XSLT Processing in .NET

August 14, 2002

Joe Feser

This article is meant to help XML developers understand the different ways XSLT transformations can be performed using the .NET framework. It alsos describe how to use various input sources for an XSLT transformation. In .NET, the System.Xml.Xsl.XslTransform class is used for transforming XML data using an XSLT stylesheet. System.Xml.Xsl.XslTransform supports the XSLT 1.0 syntax, using the http://www.w3.org/1999/XSL/Transform namespace.

Input and Output Data Sources to the XslTransform Class

There are several classes that may be used to read XML and XSLT documents for a transformation. The most versatile of these is the System.Xml.XmlReader class. Since System.Xml.XmlReader is an abstract class, another class must inherit from it. The first class of this type is System.Xml.XmlTextReader, which reads character streams and checks that XML is well-formed, but does not validate the XML against a DTD or schema.

The second class that inherits from System.Xml.XmlReader is System.Xml.XmlNodeReader, which allows data to be read from any XML Document Object Model (DOM) API; for example, System.Xml.XmlNode. The System.Xml.XmlNode does not have to be a complete XML document; it may be a child node.

The third type of XmlReader class is System.Xml.XmlValidatingReader, a validating reader capable of ensuring XML data conforms to a DTD, XML-Data Reduced (XDR) schema, or W3C XML Schema.

// Load the String into a TextReader
System.IO.TextReader tr = new System.IO.StreamReader("numbers.xsl");
// Use that TextReader as the Source for the XmlTextReader
System.Xml.XmlReader xr = new System.Xml.XmlTextReader(tr);
// Create a new XslTransform class
System.Xml.Xsl.XslTransform trans = new System.Xml.Xsl.XslTransform();
// Load the XmlReader StyleSheet into the XslTransform class
trans.Load(xr);

Another type of input class is System.Xml.XPath.XPathNavigator or any class that implements the System.Xml.XPath.IXPathNavigable interface, including System.Xml.XPath.XPathDocument, System.Xml.XmlDocument, and XMLDataDocument. The System.Xml.XPath.XPathNavigator is based on the XPath data model and provides the methods needed to implement XPath queries over any data store.

The System.Xml.XPath.XPathDocument is the fastest of such classes, because it's read only, and is the preferred class when the speed of XSLT transformations is the highest priority. System.Xml.XmlDocument is the second most efficient class; XMLDataDocument is not recommended for performing XSLT transformations. System.Xml.XPath.IXPathNavigable may be implemented against any data source, allowing any imaginable type of data to be used as the source in an XSLT transformation.

// The following code may be appended to the preceding example
// Create a new XPathDocument, loading the source from a file
System.Xml.XPath.XPathDocument xp = new 
  System.Xml.XPath.XPathDocument("numbers.xml");
// Create the Navigator
System.Xml.XPath.XPathNavigator xpn = xp.CreateNavigator();

The XslTransform class requires the source of the XML or XSLT data to be an XmlReader or System.Xml.XPath.XPathNavigator. The Load and Transform methods of the System.Xml.Xsl.XslTransform class do not provide an overloaded method for directly processing a Stream, but the example below demonstrates an indirect method to use a MemoryStream as the source location of the XML data and XSLT stylesheet.

// Create a Stream and show how it would be used as the source
// This is only a sample since no data exists in the stream.
System.IO.Stream st = new System.IO.MemoryStream();
// You would populate the Stream with the XML File here and set the position to 0
st.Position = 0;
// Using the Stream, load it into an XPathDocument
System.Xml.XPath.XPathDocument xp = new System.Xml.XPath.XPathDocument(st);
// Now do the same with the Xslt document
System.IO.Stream XSLTStream = new System.IO.MemoryStream();
// You would populate the Stream with the XSLT File here and set the position to 0
st.Position = 0;
// Use an XmlReader and pass in a Stream as the Source
System.Xml.XmlReader xsltXR = new System.Xml.XmlTextReader(XSLTStream);
// The XmlReader may be passed into the Load Method of XslTransformm

The line containing the initialization of the Stream is just to demonstrate the MemoryStream constructor; it is still necessary to populate the data. One other source we may wish to use for XML and XSLT documents is a document retrieved from a URL, in the form of a string along with an optional System.Xml.XmlResolver, which is the most efficient method to load the source from an existing file.

