barcode library c# XML AND WEB DEVELOPMENT in Font

Printing ANSI/AIM Code 39 in Font XML AND WEB DEVELOPMENT

' Set up the manager Dim mgr As New XmlNamespaceManager(nav.NameTable) mgr.AddNamespace("af", ns) The XPath expression is built using the values in the text boxes. For this purpose, you use String.Format() to replace the dynamic values and compile the expression with the navigator: Dim path As String = String.Format("/af:Friends/af:User/" + _ "af:Attended[af:YearIn>={0} and af:YearOut<={1}]", _ txtYearFrom.Text, txtYearTo.Text) Dim expr As XPathExpression = nav.Compile(path) Setting the expression context to the namespace manager and executing the Select() method is straightforward: expr.SetContext(mgr) Dim it As XPathNodeIterator = nav.Select(expr) You will notice the differences in the way attribute and child elements are accessed from an XPathNavigator inside the while loop. First, an attribute value is retrieved directly through the GetAttribute() method: row.Cells(0).Text = attended.GetAttribute("Name", String.Empty) The first argument to the method is the name of the attribute to retrieve, and the second is its namespace. The attributes don t use namespaces, which is the default behavior in both XML and the XML Schema, as you saw in the previous chapter. To process the child YearIn and YearOut elements, you need to move the navigator cursor into the children of the current Attended node. This is achieved by the following method call: ' Iterate children of current Attended element attended.MoveToFirstChild() At this point, it s important to remark that you are sure there are children, thanks to our schema. You know the document is valid, because you uploaded it with an XmlValidatingReader. You can now appreciate the simplification in code that derives from that fact. The method in this code returns a Boolean indicating whether or not it could move to the first child. You know it will always succeed; otherwise, the document would have been invalid in the first place. If you had not used an XML Schema, you would have needed to add checks everywhere to ensure that the structure conformed to that expected by your code. The code to access children values is a little more complicated than with XmlNodes, but once you get used to it, it s fairly easy. Basically, you iterate until you run out of children (Loop While attended.MoveToNext()), checking the information in the current position to see if it s a node you re interested in: Do If attended.LocalName = "YearIn" AndAlso _ attended.NamespaceURI = ns Then row.Cells(1).Text = attended.Value ElseIf attended.LocalName = "YearOut" AndAlso _

attended.NamespaceURI = ns Then row.Cells(2).Text = attended.Value End If Loop While attended.MoveToNext() Retrieving the parent User element s ID requires moving the cursor upwards. As you have already moved into Attended s children, you need to move twice in that direction, and finally retrieve the attribute with the GetAttribute() method you used before: attended.MoveToParent() attended.MoveToParent() row.Cells(3).Text = attended.GetAttribute("ID", String.Empty) Once you have the results loaded in the table, you can change the values and click the Execute button again, to reload the table with the new values

As you ve learned in this chapter, there are three basic approaches available when accessing an XML file; the XmlReader, the DOM through XmlDocument, and the XPath-optimized XPathDocument. In this section, we ll sum up what differentiates each of them, to help you make the correct decision when choosing which one to use in your own applications. What mainly differentiates XmlReader from the other two approaches is that these readers provide forward-only, read-only access to documents, with no caching, while the other approaches cache an entire document in memory. The main difference between the XmlDocument and XPathDocument approaches is the editing capability: the latter is read-only. These differences give rise to a range of pros and cons for each one, as noted in Table 8-1. This table presents an analysis of the key features of all three approaches and compares implementations. Table 8-1. Comparing XmlReader, XmlDocument, and XPathDocument