MSDN Magazine - March 2008 - (Page 30)

Figure 4 Setting the PropertyNode Variable Value If propertyNode IsNot Nothing Then Dim propertyContentNode = _ propertyNode.Elements.First If propertyContentNode.Name.LocalName = _ propertyTypeName Then ‘ Type is the same, so just change the value. propertyContentNode.Value = propertyValue.ToString() Else ‘ Type is different, so delete the node. propertyNode.Remove() propertyNode = Nothing End If End If Next, the code uses one of the best new features in Visual Basic—replacement tokens within XML literals. Visual Basic allows you to embed XML content directly in the code, so you can refer to and create XML without needing to directly program against the Document Object Model (DOM) objects. In this case, the code creates a new XML element: propertyNode = _ <property fmtid= pid= name= > < > After this statement executes, the propertyNode variable contains an XElement object containing XML that looks like this: 2 Figure 5 Creating the Node If propertyNode Is Nothing Then Const fmtid As String = _ “{D5CDD505-2E9C-101B-9397-08002B2CF9AE}” Dim pid As String = “2” Dim propertiesNode As XElement = Nothing If customDoc IsNot Nothing Then propertiesNode = customDoc. .First Dim lastChild = propertiesNode.Elements.LastOrDefault If lastChild IsNot Nothing Then pid = (Integer.Parse(lastChild.@pid) + 1).ToString() End If End If End If If the propertyNode variable is Nothing, it’s time to create the node and set its content. The code in Figure 5 starts out by creating the fmtid constant and calculates the pid value to use. To calculate the pid to use for the new element, the code retrieves a reference to the Properties element and then finds the last child of the element. If the last child exists, the code retrieves the child’s pid value, increments it, and uses the new value. Note the various LINQ to XML features used in Figure 5: the First and LastOrDefault methods provide simple ways to access child elements, given specific criteria. Next, the code uses some features specific to Visual Basic to create the new property node: Dim propertyTypeXName As XName = _ GetXmlNamespace(vt) + propertyTypeName propertyNode = _ <property fmtid= pid= name= > < > Comparing the two code fragments, it’s easy to see how Visual Basic makes the replacements: each time it encounters a replacement token (delimited with ) it replaces the named token with its value. The tokens can even contain expressions, as you’ll see later. Note that the replacement token for an element name must contain an XName reference, not a string or some other value. (I have to thank Avner Aharoni, of the Visual Basic team, for helping me work out this little detail.) The final goal, before saving the custom XML part back to the document, is to recreate the entire contents of the part. The code must create the container XML document, insert all the existing custom properties, and then append the new custom property. It’s possible to take the existing custom properties part and simply call the Add method of the Properties element to add a new property child element (and that’s how I originally wrote the code). Doing so bypasses an important feature of Visual Basic namespace handling. When you use the Add method, Visual Basic can’t determine how it should handle namespace factoring, resulting in the insertion of redundant namespace information. In this case, Visual Basic inserts an explicit default namespace reference. Although the XML it creates is perfectly valid, the XML parser in Word fails when it tries to load the custom properties part. If you use the Add method, you’ll often find that you must explicitly remove redundant namespace references. (Thanks again to Avner, who explained this behavior to me.) To avoid this problem, the sample code completely recreates the custom properties part. Using this technique allows Visual Basic to factor out redundant namespace references and creates exactly the XML that Word expects: customDoc = _ In order to create the property node’s value element, the code requires an XName object containing the full type name (vt:i4, for example). To create that XName object, you use the new GetXmlNamespace keyword. Supplying the namespace abbreviation that you created in the Imports statement enables GetXmlNamespace to retrieve the corresponding XML namespace. Visual Basic overloads the + operator when used with XML namespaces, so you can write a statement like this to create the full XName instance: Dim propertyTypeXName As XName = _ GetXmlNamespace(vt) + propertyTypeName This code doesn’t need to reference the default namespace; because Visual Basic already has the information about the default namespace from the Imports statements, it can infer that infor- 30 msdnmagazine Advanced Basics

Table of Contents Feed for the Digital Edition of MSDN Magazine - March 2008

MSDN Magazine - March 2008 Contents Toolbox CLR Inside Out Data Points Advanced Basics Office Space Introducing ASP.NET MVC Loosen Up CI Server Performance Office Development Test Run Security Briefs Extreme ASP.NET Foundations .NET Matters {End Bracket}

MSDN Magazine - March 2008

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