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Now that you have the reference, you need to add some using statements to take advantage of them in your program:
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using System.Data; using System.Data.Linq; using System.Data.Linq.Mapping;
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As we mentioned earlier, when you re using LINQ and SQL, you can treat the database tables as classes, and the columns as members. It just requires a bit of extra work on your part. You ll retrieve some simple information from Northwind s Employees table in this example.
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If you did the examples in 20, you should already have the Northwind database installed and attached on your machine. If not, turn back to 20 for detailed instructions.
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Create the following class in your application:
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[Table(Name = "Employees")] public class Employee { [Column] public int EmployeeID { get; set; } [Column] public string FirstName { get; set; } [Column] public string LastName { get; set; } }
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You ve probably noticed the extra code in square brackets that s unfamiliar to you. These are called attributes, and they re used in a lot of different places in C# to provide extra modifiers to your classes. In this case, you re using the Table attribute to indicate that this class is drawn from a table, specifically the Employees table in the associated database. Each of the public properties in the class has a Column attribute to indicate that the property is associated with a column in the table. As you probably remember from 20, you always need a data context before you can work with a database. With LINQ, creating the data context is much easier. Add the following line to Main( ):
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DataContext db = new DataContext("Data Source = .\\SQLExpress;" + "Initial Catalog=Northwind;Integrated Security=True");
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Notice that the DataContext object s constructor takes a string as its parameter the same connection string that you used to connect to the database in 20. The DataContext object has an important method, GetTable( ), which is how you retrieve the data table from the database and assign it to a generic Table collection. The collection holds the type of objects that you defined earlier in the application. So, add this Table<Employee> declaration to your application (after the DataContext line):
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Table<Employee> employees = db.GetTable<Employee>( );
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21: LINQ
That s all you need to retrieve the data. Now, though, you ll need a query. For this example, you ll simply retrieve all the employee rows in the table. That s where the LINQ syntax you saw earlier in this chapter comes in. You don t need to create a query string; just use the LINQ syntax directly:
var dbQuery = from emp in employees select emp;
Although dbQuery is declared using the var keyword, it returns an IEnumerable collection, which means that you can iterate over it with a foreach loop, just like you would any other collection. Add this code to output some of the data:
foreach (var employee in dbQuery) { Console.WriteLine("{0}\t{1} {2}", employee.EmployeeID, employee.FirstName, employee.LastName); }
Simple, right No more worrying about DataSet or DataRow objects. Example 21-6 shows the complete code for this example.
using using using using using using using System; System.Collections.Generic; System.Linq; System.Text; System.Data; System.Data.Linq; System.Data.Linq.Mapping;
namespace Example_21_6_ _ _ _LINQ_and_SQL { [Table(Name = "Employees")] public class Employee { [Column] public int EmployeeID { get; set; } [Column] public string FirstName { get; set; } [Column] public string LastName { get; set; } } class Program { static void Main( ) { DataContext db = new DataContext("Data Source = .\\SQLExpress; Initial Catalog=Northwind;Integrated Security=True"); Table<Employee> employees = db.GetTable<Employee>( ); var dbQuery = from emp in employees select emp; foreach (var employee in dbQuery) {
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