ASP.NET 4 in VB 2010 : The XML Classes (part 1) - The XML TextWriter

2/26/2013 6:26:53 PM

.NET provides a rich set of classes for XML manipulation in several namespaces that start with System.Xml. One of the most confusing aspects of using XML with .NET is deciding which combination of classes you should use. Many of them provide similar functionality in a slightly different way, optimized for specific scenarios or for compatibility with specific standards.

The majority of the examples you'll explore use the types in the core System.Xml namespace. The classes here allow you to read and write XML files, manipulate XML data in memory, and even validate XML documents.

In this article, you'll look at the following options for dealing with XML data:

  • Reading and writing XML directly, just like you read and write text files using XmlTextWriter and XmlTextReader. For sheer speed and efficiency, this is the best approach.

  • Dealing with XML as a collection of in-memory objects using the XDocument class. If you need more flexibility than the XmlTextWriter and XmlTextReader provide or you just want a simpler, more straightforward model (and you don't need to squeeze out every last drop of performance), this is a good choice.

  • Using the Xml control to transform XML content to displayable HTML. In the right situation—when all you want to do is display XML content using a prebuilt XSLT style sheet—this approach offers a useful shortcut.


When it comes to XML, Microsoft is a bit schizophrenic. The .NET Framework includes at least a dozen ways to read and manipulate XML. In the following sections, you'll spend most of your time exploring the two most practical ways to work with XML. First, you'll learn to use the basic XmlTextWriter and XmlTextReader classes, which guarantee good performance. Second, you'll explore the XDocument class, which can simplify intricate XML processing.

1. The XML TextWriter

One of the simplest ways to create or read any XML document is to use the basic XmlTextWriter and XmlTextReader classes. These classes work like their StreamWriter and StreamReader relatives, except that they write and read XML documents instead of ordinary text files. First, you create or open the file. Then, you write to it or read from it, moving from top to bottom. Finally, you close it and get to work using the retrieved data in whatever way you'd like.

Before beginning this example, you'll need to import the namespaces for file handling and XML processing:

Imports System.IO
Imports System.Xml

Here's an example that creates a simple version of the SuperProProductList document:

' Place the file in the App_Data subfolder of the current website.
' The System.IO.Path class makes it easy to build the full file name.
Dim file As String = Path.Combine(Request.PhysicalApplicationPath, _

Dim fs As New FileStream(file, FileMode.Create)
Dim w As New XmlTextWriter(fs, Nothing)

w.WriteComment("This file generated by the XmlTextWriter class.")

' Write the first product.
w.WriteAttributeString("ID", "", "1")
w.WriteAttributeString("Name", "", "Chair")



' Write the second product.
w.WriteAttributeString("ID", "2")
w.WriteAttributeString("Name", "Car")




' Write the third product.
w.WriteAttributeString("ID", "3")
w.WriteAttributeString("Name", "Fresh Fruit Basket")



' Close the root element.


1.1. Dissecting the Code . . .
  • You create the entire XML document by calling the methods of the XmlTextWriter, in the right order. To start a document, you always begin by calling WriteStartDocument(). To end it, you call WriteEndDocument().

  • The next step is writing the elements you need. You write elements in three steps. First, you write the start tag (like <Product>) by calling WriteStartElement(). Then you write attributes, elements, and text content inside. Finally, you write the end tag (like </Product>) by calling WriteEndElement().

  • The methods you use always work with the current element. So if you call WriteStartElement() and follow it up with a call to WriteAttributeString(), you are adding an attribute to that element. Similarly, if you use WriteString(), you insert text content inside the current element, and if you use WriteStartElement() again, you write another element, nested inside the current element.

In some ways, this code is similar to the code you used to write a basic text file. It does have a few advantages, however. You can close elements quickly and accurately, the angle brackets (< >) are included for you automatically, and some errors (such as closing the root element too soon) are caught automatically, thereby ensuring a well-formed XML document as the final result.

To check that your code worked, open the file in Internet Explorer, which automatically provides a collapsible view for XML documents (see Figure 1).

Figure 1. SuperProProductList.xml


By default, the XmlTextWriter will create an XML file that has all its elements lumped together in a single line without any helpful carriage returns or indentation. You don't see this limitation in Figure 18-1, because Internet Explorer uses a style sheet to give the XML a more readable (and more colorful) appearance. However, if you open the XML document in Notepad, you'll see the difference.

Although additional formatting isn't required (and doesn't change how the data will be processed), it can make a significant difference if you want to read your XML files in Visual Studio, Notepad, or another text editor. Fortunately, the XmlTextWriter supports formatting; you just need to enable it, as follows:

' Set it to indent output.
w.Formatting = Formatting.Indented

' Set the number of indent spaces.
w.Indentation = 5
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