Figure 2-5. A new row has been added from the code. in Visual Basic .NET

Generating QR Code 2d barcode in Visual Basic .NET Figure 2-5. A new row has been added from the code.

Figure 2-5. A new row has been added from the code.
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Now you ll learn to modify and delete rows. To modify a row, we first have to retrieve the row and change the values, as in Listing 2-6.
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Listing 2-6. Modifing a Row with LINQ to SQL
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Public Class UpdateExample Public Shared Function whereCondition(ByVal p As Person) As Boolean Return p.ID = 5 End Function Sub Listing2_6() Dim people = New PeopleDataContext() Dim predicateDelegate As New Func(Of Person, Boolean)(AddressOf whereCondition) Dim person = people.People.Single(predicateDelegate) person.FirstName = "Name" person.LastName = "Modified" Console.WriteLine(people.GetChangeText()) people.SubmitChanges() End Sub
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firstPress: LINQ to ADO.NET
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End Class
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In the code snippet, using the Single method we retrieve the unique row whose ID is equal to 5. Then we change some attributes and call SubmitChanges() to update the database table. Figure 2-6 shows the SQL generated by LINQ.
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Figure 2-6. The UPDATE statement built by LINQ to SQL
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As you can see, LINQ produces an UPDATE statement containing only the columns changed in the code. The last SELECT is useful for retrieving the number of rows affected by the updating process. Deleting a row is an easier process that doesn t involve a round trip to the database to first retrieve the row. We can use the Remove method of the DataContext class, and specify the object previously set with the identifier value that we want to remove from the table. Listing 2-7 shows the code.
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Listing 2-7. Using the Remove Method to Delete a Row from the Database
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Dim people = New PeopleDataContext() Dim person As New Person() person.ID = 5 people.People.Remove(person) Console.WriteLine(people.GetChangeText()) people.SubmitChanges()
The code creates a new Person object and sets its primary key to the value of the row we are going to remove. Finally, the object is passed to the Remove method and the changes are submitted with the SubmitChanges() method call.
firstPress: LINQ to ADO.NET
DataContext: Advanced Features
We ve focused our attention on the basic features provided by LINQ to SQL (in the System.Data.DLinq.dll assembly). Data contexts have even more features. In the next sections you ll see how to define relationships between entities, and the benefits of doing that.
Defining Relationships Between Entities
The first feature we ll look at regards relationships between tables. A relational database such as Microsoft SQL Server provides the capability to define a relationship between two tables using primary and foreign keys. For example, a table containing a list of orders could have a foreign key pointing to a customers table. Using this relationship we can easily retrieve all the records for a specific customer. Moreover, we can define the rules to apply to the rows of related tables when some action occurs. For example, we can inform the database to remove every order row for a customer when the related customer is removed. The relationships between objects are defined in a different way. Usually a class contains a collection of related objects from another class. LINQ to SQL provides a relational-like way to define a relationship between two entity classes. Thanks to new generic types such as EntitySet(Of T) and EntityRef(Of T), it s possible to define the class members that are involved in relationships. The steps to implement relationships between entity classes are as follows: 1. Add an EntitySet(Of T) private field in the parent entity class to collect the objects belonging to the child entity class. 2. Add the property that encapsulates the access to this private field. Additionally, we have to add the Association decoration to specify some properties, such as the relation name and the keys involved in the relation. 3. Add the initialization of this private field using its two-parameter constructor. 4. Add an EntityRef(Of T) private field in the child entity class to retrieve the instance of the parent entity object. 5. Add the property that encapsulates the access to this private field. Again, we have to add the Association attribute to the property. 6. Add the initialization of this private field using the default constructor.
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