SAVE XML TO A FILE

You will sometimes need to persist data as XML. In ADO.NET, the persistence mechanism for DataSets is XML. XML provides an excellent way to save and

retrieve data without a database server.

One of the fastest ways to write data is by using the

XMLTextWriter class that is part of the System.Xml namespace. This writer provides a fast, forward-only way of generating XML and helps you to build XML documents that conform to the W3C Extensible Markup Language (XML) 1.0 and the Namespaces in XML specifications. You can find the latest XML specification at www.w3c.org.

The XMLTextWriter is an implementation of the XMLWriter abstract class. You can write your own

implementation of this abstract class, but if the XMLTextWriter has what you need, you use this .NET Framework class. Typically, you use an XMLTextWriter if you need to quickly write XML to file, stream, or a TextWriter, and do not need to use the Document

Object Model (DOM).

The XMLTextWriter has formatting capabilities to assist in giving a file with nice indentions that are handy when reading the documents in a text viewer. When you construct your XML, you use one of several Write methods, depending on what part of the XML document you are constructing (element, attribute, or comment).

° Set the XmlTextWriter variable equal to a new

XmlTextWriter, using the location of the XML file.

· Begin the XML document using the

WriteStartDocument method.

You can use verbatim strings to handcraft XML and set the indention in your code. Remember that you will have to double up your quotes inside of the string.

TYPE THIS:

using System; using System.IO; using System.Xml; public class Sample

{public static void Main()

{ XmlDocument doc = new XmlDocument();

string sXML =

@""<?xml version=""1.0"" standalone=""no""?> <!—This file represents a list of favorite photos—> <photofavorites owner=""Frank Ryan"">

<photo cat=""vacation"" date=""2000""> <title>Maddie with Minnie</title>

</photo>

</photofavorites>"; // end of string doc.LoadXml(sXML); doc.Save("data.xml");

}}

RESULT:

XML document created in the internals of the class and echoed out to the console.

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XML is great for portable data. If you want a quick way to query XML documents for pieces of data relevant to your application, XPath is a high-performance

mechanism to get this done. XPath is specified by W3C and is a general query language specification for extracting information from XML documents. XPath functionality has its own namespace in the .NET Framework. The System.Xml.XPath namespace has four classes that work together to provide efficient XML data searches.

The classes provided by System.Xml.XPath are:

XPathDocument, XPathExpression, XPathNavigator, and XPathNodeIterator. XPathDocument is used to cache your XML document in a high-performance oriented cache for XSLT processing. To query this cache, you will

need an XPath expression. This can be done with just a string that contains an XPath expression or you can use the XPathExpression class. If you want performance, you will use the XPathExpression class because it

compiles once and can be rerun without requiring subsequent compiles. The XPath expression is provided to a select method on the XPathNavigator class. The XPathNavigator object will return an XPathNodeIterator object from executing the Select method. After calling this method, the XPathNodeIterator returned represents the set

of selected nodes. You can use MoveNext on the XPathNodeIterator to walk the selected node set.

· Create an

XPathNodeIterator variable that will contain the result of running an XPath query that returns all of the photo/title elements.

‚ Add a while loop to output the name and the value of the node to

the console.

— Set a debug stop.

± Press F5 to save, build, and run the console application.

■ A message appears that shows the name and the value for the two elements that match the XPath query.