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Connecting to and Querying Data with LINQ
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Notice in this example that you are using the strong types for tables, collections, and fields. This makes the programming easier, faster, and more readable because types are checked at compile time.
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Inserting, Updating, and Deleting with LINQ to SQL
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LINQ to SQL makes inserting, updating, and deleting data in your database a very simple process. It creates the connections between your O/R map and your database. You simply need to make a modification to object instances and then save the changes. You work with your object model to add a new instance to a collection, modify an object, or remove an object from the collection. When your change is complete, you call the SubmitChanges method of the DataContext object to write the results back to the database. For example, if you want to add a new author to the pubs database, you start by creating a new author object. You then add it to the Authors table by using the InsertOnSubmit method. When you are ready to submit all changes, you call SubmitChanges. The following code shows this example.
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Sample of Visual Basic Code Dim pubs As PubsDatabaseModelDataContext = _ New PubsDatabaseModelDataContext() Dim auth As New author auth.au_id = "000-00-0001" auth.au_fname = "Monica" auth.au_lname = "Brink" auth.address = "555 Some St." auth.state = "WA" auth.city = "Redmond" auth.contract = False auth.phone = "555-1212" auth.zip = "12345" pubs.authors.InsertOnSubmit(auth) pubs.SubmitChanges() Sample of C# Code PubsDatabaseModelDataContext pubs = new PubsDatabaseModelDataContext(); author auth = new author(); auth.au_id = "000-00-0001"; auth.au_fname = "Monica"; auth.au_lname = "Brink"; auth.address = "555 Some St."; auth.state = "WA"; auth.city = "Redmond"; auth.contract = false; auth.phone = "555-1212"; auth.zip = "12345"; pubs.authors.InsertOnSubmit(auth); pubs.SubmitChanges();
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Lesson 2: LINQ and ADO.NET
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Deleting data works much the same way. You call the DeleteOnSubmit method instead. You pass the instance you want to delete to this method. You then call SubmitChanges. To update data, you first retrieve the data by using a query (or by using the GetTable method of the DataContext object). You then update the data and call SubmitChanges to save your changes to the database.
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LINQ to Entities works in a manner that is similar to that of LINQ to SQL. You define a model to represent your application domain. You then create a map between this model and your actual data source. You can then load data into the model and query against it by using LINQ. However, LINQ to Entities uses the ADO.NET Entity Framework as a basis for the models. These models can be created for any data source (not just for SQL Server). The ADO.NET Entity Framework allows you to create a conceptual model of your application domain. This ensures that you write application code against the conceptual domain objects rather than directly against a relational database. This allows your model and database to evolve independently. If changes need to be made to either, you only need to change the mappings. In this way, you can even change your storage schemas without having to change your actual application code. You would just change the mapping.
Creating an Entity Model
You create an Entity Framework model by using the ADO.NET Entity Data Model template from the Add New Item dialog box in Visual Studio. This file is an EDMX file. Like a LINQ to SQL DBML file, this file has an XML representation of the schema (which can be edited in the designer). It also includes a code-behind class file that contains the actual entity objects in your model as well as the database connectivity and the table and column mappings. When you add a new ADO.NET Entity Data Model to your website, Visual Studio walks you through the Entity Data Model Wizard. Here you can choose to create your model from scratch or generate it from an existing database structure. You can also select the connection to an actual database. Finally, you select the items you want to include in your model. Figure 11-5 shows an example. Notice that you can also pluralize or singularize object names to make sure individual objects are singular and collections of those objects have plural names. When you have finished using the wizard to associate your entities with their data source counterparts, Visual Studio generates the EDMX file. Again, this includes a definition of the current database structure, an object structure, and a mapping between these two structures. Figure 11-6 shows an example of the pubs database generated as an Entity Framework model.
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