Microsoft 9GD00001 manual Performing Queries on Multiple Tables, 205

Chapter 7: Working with Data 205

A data context is the code that is generated by VS when you run the LINQ to SQL item wizard. The Main method instantiates MyShopDataContext, which is the data context. The name came from when the LINQ to SQL item wizard ran and your naming of the *.dbml file.

LINQ to SQL queries are made with the data context, which contains a property that holds a collection of the class type that the property is named after, myShop.Customers and myShop.Orders in this case. The LINQ query in the Main method uses the myShop data context instance to access the Customers collection in the from portion of the query.

NOTE

The LINQ to SQL provider uses pluralized data context properties. However, the results are not perfect; for example, Deer becomes Deers, which is incorrect in English. Additionally, pluralization is designed for English and will produce strange results in languages other than English. If the pluralization generated by the LINQ of a class is incorrect, you can either double-click the class name in the Designer or change the class name via the Properties window.

This section introduced you to what goes into creating a LINQ to SQL query, but your queries will likely need to work with multiple tables, as discussed in the next section.

Performing Queries on Multiple Tables

Until now, all queries have been from a single data source or table, like Customers in Listing 7-3. Often, you need to combine the results from multiple tables, which is where select many and join queries are useful. To demonstrate how joins work, we’ll define

a scenario where you need to know the dates of all orders made and the name of the customer who made the order.

The select many lets you join tables based on associations in the LINQ to SQL Designer. From the parent object, you navigate to the child object and are able to access the properties of both parent and child. The following code shows how to perform a select many query that gets data from the Customer and Order tables and repackages it into a collection of data transfer objects:

C#:

var myShop = new MyShopDataContext();

var customers =

from cust in myShop.Customers from ord in cust.Orders select new

{

Name = cust.Name, Date = ord.OrderDate

};