Tài liệu Use XMLReader to Read an XML Document ppt

12.2 Use XMLReader to Read an XML Document In How-To 12.1, I learned how to write out data to an XML document by using the XMLWriter.. How do I read data using the XMLReader?. For this H

12.2 Use XMLReader to Read an XML Document

In How-To 12.1, I learned how to write out data to an XML document by using the

XMLWriter How do I read data using the XMLReader?

Technique

Whereas the XMLWriter has one implementation (class), the XMLReader has three, depending on the task you need to perform Those classes are listed in Table 12.4

Table 12.4 XMLWriter Implementations Class Name Purpose/Descriptions

XMLTextReader Reads character streams This is a forward-only reader that has

methods returning data on content and node types No validation occurs

XMLNodeReader Provides a parser over an XML Document Object Model

(DOM) API, similar to the XMLNode tree

XMLValidatingReader Takes an XMLTextReader adding validation In doing so, it

provides a fully compliant validating or non-validating XML parser with DTD, XSD schema, or XML-Data Reduced (XDR) schema support

For this How-To, you will be using the XMLTextReader to read the file and display the results To do this, you will use the following methods:

• Read Reads a line of the character stream out of the XMLReadReader

• AttributeCount Gives the count of attributes This depends on the type of data that

is being read In this case, AttributeCount is used to display the individual

attributes, which will be columns

• NodeType Allows you to specify node types in the XML document In the case of

this How-To, the System.Xml.XmlNodeType.XmlDeclaration is checked for so that no information is printed for that type of node You can see the other node types by looking at intellisense for the NodeType property

• Value Represents the actual value of the line in the stream

• Close Closes the stream

Steps

Open and run the Visual Basic NET-Chapter 12 solution From the main Web page, click on the hyperlink with the caption How-To 12.2: Use XMLReader to Read an XML

Document When the page loads, click the button labeled Read XML File The example then reads the XML file that is specified in the text box labeled File to Read and displays the information in the text area located at the bottom of the form (see Figure 12.2)

1 Create a Web Form Then place the Label, TextBox, and Button objects as seen in Figure 12.2 on the form with the following properties set as in Table 12.5

Table 12.5 Label, TextBox, and Button Control Property Settings

Object Property Setting

NavigateURL wfrmMain.aspx

2 Note

You will find the TextArea control in the HTML Components section of the toolbox After you have dragged this control onto the Web Form, right-click and choose Run as Server Control from the pop-up menu This will then run this control as a server side control, and you will be able to work with its properties and methods in code behind

3 Add the code in Listing 12.5 to the Click event of btnReadFile To start off, the XXLTextReader is initialized with the XML document specified in txtFileToRead Then each node of the document is read using the Read method The node type is compared to make sure the current node is not the XMLDeclaration If it's not, then each of the attributes is displayed; in this case, the columns of each name are entered After each of the nodes (rows) have been read and added to strOut, then that string is assigned to the InnerText property of the TextArea object called taOutput Last, the XMLTextReader object is closed

Listing 12.5 wfrmHowTo12_2.aspx.vb: Reading an XML Document Using XMLTextReader

Private Sub btnReadFile_Click(ByVal sender As System.Object,

ByVal e As System.EventArgs) Handles btnReadFile.Click

Dim xtrNames As System.Xml.XmlTextReader

Dim strOut As String

Dim intAtts As Integer

Dim intCurrAtt As Integer

Try

strOut = "Reading file " & _

Me.txtFileToRead.Text & " " & vbCrLf

xtrNames = New System.Xml.XmlTextReader(Me.txtFileToRead.Text)

While xtrNames.Read()

intAtts = xtrNames.AttributeCount

If xtrNames.NodeType <> System.Xml.XmlNodeType.XmlDeclaration Then

If intAtts > 0 Then

For intCurrAtt = 0 To intAtts - 1

strOut &= xtrNames(intCurrAtt) & " "

Next

Else

strOut &= xtrNames.Value

End If

End If

End While

Catch excp As Exception

strOut &= "Following Error Occurred: " & excp.Message

Finally

strOut &= vbCrLf & "Done Processing " & Me.txtFileToRead.Text & "" taOutput.InnerText = strOut

If Not xtrNames Is Nothing Then

xtrNames.Close()

End If

End Try

End Sub

Figure 12.2 The information displayed here was read from an XML document

using XMLReader

Comments

Again, as with XMLTextReader, this falls into the middle of complexity when it comes

to reading XML documents If you want to actually validate the data, then you should use the XMLValidatingReader implementation of the XMLReader, rather than the

XMLTextReader