XML Files

Introduction to XML

As noted earlier, binary files are falling out of favor and being replaced by text-based formats like XML and JSON for transferring and storing data. Both are human-readable, making them easy to understand and edit. In this lesson, we’ll explore XML, a widely used format for data exchange.

XML stands for Extensible Markup Language and is used to structure, store, and transport data. It is both machine-readable and human-readable, making it a popular choice in many applications, such as configuration files, web services, and document storage.

Why Use XML?

• Structured Data: XML allows for a well-defined structure with nested elements.
• Self-Descriptive: Tags and attributes describe the data, making it easy to understand.
• Platform Independent: It can be used across different systems and technologies.

Let’s look at a simple example to understand the structure of an XML file.

XML Structure Example

Using our book example, the XML representation for a book collection might look like this:

<books>
    <book id="1">
        <title>Wind in the Willows</title>
        <author>Kenneth Grahame</author>
        <price>14.99</price>
        <year>1908</year>
    </book>
    <book id="2">
        <title>Alice in Wonderland</title>
        <author>Lewis Carroll</author>
        <price>9.99</price>
        <year>1865</year>
    </book>
</books>

Explanation

• Tags: Elements are defined with tags (<title>, <author>), similar to HTML.
• Attributes: Elements can have attributes, such as id="1", which provide additional information.
• Hierarchy: XML is structured as a tree, with a root element (<books>) and nested child elements (<book>).

Common XML Errors

1. Unclosed Tags: Every opening tag <title> must have a corresponding closing tag </title>.
2. Case Sensitivity: Tags are case-sensitive, so <Book> and <book> are different.
3. Invalid Characters: Avoid using characters like < and & inside elements; use escape sequences like < and &.

Working with XML in C#

C# provides several classes in the System.Xml namespace for handling XML files. The most commonly used classes include:

• XmlDocument: Reads and writes the entire XML document.
• XmlReader: Reads XML data one element at a time, useful for large files.
• XmlWriter: Writes XML data to a file, stream, or StringBuilder.

Writing to XML Files

To write to an XML file, we use the XmlWriter class. Remember to include the System.Xml namespace at the top of your program.

using System;
using System.Xml;

class Program
{
    static void Main(string[] args)
    {
        Console.WriteLine("Writing to an XML file");

        // Create a new book object
        Book willows = new Book(1, "Wind in the Willows", "Kenneth Grahame", 14.99, 1908);

        // Create an XmlWriter object
        using(XmlWriter xmlWriter = XmlWriter.Create("books.xml"))
        {
            xmlWriter.WriteStartDocument();
            xmlWriter.WriteStartElement("books");

            // Write book element
            xmlWriter.WriteStartElement("book");
            xmlWriter.WriteAttributeString("id", willows.Id.ToString());

            xmlWriter.WriteStartElement("title");
            xmlWriter.WriteString(willows.Title);
            xmlWriter.WriteEndElement(); // Close <title>

            xmlWriter.WriteStartElement("author");
            xmlWriter.WriteString(willows.Author);
            xmlWriter.WriteEndElement(); // Close <author>

            xmlWriter.WriteStartElement("price");
            xmlWriter.WriteString(willows.Price.ToString());
            xmlWriter.WriteEndElement(); // Close <price>

            xmlWriter.WriteStartElement("year");
            xmlWriter.WriteString(willows.Year.ToString());
            xmlWriter.WriteEndElement(); // Close <year>

            xmlWriter.WriteEndElement(); // Close <book>
            xmlWriter.WriteEndElement(); // Close <books>
            xmlWriter.WriteEndDocument();
        }

        Console.WriteLine("XML file written successfully.");
    }
}

Key Points

• Start with WriteStartDocument() and the root element.
• For each book, use WriteStartElement() for elements and WriteAttributeString() for attributes.
• Use WriteEndElement() to properly close each element.
• Convert all data to strings before writing.

