Use XMLReader to Read an XML Document

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Sử dụng XMLReader để hiểu một tài liệu XML Trong thế nào-Để 12,1, tôi đã học được cách để viết ra dữ liệu đến một tài liệu XML bằng cách sử dụng XMLWriter. Làm thế nào để đọc dữ liệu bằng cách sử dụng XMLReader?

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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 Label Text File to Read TextBox ID txtFileToRead Button ID btnReadFile Text Read XML File TextArea ID taOutput HyperLink ID hplReturnToMain 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.