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Mapping Classe s to Table s
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Although this chapter uses C# to illustrate concepts and usage patterns, LINQ to SQL is language-agnostic It can be used in any managed language that supports LINQ In NET Framework 35, similar code can be written in VBNET Other languages are also adding LINQ support and will be able to enable the use of LINQ to SQL Let s look at the key components of a program that uses LINQ to SQL to access relational data We will begin by exploring a simple class that maps to a table
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Mapping Classes to Tables
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LINQ to SQL needs to know how your class maps to a database table Then you can use the LINQ to SQL API primarily through a class called to query for objects of a mapped class and save the changes class next We ll look at a mapped class and the
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Creating Entity Classes The following code fragment shows a simple class, , that is mapped to the Customers table in the Northwind sample database The declarations contain the LINQ to SQL namespaces added with the API used for data access and mapping, respectively
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A class is mapped to a table by adding the attribute The attribute has a property that speci es the name of the database table property is supplied, the database table name is assumed to be If no the same name as the class Instances of classes declared with the
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7: A Quick Tour of LINQ to SQL
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attribute are considered persistent and are known as entities; the classes themselves are called entity classes In addition to mapping classes to tables, each eld or property must be attribmapped to a database column This can be done by using the ute If you omit this attribute when declaring a eld of your class, this class member is assumed to be unmapped or nonpersistent This allows you to choose which members are persistent and augment the data from the database with additional, nonpersistent members In the preceding class class are mapped to the cordeclaration, both of the elds of the responding columns in the Customers table attribute has a variety of properties you can use to customize The the exact mapping between elds and the database s columns One propproperty It indicates that the database colerty of note is the umn is a member of the table s primary key attribute, you need to supply information in the As with the attribute only if it differs from what can be deduced from your eld property on the or property declaration In this example, the attribute conveys that the eld is part of the table s primary key Note that you don t have to specify the exact name or type if the defaults work
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The DataContext The is the main conduit by which you retrieve objects from the database and submit changes back to the database It encapsulates an property that is initialized with an instance of a conADONET nection object or a connection string that you supply in the constructor You to help retrieve customer objects by writing the can use the following code:
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This code creates an instance of the and then retrieves a set objects corresponding to rows where the eld is equal to of The results bring back information for six customers:
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Looking more closely at this code, you can see that a database table is accessible via the represented as a property of type method using its entity class to identify it:
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is a LINQ to SQL class that implements the key LINQ inter That is how the LINQ query for customers from Lonface don is implemented by LINQ to SQL Rather than use this code in the midst of your query, it is recommended instead of relying on the that you declare a strongly typed class and the method A strongly typed basic declares all properties as members
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The query for customers from London can then be expressed more simply:
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