee>

<Name>Chris</Name>

<Street>124 Kutbay</Street>

<City>Montara</City>

</Address>

</Employee>

</Employees>

The application is a console application targeting .NET 3.5 framework. I have also used query expressions 'at places' instead of Lambda expression in these samples. It is just a matter of preference and you are free to use any of these. Use the following namespaces while testing the samples: System; System.Collections.Generic; System.Linq; System.Text; System.Xml; System.Xml.Linq;

1. How Do I Read XML using LINQ to XML

There are two ways to do so: Using the XElement class or the XDocument class. Both the classes contain the ‘Load()’ method which accepts a file, a URL or XMLReader and allows XML to be loaded. The primary difference between both the classes is that an XDocument can contain XML declaration, XML Document Type (DTD) and processing instructions. Moreover an XDocument contains one root XElement.

Using XElement

XElement xelement = XElement.Load("..//..//Employees.xml");

IEnumerable<XElement> employees = xelement.Elements();

// Read the entire XML

foreach (var employee in employees)

{

Console.WriteLine(employee);

}

VB.NET

Dim xelement As XElement = XElement.Load("../../Employees.xml")

Dim employees As IEnumerable(Of XElement) = xelement.Elements()

' Read the entire XML

For Each employee In employees

Console.WriteLine(employee)

Next employee

Output:

Using XDocument

XDocument xdocument = XDocument.Load("..//..//Employees.xml");

IEnumerable<XElement> employees = xdocument.Elements();

foreach (var employee in employees)

{

Console.WriteLine(employee);

}

VB.NET

Dim xdocument As XDocument = XDocument.Load("../../Employees.xml")

Dim employees As IEnumerable(Of XElement) = xdocument.Elements()

For Each employee In employees

Console.WriteLine(employee)

Next employee

Output:

Note 1: As you can observe, XDocument contains a single root element (Employees).

Note 2: In order to generate an output similar to the one using XElement, use “xdocument.Root.Elements()” instead of “xdocument.Elements()”

Note 3: VB.NET users can use a new feature called XML Literals which allows you to incorporate XML directly.

2. How Do I Access a Single Element using LINQ to XML

Let us see how to access the name of all the Employees and list them over here

XElement xelement = XElement.Load("..//..//Employees.xml");

IEnumerable<XElement> employees = xelement.Elements();

Console.WriteLine("List of all Employee Names :");

foreach (var employee in employees)

{

Console.WriteLine(employee.Element("Name").Value);

}

VB.NET

Dim xelement As XElement = XElement.Load("../../Employees.xml")

Dim employees As IEnumerable(Of XElement) = xelement.Elements()

Console.WriteLine("List of all Employee Names :")

For Each employee In employees

Console.WriteLine(employee.Element("Name").Value)

Next employee

Output:

3. How Do I Access Multiple Elements using LINQ to XML

Let us see how to access the name of all Employees and also list the ID along with it

XElement xelement = XElement.Load("..//..//Employees.xml");

IEnumerable<XElement> employees = xelement.Elements();

Console.WriteLine("List of all Employee Names along with their ID:");

foreach (var employee in employees)

{

Console.WriteLine("{0} has Employee ID {1}",

employee.Element("Name").Value,

employee.Element("EmpId").Value);

}

VB.NET

Dim xelement As XElement = XElement.Load("../../Employees.xml")

Dim employees As IEnumerable(Of XElement) = xelement.Elements()

Console.WriteLine("List of all Employee Names along with their ID:")

For Each employee In employees

Console.WriteLine("{0} has Employee ID {1}", employee.Element("Name").Value, employee.Element("EmpId").Value)

Next employee

Output:

4. How Do I Access all Elements having a Specific Attribute using LINQ to XML

Let us see how to access details of all Female Employees

XElement xelement = XElement.Load("..//..//Employees.xml");

var name = from nm in xelement.Elements("Employee")

where (string)nm.Element("Sex") == "Female"

select nm;

