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In a relational database system, data is organized into nicely normalized tables and accessed with a very simple but powerful query language SQL. SQL can work with any set of data in a database because the data is organized into tables, following strict rules. In a program, as opposed to a database, however, data is stored in class objects or structs that are all vastly different. As a result, there s been no general query language for retrieving data from data structures. The method of retrieving data from objects has always been custom-designed as part of the program. LINQ, however, makes it easy to query collections of objects. The following are the important high-level characteristics of LINQ: LINQ stands for Language Integrated Query and is pronounced link. LINQ is an extension of the .NET Framework that allows you to query collections of data in a manner similar to using SQL to query databases. With LINQ you can query data from databases, collections of program objects, XML documents, and more.

<ListBoxItem>Princess</ListBoxItem> <ListBoxItem>Avonlea</ListBoxItem> <ListBoxItem>Brumby</ListBoxItem> </ListBox> </StackPanel> The code-behind contains a simple event handler named lstbxDogs_SelectionChanged, as shown here.

.NET 4.0 introduces a new method to the File class called File.ReadLines()(), which returns IEnumerable<string> rather than a string array. Reading a file one line at a time is much more efficient than loading the entire contents into memory (File.ReadAllLines()). This technique also has the advantage that you can start working with a file immediately and bail out of the loop when you find what you are looking for without having to read the entire file. ReadLines()() is now the preferred method for working with very large files. IEnumerable<string> FileContent=File.ReadLines("MyFile.txt"); foreach(string Line in FileContent) { Console.Write(Line); } File.WriteAllLines()() and File.AppendAllLines()() now take an IEnumerable<string> as an input parameter. System.IO.Directory has the following new static methods that return IEnumerable<string>: EnumerateDirectories(path) EnumerateFiles(path) EnumerateFileSystemEntries(path)

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The following code shows a simple example of using LINQ. In this code, the data source being queried is simply an array of ints. The definition of the query is the statement with the from and select keywords. Although the query is defined in this statement, it is actually performed and used in the foreach statement at the bottom. static void Main() { int[] numbers = { 2, 12, 5, 15 }; IEnumerable<int> lowNums = from n in numbers where n < 10 select n; foreach (var x in lowNums) Console.Write("{0}, ", x); } This code produces the following output: 2, 5,

private void lstbxDogs_SelectionChanged( object sender, SelectionChangedEventArgs e ) { ListBox lb = sender as ListBox; ListBoxItem lbi = lb.SelectedItem as ListBoxItem; MessageBox.Show( lbi.Content.ToString(), "Dog Selected" ); }

System.IO.DirectoryInfo has the following new static methods: EnumerateDirectories(path) returns IEnumerable<DirectoryInfo> EnumerateFiles(path) returns IEnumerable<FileInfo> EnumerateFileSystemInfos(path) returns IEnumerable<FileSystemInfo>

In the previous example, the data source was simply an array of ints, which is an in-memory object of the program. LINQ, however, can work with many different types of data sources, such as SQL databases, XML documents, and a host of others. For every data source type, however, under the covers there must be a module of code that implements the LINQ queries in terms of that data source type. These code modules are called LINQ providers. The important points about LINQ providers are the following: Microsoft provides LINQ providers for a number of common data source types, as shown in Figure 21-1. You can use any LINQ-enabled language (C# in our case) to query any data source type for which there is a LINQ provider. New LINQ providers are constantly being produced by third parties for all sorts of data source types.

Figure 21-1. The architecture of LINQ, the LINQ-enabled languages, and LINQ providers There are entire books dedicated to LINQ in all its forms and subtleties, but that s clearly beyond the scope of this chapter. Instead, this chapter will introduce you to LINQ and explain how to use it with program objects (LINQ to Objects) and XML (LINQ to XML).

Enumerating files within a directory is also now more efficient than previously because file metadata such as a creation date is queried during enumeration. This means that Windows API calls should be unnecessary because the information is already retrieved.

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