The xsl:output element is sometimes ignored depending on the type of output class used; if, for example, the output class is either an XmlWriter or an XmlReader class. For a detailed description of which attributes are supported on xsl:output and the classes that support them please read the "Outputs from an XslTransform" section of the MSDN documentation.

The minimum requirements to perform an XSLT transformation are an XML document, an XSLT document, and a valid object to handle the output. In the example below, an XPathDocument and stylesheet will be loaded from a file, and output directed to the system console.

// Create the XslTransform.
System.Xml.Xsl.XslTransform xslt = new System.Xml.Xsl.XslTransform();
// Load the stylesheet.
xslt.Load("numbers.xsl");
// Load the XML data file.
System.Xml.XPath.XPathDocument doc = new 
  System.Xml.XPath.XPathDocument("numbers.xml");
// Create the XmlTextWriter to output to the console.
System.Xml.XmlTextWriter writer = new 
  System.Xml.XmlTextWriter(System.Console.Out);
// Transform the file.
xslt.Transform(doc, null, writer);
// Be careful when using the console,
// closing it will not allow you to write any 
// additional information to it
writer.Close();

XSLT Transformations Involving Streams

It's also possible to output the results of a transformation directly to a System.IO.Stream object. In the following example, System.IO.MemoryStream will be used for output, and then a System.IO.StreamReader used to read the information back as a string and return the value from the function.

// Create the XslTransform.
System.Xml.Xsl.XslTransform xslt = new System.Xml.Xsl.XslTransform();
// Load the stylesheet.
xslt.Load("numbers.xsl");
// Load the XML data file.
System.Xml.XPath.XPathDocument doc = new 
  System.Xml.XPath.XPathDocument("numbers.xml");
// Create the Stream to place the output.
System.IO.Stream str = new System.IO.MemoryStream();
// Transform the file.
xslt.Transform(doc, null, str);
// Remember to Flush the Stream and set the position to 0
str.Flush();
str.Position = 0;

A System.Xml.XmlDocument or System.Xml.XPath.XPathDocument that is already loaded into memory is also on option for input. Both objects implement the System.Xml.XPath.IXPathNavigable interface, which allows each to be passed as parameters directly into the Transform method of the System.Xml.Xsl.XslTransform class.

// Create the XslTransform.
System.Xml.Xsl.XslTransform xslt = new System.Xml.Xsl.XslTransform();
// Load the stylesheet that creates XML Output.
xslt.Load("numbersXML.xsl");
// Load the XML data file into an XPathDocument.
System.Xml.XPath.XPathDocument doc = new 
  System.Xml.XPath.XPathDocument("numbers.xml");
// Create the Stream to place the output.
System.IO.Stream str = new System.IO.MemoryStream();
System.Xml.XmlWriter xw = new 
  System.Xml.XmlTextWriter(str,System.Text.Encoding.UTF8);
// Transform the file.
xslt.Transform(doc, null, xw);
// Flush the XmlWriter and set the position of the Stream to 0
xw.Flush();
str.Position = 0;

Stylesheet Parameters and Extension Objects

Parameters or extension objects may also be passed to the stylesheet. This is accomplished using the System.Xml.Xsl.XsltArgumentList class. Passing a parameter to a stylesheet gives the programmer the ability to initialize a globally scoped variable, which is defined as any xsl:variable that is a child of the xsl:stylesheet and not contained inside a xsl:template. A parameter may be added to the System.Xml.Xsl.XsltArgumentList class by calling the AddParam method providing a qualified name, the namespace URI and value. If the parameter value is not a String, Boolean, Number, Node Fragment, or NodeSet, it will be forced to a double or string. An extension object is any .NET class that provides methods that return an XSLT data type. Extension objects are added to the System.Xml.Xsl.XsltArgumentList using the AddExtensionObject method providing a qualified name and namespace URI.

