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CHAPTER 29 LINQ TO XML
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<ItemCount>300</ItemCount> </Fruit> </Fruits> Press enter to finish
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You can use the System.Xml.Linq namespace classes on their own. These classes can stand on their own, and there are no additional features that come from using these classes with LINQ queries. But to use these classes without LINQ queries is to miss out on the flexibility and simplicity that combining them can bring. In the following sections, I ll show you some different techniques for using queries on XML data. All of these examples could be achieved without the query, but the code required would be more verbose and more error-prone. Most of the examples in this section use the following XML file: < xml version="1.0" encoding="utf-8" > <Fruits> <Fruit> <Name>Cherry</Name> <Color>Red</Color> <StockLevel>500</StockLevel> </Fruit> <Fruit> <Name>Apple</Name> <Color>Green</Color> <StockLevel>230</StockLevel> </Fruit> <Fruit> <Name>Plum</Name> <Color>Red</Color> <StockLevel>300</StockLevel> </Fruit> <Fruit> <Name>Banana</Name> <Color>Yellow</Color> <StockLevel>100</StockLevel> </Fruit> <Fruit> <Name>Grape</Name> <Color>Green</Color> <StockLevel>400</StockLevel> </Fruit> </Fruits> This file is included in the Visual Studio projects as data.xml and loaded at the start of each example that uses it.
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CHAPTER 29 LINQ TO XML
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A good place to start is to query an XML document to extract data that matches something we are interested in. Listing 29-15 provides a very simple example. Listing 29-15. Performing a Query on XML Data using using using using System; System.Collections.Generic; System.Linq; System.Xml.Linq;
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class Listing 15 { static void Main(string[] args) { // load the XML data XElement rootNode = XElement.Load(@"..\..\data.xml"); // perform a query on the XML var results = from fruit in rootNode.Elements() let stockLevel = int.Parse(fruit.Element("StockLevel").Value) where stockLevel < 250 select new { Name = fruit.Element("Name").Value, Count = stockLevel }; // write out the results Console.WriteLine("Fruits that are low on stock:"); foreach (var item in results) { Console.WriteLine("Name: {0}, Current Stock: {1}", item.Name, item.Count); } // wait for input before exiting Console.WriteLine("Press enter to finish"); Console.ReadLine(); } } This query enumerates the elements that are immediate descendants of the root node. An int range variable is defined using the let keyword and assigned the value of parsing the Value property of the StockLevel element. A where clause filters for items that have a low stock level, and the select clause projects an anonymous type containing the Value property of the first-level element and the value of the int range variable. With a simple query, we have created a stock reorder report. Compiling and running Listing 29-15 produces the following results: Fruits that are low on stock: Name: Apple, Current Stock: 230
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CHAPTER 29 LINQ TO XML
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Name: Banana, Current Stock: 100 Press enter to finish
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Creating XML from LINQ Queries
LINQ to XML makes it very easy to create XML from a set of objects. Listing 29-16 provides a demonstration. Listing 29-16. Creating XML Using a LINQ Query using using using using System; System.Collections.Generic; System.Linq; System.Xml.Linq;
class Fruit { public Fruit(string nameParam, string colorParam, int stockParam) { Name = nameParam; Color = colorParam; ItemsInStock = stockParam; } public string Name { get; set; } public string Color { get; set; } public int ItemsInStock { get; set; } } class Listing 16 { static void Main(string[] args) { // create a data source containing Fruit objects List<Fruit> myFruitList = new List<Fruit>() { new Fruit("Cherry", "Red", 500), new Fruit("Apple", "Green", 230), new Fruit("Plum", "Red", 300), new Fruit("Banana", "Yellow", 100), new Fruit("Grape", "Green", 400) }; // perform a query to generate XElements IEnumerable<XElement> elements = from e in myFruitList select new XElement("Fruit", new XAttribute("Name", e.Name), new XAttribute("Color", e.Color), new XAttribute("StockLevel", e.ItemsInStock) );
CHAPTER 29 LINQ TO XML
// create a root node to contain the query results XElement rootNode = new XElement("Fruits", elements); // print out the XML data Console.WriteLine(rootNode); // wait for input before exiting Console.WriteLine("Press enter to finish"); Console.ReadLine(); } } Listing 29-16 defines the Fruit class, which has three attributes. In the Main method, I create a List<Fruit> and populate it with some Fruit objects. I then perform a LINQ query and use the select clause to project an XElement object for each Fruit object in the data source. I use the constructor parameter array of the XElement class to supply a set of three attributes for the XElement objects, each of which corresponds to a property value from the Fruit range variable. The result from my LINQ query is an IEnumerable<XElement> containing one XElement for each Fruit in the data source. At the moment, these are independent of one another. To create a hierarchy, I create a new XElement object to be the root node and pass the IEnumerable<XElement> query results as a constructor argument. I then print out the contents of the root node to the console, producing the following XML output: <Fruits> <Fruit Name="Cherry" Color="Red" StockLevel="500" /> <Fruit Name="Apple" Color="Green" StockLevel="230" /> <Fruit Name="Plum" Color="Red" StockLevel="300" /> <Fruit Name="Banana" Color="Yellow" StockLevel="100" /> <Fruit Name="Grape" Color="Green" StockLevel="400" /> </Fruits> Press enter to finish You ll appreciate how simple it is to use LINQ to create XML like this if you ve ever tried to do the same thing using another language or using another C# XML API. Combining LINQ queries with the System.Xml.Linq classes make generating XML simple and quick. In Listing 29-16, I made each Fruit property into an attribute, but I could as easily have created nested elements, comments, text blocks, and so on, by creating different objects when projecting the XElement for the range variable. Listing 29-17 provides an example. Listing 29-17. Creating Different Kinds of XML Element in a LINQ Query using using using using System; System.Collections.Generic; System.Linq; System.Xml.Linq;
class Listing 17 { static void Main(string[] args) {
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