【转】LINQ To XML 入门(2)

Some Common Operations using LINQ To XML - Part II

 

In this article, we will explore some common ‘How Do I’ kind of examples using LINQ to XML. This article is the Part II of the 3-part LINQ to XML series. I hope you have read the previous article Some Common Operations using LINQ To XML - Part I. 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.

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

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

<Employees>

 <Employee>

    <EmpId>1</EmpId>

    <Name>Sam</Name>   

    <Sex>Male</Sex>

    <Phone Type="Home">423-555-0124</Phone>

    <Phone Type="Work">424-555-0545</Phone>

    <Address>

      <Street>7A Cox Street</Street>

      <City>Acampo</City>

      <State>CA</State>

      <Zip>95220</Zip>

      <Country>USA</Country>

    </Address>

 </Employee>

 <Employee>

    <EmpId>2</EmpId>

    <Name>Lucy</Name>

    <Sex>Female</Sex>

    <Phone Type="Home">143-555-0763</Phone>

    <Phone Type="Work">434-555-0567</Phone>

    <Address>

      <Street>Jess Bay</Street>

      <City>Alta</City>

      <State>CA</State>

      <Zip>95701</Zip>

      <Country>USA</Country>

    </Address>

 </Employee>

 <Employee>

    <EmpId>3</EmpId>

    <Name>Kate</Name>

    <Sex>Female</Sex>

    <Phone Type="Home">166-555-0231</Phone>

    <Phone Type="Work">233-555-0442</Phone>

    <Address>

      <Street>23 Boxen Street</Street>

      <City>Milford</City>

      <State>CA</State>

      <Zip>96121</Zip>

      <Country>USA</Country>

    </Address>

 </Employee>

 <Employee>

    <EmpId>4</EmpId>

    <Name>Chris</Name>

    <Sex>Male</Sex>

    <Phone Type="Home">564-555-0122</Phone>

    <Phone Type="Work">442-555-0154</Phone>

    <Address>

      <Street>124 Kutbay</Street>

      <City>Montara</City>

      <State>CA</State>

      <Zip>94037</Zip>

      <Country>USA</Country>

    </Address>

 </Employee>

</Employees>

 

The application is a console application targeting .NET 3.5 framework. I have used query expressions 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; System.IO;

9. Create an XML Document with Xml Declaration/Namespace/Comments using LINQ to XML

When you need to create an XML document containing XML declaration, XML Document Type (DTD), Processing instructions, Comments, Namespaces; you should go in for the XDocument class.

C#

XNamespace empNM = "urn:lst-emp:emp";

 

XDocument xDoc = new XDocument(

            new XDeclaration("1.0", "UTF-16", null),

            new XElement(empNM + "Employees",

                new XElement("Employee",

                    new XComment("Only 3 elements for demo purposes"),

                    new XElement("EmpId", "5"),

                    new XElement("Name", "Kimmy"),

                    new XElement("Sex", "Female")

                    )));

 

StringWriter sw = new StringWriter();

xDoc.Save(sw);

Console.WriteLine(sw);

VB.NET

        Dim empNM As XNamespace = "urn:lst-emp:emp"

 

        Dim xDoc As New XDocument(New XDeclaration("1.0", "UTF-16", Nothing), _

                    New XElement(empNM + "Employees", _

                    New XElement("Employee", _

                    New XComment("Only 3 elements for demo purposes"), _

                    New XElement("EmpId", "5"), _

                    New XElement("Name", "Kimmy"), _

                    New XElement("Sex", "Female"))))

 

        Dim sw As New StringWriter()

        xDoc.Save(sw)

        Console.WriteLine(sw)

10. Save the XML Document to a XMLWriter or to the disk using LINQ to XML

Use the following code to save the XML to a XMLWriter or to your physical disk

C#

XNamespace empNM = "urn:lst-emp:emp";

 

XDocument xDoc = new XDocument(

            new XDeclaration("1.0", "UTF-16", null),

            new XElement(empNM + "Employees",

                new XElement("Employee",

                    new XComment("Only 3 elements for demo purposes"),

                    new XElement("EmpId", "5"),

                    new XElement("Name", "Kimmy"),

                    new XElement("Sex", "Female")

                    )));

 

StringWriter sw = new StringWriter();

XmlWriter xWrite = XmlWriter.Create(sw);

xDoc.Save(xWrite);

xWrite.Close();

 

// Save to Disk

xDoc.Save("C:\\Something.xml");

Console.WriteLine("Saved");

VB.NET

       Dim empNM As XNamespace = "urn:lst-emp:emp"

 

Dim xDoc As New XDocument(New XDeclaration("1.0", "UTF-16", Nothing),_

        New XElement(empNM + "Employees", _

        New XElement("Employee", _

        New XComment("Only 3 elements for demo purposes"), _

        New XElement("EmpId", "5"), _

        New XElement("Name", "Kimmy"), _

        New XElement("Sex", "Female"))))

 

        Dim sw As New StringWriter()

        Dim xWrite As XmlWriter = XmlWriter.Create(sw)

        xDoc.Save(xWrite)

        xWrite.Close()

 

        ' Save to Disk

        xDoc.Save("C:\Something.xml")

        Console.WriteLine("Saved")

11. Load an XML Document using XML Reader using LINQ to XML

Use the following code to load the XML Document into an XML Reader

C#

XmlReader xRead = XmlReader.Create(@"..\\..\\Employees.xml");

