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Create Code-39 in Visual C#.NET CH APT ER 4 AC CE SSI NG XM L DO C UMENT S BY USI NG TH E XPAT H DA TA MO D EL

CH APT ER 4 AC CE SSI NG XM L DO C UMENT S BY USI NG TH E XPAT H DA TA MO D EL
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This chapter began with a brief overview of XPath and its vocabulary, including terms such as axis, node test, predicate, and function. You might be wondering where they come into the picture. It s time now to see those features in action. To test various XPath expressions, we will create a simple application that looks like the one shown in Figure 4-3.
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Figure 4-3. Executing XPath expressions The application consists of a text box positioned at the top to enter XPath expressions. After you click the Execute button, the given expression is executed and its results are displayed in another text box at the bottom. The label at the bottom displays the total number of rows returned by the expression. Listing 4-3 shows the Click event handler of the Execute button. Listing 4-3. Using the Select() Method private void button1_Click(object sender, EventArgs e) { XPathDocument doc = new XPathDocument(Application.StartupPath + @"\employees.xml"); XPathNavigator navigator = doc.CreateNavigator(); XPathNodeIterator iterator = navigator.Select(textBox1.Text); try { label3.Text = "The expressions returned " + iterator.Count + " nodes"; if (iterator.Count > 0) { while (iterator.MoveNext())
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CHAPTER 4 AC CES SING XML DOCUM ENTS BY USING THE XPATH DA TA MODEL
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{ textBox2.Text = iterator.Current.OuterXml; } } else { textBox2.Text = "No results"; } } catch (Exception ex) { MessageBox.Show(ex.Message); } } The code creates an instance of XPathDocument by passing the path of the Employees.xml file. Then an XPathNavigator is obtained by using the CreateNavigator() method of XPathDocument. The code then calls the Select() method of XPathNavigator, which accepts an XPath expression and returns an instance of XPathNodeIterator. The XPathNodeIterator class allows you to iterate through the returned nodes and has a number of properties and methods to assist you. To start with, the Count property tells you how many nodes were selected by the Select() method. After you are satisfied that there were some results, you can iterate through the selected nodes by using the MoveNext() method. On each node, you then use the Current property to give you a reference to the XPathNavigator that is positioned at the current node. You can then call any of the methods and properties of XPathNavigator. In our example, we simply display the OuterXml property of the underlying XPathNavigator in a text box. Though not used in our example, the CurrentPosition property of XPathNodeIterator returns the current index of the node being accessed. Now let s try some XPath expressions by using our application. Some XPath expressions relevant to our XML document (Employees.xml) are given in Table 4-6. Table 4-6. Examples of XPath Expressions
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To select an employee whose employee ID is 1 To select the employee whose first name is Andrew To select the last employee from the document To select the employee whose index is 2 To select an employee whose name contains Nancy To select the name of the first employee
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employees/employee[@employeeid=1] employees/employee[firstname/text()='Andrew'] employees/employee[last()] employees/employee[position()=2] employees/employee[contains(firstname,'Nancy')] employees/employee/firstname[text()]
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CH APT ER 4 AC CE SSI NG XM L DO C UMENT S BY USI NG TH E XPAT H DA TA MO D EL
Selecting Single Nodes
The Select() method returns all the nodes that are obtained after evaluating the XPath expression. There is a method called SelectSingleNode() that executes the supplied XPath expression and returns an XPathNavigator object (and not an XPathNodeIterator) that contains the first matching node for the specified expression. You can then use the XPathNavigator object to navigate through the nodes. SelectSingleNode() comes in handy when you know that your XPath expression is going to return just one node. For example, in our document we can use SelectSingleNode() to extract an employee matching a specific employee ID. To illustrate the use of SelectSingleNode(), you need to develop an application like the one shown in Figure 4-4.
Figure 4-4. Using the SelectSingleNode() method The application contains a text box to accept an employee ID and nine labels. Clicking the Show button displays details of an employee, and because the employee ID is unique in Employees.xml, we can safely use SelectSingleNode() here. Listing 4-4 shows the relevant code. Listing 4-4. Calling the SelectSingleNode() Method private void button1_Click(object sender, EventArgs e) { XPathDocument doc = new XPathDocument(Application.StartupPath + @"\employees.xml"); XPathNavigator navigator = doc.CreateNavigator(); XPathNavigator result = navigator.SelectSingleNode(@"employees/employee[@employeeid=" + textBox1.Text + "]"); result.MoveToFirstChild();
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