Querying Document Object Model (DOM) Documents in Font

Make Code 39 in Font Querying Document Object Model (DOM) Documents

XmlReader-based objects provide forward-only access to the underlying XML data, which means that as soon as you move forward, you lose all information pertaining to the previous element. Clearly, such an approach is unsuited to querying a document, because you would end up reading the entire file for every query you perform. Even if the element you were looking for was the first one in the document, there would be no way to know that for sure, and you would need to read it in its entirety to be certain. To perform queries effectively, you really need the complete document in memory, so that you can perform all the queries without needing to reparse the file. The W3C again has an answer: the Document Object Model (or DOM). The DOM defines the way an XML document is stored in memory, and how its nodes are loaded, accessed, and changed using a collection approach: each node contains other nodes

as children, and these, in turn, can contain other nodes, and so on. The DOM allows you to navigate back and forth between child and parent elements, too, tinkering with them as you go. It is neither forward-only nor read-only. The DOM is built on several key building blocks. The fundamental one is the concept of the Document, which is to DOM what the Schema element is for XSD. This important object is implemented by the .NET Framework in the System.Xml.XmlDocument class. Try It Out: Query a DOM Document With the information about the DOM in mind, you re ready to build the code for performing XPath queries, as we outlined in the previous section. 1. Open the code-behind page for the UploadListReport.aspx web form and import the following namespaces at the top of the file: Imports Imports Imports Imports System.IO System.Xml System.Xml.XPath System.Text

2. Add the GetReader() helper method to the UploadListReport class. This method will serve the same purpose as the function by the same name in the UploadList.aspx page: Private Function GetReader() As XmlReader If Session("xml") Is Nothing Then Throw New InvalidOperationException( _ "No XML file has been uploaded yet.") End If ' Build the XmlTextReader from the in-memory string saved before Dim xmlinput As New StringReader(CType(Session("xml"), String)) Return New XmlTextReader(xmlinput) End Function 3. Locate the Page_Load() method and place the following code inside it: Private Sub Page_Load(ByVal sender As System.Object, _ ByVal e As System.EventArgs) Handles MyBase.Load ' Configure header MyBase.HeaderIconImageUrl = "~/Images/print.gif" MyBase.HeaderMessage = "Upload Attendees - Report" Dim ns As String = "http://www.apress.com/schemas/friendsreunion" Try ' Retrieve the reader object and initialize the DOM document Dim reader As XmlReader = GetReader() Dim doc As New XmlDocument doc.Load(reader)

' Initialize the namespace manager for the document Dim mgr As New XmlNamespaceManager(doc.NameTable) mgr.AddNamespace("af", ns) ' List of new users Dim nodes As XmlNodeList = doc.SelectNodes("/af:Friends/af:User", mgr) Dim row As TableRow = New TableRow Dim cell As TableCell = New TableCell cell.Text = "Users: " + nodes.Count.ToString() row.Cells.Add(cell) Dim sb As StringBuilder = New StringBuilder For Each node As XmlNode In nodes sb.AppendFormat("{0}, {1} ({2})<br/>", _ node("LastName", ns).InnerText, _ node("FirstName", ns).InnerText, _ node("Email", ns).InnerText) Next ' Add the cell with the accumulated list cell = New TableCell cell.Text = sb.ToString() row.Cells.Add(cell) tbReport.Rows.Add(row) Catch ex As Exception lblError.Text = ex.Message pnlError.Visible = True End Try If tbReport.Rows.Count = 1 Then tbReport.Visible = False End If End Sub 4. Double-click the btnBackImg and btnBackLink controls in the designer to create Click event handlers for each of these. Add the following line of code to each handler to allow the user to navigate back to the UploadList form: Private Sub btnBackImg_Click(ByVal sender As System.Object, _ ByVal e As System.Web.UI.ImageClickEventArgs) Handles btnBackImg.Click Response.Redirect("UploadList.aspx") End Sub Private Sub btnBackLink_Click(ByVal sender As System.Object, _ ByVal e As System.EventArgs) Handles btnBackLink.Click Response.Redirect("UploadList.aspx") End Sub