XML and Web Services
XML (eXtensible Markup Language) is a platform-independent format for storing and transporting data. It is used extensively in .NET for configuration, data interchange, and web services.
XML Fundamentals
XML uses custom tags to describe data structure. A well-formed XML document has a declaration (<?xml version="1.0"?>), a single root element, properly nested and closed tags, and case-sensitive names. Elements contain text, attributes, or child elements. Comments use <!-- -->.
DTD and XML Schema
A DTD (Document Type Definition) defines the structure of an XML document — elements, attributes, and nesting rules. XML Schema (XSD) is more powerful, supporting data types, namespaces, and complex constraints. XSD uses XML syntax itself, making it easier to process programmatically.
XSLT
XSLT (eXtensible Stylesheet Language Transformations) transforms XML documents into other formats (HTML, text, different XML). It uses templates and XPath expressions to match and transform elements. XSLT processors apply stylesheets to source documents to produce output.
XPath
XPath navigates XML documents using path expressions. /root/child selects from root; //element selects anywhere; @attr selects attributes; predicates ([condition]) filter nodes. XPath is used in XSLT, XQuery, and XML processing libraries.
XML in .NET
.NET provides multiple XML APIs. XmlDocument (DOM) loads the entire document into memory for random access. XmlReader/XmlWriter provide fast, forward-only streaming. XDocument (LINQ to XML) combines DOM convenience with LINQ querying. Serialization converts objects to/from XML.
SOAP Web Services
SOAP (Simple Object Access Protocol) is an XML-based messaging protocol. A SOAP message has an Envelope containing a Header and Body. WSDL (Web Services Description Language) describes service operations. ASMX and WCF implement SOAP services in .NET.
RESTful Services
REST (Representational State Transfer) uses HTTP methods (GET, POST, PUT, DELETE) to operate on resources identified by URLs. REST services typically use JSON (lighter than XML). ASP.NET Web API and ASP.NET Core create RESTful services with routing, model binding, and content negotiation.
Summary
XML is fundamental to data interchange and configuration in .NET. Understanding XML structure, validation (DTD/XSD), transformation (XSLT), and both SOAP and RESTful web services equips students for enterprise application development.