Console.WriteLine("Details of Female Employees:");

foreach (XElement xEle in name)

Console.WriteLine(xEle);

VB.NET

Dim xelement As XElement = XElement.Load("../../Employees.xml")

Dim name = _

From nm In xelement.Elements("Employee") _

Where CStr(nm.Element("Sex")) = "Female" _

Select nm

Console.WriteLine("Details of Female Employees:")

For Each xEle As XElement In name

Console.WriteLine(xEle)

Next xEle

Output:

5. How Do I Access Specific Element having a Specific Attribute using LINQ to XML

Let us see how to list all the Home Phone Nos.

XElement xelement = XElement.Load("..//..//Employees.xml");

var homePhone = from phoneno in xelement.Elements("Employee")

where (string)phoneno.Element("Phone").Attribute("Type") == "Home"

select phoneno;

Console.WriteLine("List HomePhone Nos.");

foreach (XElement xEle in homePhone)

{

Console.WriteLine(xEle.Element("Phone").Value);

}

VB.NET

Dim xelement As XElement = XElement.Load("../../Employees.xml")

Dim homePhone = _

From phoneno In xelement.Elements("Employee") _

Where CStr(phoneno.Element("Phone").Attribute("Type")) = "Home" _

Select phoneno

Console.WriteLine("List HomePhone Nos.")

For Each xEle As XElement In homePhone

Console.WriteLine(xEle.Element("Phone").Value)

Next xEle

Output:

6. How Do I Find an Element within another Element using LINQ to XML

Let us see how to find the details of Employees living in 'Alta' City

XElement xelement = XElement.Load("..//..//Employees.xml");

var addresses = from address in xelement.Elements("Employee")

where (string)address.Element("Address").Element("City") == "Alta"

select address;

Console.WriteLine("Details of Employees living in Alta City");

foreach (XElement xEle in addresses)

Console.WriteLine(xEle);

VB.NET

Dim xelement As XElement = XElement.Load("../../Employees.xml")

Dim addresses = _

From address In xelement.Elements("Employee") _

Where CStr(address.Element("Address").Element("City")) = "Alta" _

Select address

Console.WriteLine("Details of Employees living in Alta City")

For Each xEle As XElement In addresses

Console.WriteLine(xEle)

Next xEle

Output:

7. How Do I Find Nested Elements (using Descendants Axis) using LINQ to XML

Let us see how to list all the zip codes in the XML file

XElement xelement = XElement.Load("..//..//Employees.xml");

Console.WriteLine("List of all Zip Codes");

foreach (XElement xEle in xelement.Descendants("Zip"))

{

Console.WriteLine((string)xEle);

}

VB.NET

Dim xelement As XElement = XElement.Load("../../Employees.xml")

Console.WriteLine("List of all Zip Codes")

For Each xEle As XElement In xelement.Descendants("Zip")

Console.WriteLine(CStr(xEle))

Next xEle

Output:

8. How do I apply Sorting on Elements using LINQ to XML

Let us see how to List and Sort all Zip Codes in ascending order

XElement xelement = XElement.Load("..//..//Employees.xml");

IEnumerable<string> codes = from code in xelement.Elements("Employee")

let zip = (string)code.Element("Address").Element("Zip")

orderby zip

select zip;

Console.WriteLine("List and Sort all Zip Codes");

foreach (string zp in codes)

Console.WriteLine(zp);

VB.NET

Dim xelement As XElement = XElement.Load("../../Employees.xml")

Dim codes As IEnumerable(Of String) = _

From code In xelement.Elements("Employee") _

Let zip = CStr(code.Element("Address").Element("Zip")) _

Order By zip _

Select zip

Console.WriteLine("List and Sort all Zip Codes")

For Each zp As String In codes

Console.WriteLine(zp)

Next zp

Output:

Well those were some commonly used operations while using LINQ to XML. In the Part 2 of this series, we will see some more operations and also explore how to create XML content and also save them. I hope you liked the article and I thank you for viewing it.

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