XElement xEle = XElement.Load(xRead);

Console.WriteLine(xEle);

xRead.Close();

VB.NET

Dim xRead As XmlReader = XmlReader.Create("..\\..\\Employees.xml")

Dim xEle As XElement = XElement.Load(xRead)

Console.WriteLine(xEle)

xRead.Close()

12. Find Element at a Specific Position using LINQ to XML

Find the 2^nd Employee Element

C#

// Using XElement

Console.WriteLine("Using XElement");

XElement xEle = XElement.Load("..\\..\\Employees.xml");

var emp = xEle.Descendants("Employee").ElementAt(1);

Console.WriteLine(emp);

 

Console.WriteLine("------------");

 

//// Using XDocument

Console.WriteLine("Using XDocument");

XDocument xDoc = XDocument.Load("..\\..\\Employees.xml");

var emp1 = xDoc.Descendants("Employee").ElementAt(2);

Console.WriteLine(emp);

VB.NET

' Using XElement

Console.WriteLine("Using XElement")

Dim xEle As XElement = XElement.Load("..\..\Employees.xml")

Dim emp = xEle.Descendants("Employee").ElementAt(1)

Console.WriteLine(emp)

 

Console.WriteLine("------------")

 

'// Using XDocument

Console.WriteLine("Using XDocument")

Dim xDoc As XDocument = XDocument.Load("..\..\Employees.xml")

Dim emp1 = xDoc.Descendants("Employee").ElementAt(2)

Console.WriteLine(emp)

13. List the First 2 Elements using LINQ to XML

List the details of the first 2 Employees

C#

XElement xEle = XElement.Load("..\\..\\Employees.xml");

var emps = xEle.Descendants("Employee").Take(2);

foreach (var emp in emps)

    Console.WriteLine(emp);

VB.NET

Dim xEle As XElement = XElement.Load("..\..\Employees.xml")

Dim emps = xEle.Descendants("Employee").Take(2)

For Each emp In emps

      Console.WriteLine(emp)

Next emp

14. List the 2nd and 3rd Element using LINQ to XML

List the 2nd and 3rd Employees

C#

XElement xEle = XElement.Load("..\\..\\Employees.xml");

var emps = xEle.Descendants("Employee").Skip(1).Take(2);

foreach (var emp in emps)

    Console.WriteLine(emp);

VB.NET

Dim xEle As XElement = XElement.Load("..\..\Employees.xml")

Dim emps = xEle.Descendants("Employee").Skip(1).Take(2)

For Each emp In emps

      Console.WriteLine(emp)

Next emp

15. List the Last 2 Elements using LINQ To XML

We have been posting the entire elements as output in our previous examples. Let us say that you want to display only the Employee Name, use this query:

C#

XElement xEle = XElement.Load("..\\..\\Employees.xml");

var emps = xEle.Descendants("Employee").Reverse().Take(2);

foreach (var emp in emps)

    Console.WriteLine(emp.Element("EmpId") + "" + emp.Element("Name"));

VB.NET

Dim xEle As XElement = XElement.Load("..\..\Employees.xml")

Dim emps = xEle.Descendants("Employee").Reverse().Take(2)

For Each emp In emps

      Console.WriteLine(emp.Element("EmpId") + emp.Element("Name"))

Next emp

To display only the values without the XML tags, use the ‘Value’ property

C#

XElement xEle = XElement.Load("..\\..\\Employees.xml");

var emps = xEle.Descendants("Employee").Reverse().Take(2);

foreach (var emp in emps)

    Console.WriteLine(emp.Element("EmpId").Value + ". " + emp.Element("Name").Value);

 

VB.NET

Dim xEle As XElement = XElement.Load("..\..\Employees.xml")

Dim emps = xEle.Descendants("Employee").Reverse().Take(2)

For Each emp In emps

      Console.WriteLine(emp.Element("EmpId").Value & ". " & emp.Element("Name").Value)

Next emp

If you notice, the results are not ordered i.e. the Employee 4 is printed before 3. To order the results, one of the ways is to call Reverse() again while filtering as shown below. Note that I have not tested performance while using this query on a large document. If I find any performance issues, I will update this section:

C#

XElement xEle = XElement.Load("..\\..\\Employees.xml");

var emps = xEle.Descendants("Employee").Reverse().Take(2).Reverse();

foreach (var emp in emps)

    Console.WriteLine(emp.Element("EmpId").Value + ". " + emp.Element("Name").Value);

VB.NET

Dim xEle As XElement = XElement.Load("..\..\Employees.xml")

Dim emps = xEle.Descendants("Employee").Reverse().Take(2).Reverse()

For Each emp In emps

      Console.WriteLine(emp.Element("EmpId").Value & ". " & emp.Element("Name").Value)

Next emp

16. Find the Element Count based on a condition using LINQ to XML

Count the number of Employees living in the state CA

C#

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

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

            where (string)address.Element("Address").Element("State") == "CA"

            select address;

Console.WriteLine("No of Employees living in CA State are {0}", stCnt.Count());

VB.NET

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

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

            where (string)address.Element("Address").Element("State") == "CA"

            select address;

Console.WriteLine("No of Employees living in CA State are {0}", stCnt.Count());

Well those were some 'more' commonly used 'How Do I' operations while using LINQ to XML. In the Part 3 of this series, we will see how to manipulate XML content and persist the changes. The entire source of the article in C# and VB.NET can be downloaded over here. I hope you liked the article and I thank you for viewing it.