// Using the TextReader, load it into an XPathDocument
System.Xml.XPath.XPathDocument xp = new 
  System.Xml.XPath.XPathDocument("numbers.xml");
// Create a new XslTransform class and load the stylesheet
System.Xml.Xsl.XslTransform trans = new System.Xml.Xsl.XslTransform();
// Load the XmlReader StyleSheet into the Transformation
trans.Load("numbersExtension.xsl");

// Create the XsltArgumentList class and add the 
// parameter using AddParam
System.Xml.Xsl.XsltArgumentList xslArg = new 
  System.Xml.Xsl.XsltArgumentList();
xslArg.AddParam("displayme","","Is this fun?");

// Create and add the extension object using AddExtensionObject
SayHello hi = new SayHello();
xslArg.AddExtensionObject("urn:SayHello",hi);

// Create the System.IO.Stream to place the output.
System.IO.Stream str = new System.IO.MemoryStream();

// Transform the file.
trans.Transform(xp, xslArg, str);

// Flush the XmlWriter and set the position of the Stream to 0
str.Flush();
str.Position = 0;

// Create a StreamReader to Read the Stream and Return the String
System.IO.StreamReader sr = new System.IO.StreamReader(str);
string xmlOut = sr.ReadToEnd();
// Close the StreamReader and the base Stream
sr.Close();
// Write the Results to the Console
Console.Write(xmlOut);

// Extension Object to return Hello Name
public class SayHello
{
  public string HelloName(string name) {
    return "Hello " + name;
}
}

There are advantages to using an extension object instead of embedding all the instructions within a script block, including better reuse of the classes and smaller, easier to maintain stylesheets.

Embedding Script or Code in XSLT

Programming and scripting language constructs may also be embedded and utilized in XSLT stylesheets by using the msxsl:script element. The prefix "msxsl" is assumed to be bound to the urn:schemas-microsoft-com:xslt namespace. Languages supported by the script tag include C#, VB.NET, and JScript.NET, which is the default. An implements-prefix attribute that contains the prefix representing the namespace associated with the script block must also exist in the msxsl:script element. Multiple script blocks may exist in a stylesheet, but only one language may be used per namespace.

<xsl:stylesheet version='1.0' 
     xmlns:xsl='http://www.w3.org/1999/XSL/Transform'
     xmlns:msxsl='urn:schemas-microsoft-com:xslt'
     xmlns:theScript='urn:CustomScript'>
  <xsl:output omit-xml-declaration='yes' method='text'
       media-type='text/plain' indent='no' />
  <xsl:variable name='displayme' />
 
  <msxsl:script implements-prefix='theScript' language='C#'>
  <![CDATA[
  public string HelloName(string name)
  {
    return "Hello " + name;
  }
  ]]>
  </msxsl:script>
  
  <xsl:template match='/'>
    <xsl:text disable-output-escaping='yes'>Print Integers > 3</xsl:text> 
    <xsl:apply-templates select='Root/Numbers' />
    Script Result: <xsl:value-of select='theScript:HelloName("Joe")' />
    Done: <xsl:value-of select='$displayme' />
  </xsl:template>
  
  <xsl:template match='Numbers'>
    Numbers:<xsl:apply-templates select='Integer[@value > 3]' />
  </xsl:template>
  
  <xsl:template match='Integer'>
    Integer: <xsl:value-of select='@value' />
  </xsl:template>
</xsl:stylesheet>

Resources

• Sample code from this article
• W3C XSL Transformations (XSLT) v1.0 Specifications
• W3C XML Path Language (XPath) v1.0 Specifications
• System.Xml.Xsl.XslTransform additional information
• System.Xml.XmlReader additional information
• System.IO.Stream additional information
• Code Samples in C# and VB.NET
• O'Reilly Network .NET DevCenter
• XML.com Column: Transforming XML

Acknowledgments:

I would like to thank Asad Jawahar, Arpan Desai and Praj Joshi for their help in developing this article.