Reading from XML Files

To read from an XML file, we can use the XmlReader class. The Read() method allows us to traverse the XML file element by element.

using System;
using System.Xml;

class Program
{
    static void Main(string[] args)
    {
        Console.WriteLine("Reading from an XML file");

        using (XmlReader xmlReader = XmlReader.Create("books.xml"))
        {
            while (xmlReader.Read())
            {
                if (xmlReader.IsStartElement())
                {
                    switch (xmlReader.Name)
                    {
                        case "title":
                            Console.WriteLine("Title: " + xmlReader.ReadString());
                            break;
                        case "author":
                            Console.WriteLine("Author: " + xmlReader.ReadString());
                            break;
                        case "price":
                            Console.WriteLine("Price: " + xmlReader.ReadString());
                            break;
                        case "year":
                            Console.WriteLine("Year: " + xmlReader.ReadString());
                            break;
                    }
                }
            }
        }
    }
}

Explanation

• XmlReader: Reads the XML file one element at a time.
• IsStartElement(): Checks if the current element is a starting tag.
• ReadString(): Reads the value inside an element, like "Wind in the Willows."

Using a List of Books

For more complex data, you can use a list of books:

using System.Collections.Generic;

List<Book> books = new List<Book>
{
    new Book(1, "Wind in the Willows", "Kenneth Grahame", 14.99, 1908),
    new Book(2, "Alice in Wonderland", "Lewis Carroll", 9.99, 1865)
};

// Writing multiple books
using(XmlWriter xmlWriter = XmlWriter.Create("books.xml"))
{
    xmlWriter.WriteStartDocument();
    xmlWriter.WriteStartElement("books");

    foreach (var book in books)
    {
        xmlWriter.WriteStartElement("book");
        xmlWriter.WriteAttributeString("id", book.Id.ToString());

        xmlWriter.WriteStartElement("title");
        xmlWriter.WriteString(book.Title);
        xmlWriter.WriteEndElement();

        xmlWriter.WriteStartElement("author");
        xmlWriter.WriteString(book.Author);
        xmlWriter.WriteEndElement();

        xmlWriter.WriteStartElement("price");
        xmlWriter.WriteString(book.Price.ToString());
        xmlWriter.WriteEndElement();

        xmlWriter.WriteStartElement("year");
        xmlWriter.WriteString(book.Year.ToString());
        xmlWriter.WriteEndElement();

        xmlWriter.WriteEndElement(); // Close <book>
    }

    xmlWriter.WriteEndElement(); // Close <books>
    xmlWriter.WriteEndDocument();
}

Reading Multiple Books

To read multiple books into a list, use a similar approach:

List<Book> books = new List<Book>();
Book currentBook = null;

using (XmlReader xmlReader = XmlReader.Create("books.xml"))
{
    while (xmlReader.Read())
    {
        if (xmlReader.IsStartElement())
        {
            switch (xmlReader.Name)
            {
                case "book":
                    currentBook = new Book();
                    books.Add(currentBook);
                    break;
                case "title":
                    currentBook.Title = xmlReader.ReadString();
                    break;
                case "author":
                    currentBook.Author = xmlReader.ReadString();
                    break;
                case "price":
                    currentBook.Price = double.Parse(xmlReader.ReadString());
                    break;
                case "year":
                    currentBook.Year = int.Parse(xmlReader.ReadString());
                    break;
            }
        }
    }
}

Conclusion

XML is a versatile format for storing structured data. Understanding how to read and write XML files in C# is an essential skill for working with data. Try creating and manipulating your own XML files to get comfortable with these concepts!

Exercises

1. Create an XML file representing a list of your favorite movies. Include elements like title, director, year, and rating.
2. Write a program to read this XML file and display the movies directed by a specific director.
3. Extend the Book example to include a publisher element. Modify the code to handle this new element.

Further Reading

• XML Basics
• Working with XML in C#

Explore more advanced topics like querying XML using LINQ to XML for more powerful data manipulation!