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<Column(Id:=True)> _ Public customerId As String
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because CustomerID is the primary key of the Customers table. LINQ will deduce whatever it can about a column, but you need to tell it explicitly that a column is part of a primary key. Note that you made only four of the eleven columns in the Customers table available to LINQ.
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s SQLMetal is a tool that comes with LINQ that can generate entity class declarations from a SQL Tip
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Server database. It s described in section 6.3 of the DLinq Overview for VB Developers.doc file, which comes with LINQ and is the basic documentation for LINQ to SQL.
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You created a data context:
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' Create data context Dim db As DataContext = New DataContext(connString)
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A data context does what an ADO.NET connection does, but it also does things that a data provider handles. It not only manages the connection to a data source but it translates LINQ requests (expressed in SQO) into SQL, passes the SQL to the database server, and creates objects from the result set.
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CHAPTER 18 s INTRODUCING LINQ
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You created a typed table:
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' Create typed table Dim customers As Table(Of Customers) = db.GetTable(Of Customers)()
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A typed table is a collection (of type System.Data.Dlinq.Table(Of T)) whose elements are of a specific type. The GetTable method of the data context specifies the data context to access and where to put the results. Here, you got all the rows (but only four columns) from the Customers table and the data context created an object for each row in the customers typed table.
s Data contexts can be strongly typed, and that s the recommended way to use them, but we intentionTip
ally didn t do that so we d have to explicitly create a typed table and show you how to load it. See DLinq Overview for VB Developers.doc, section 2.2, to learn how to define a strongly typed data context.
You declared a VB 9.0 implicitly typed local variable, custs, of type var:
' Query database Dim custs = _
An implicitly typed local variable is just what its name implies. When VB sees a declaration without a type, it infers the type of the local variable based on the type of the expression in the initializer to the right of the = sign. You initialized the local variable with a query expression:
From c In customers _ Select _ c
A query expression is composed of a From clause and a query body. We ve used the simplest form of the From clause and query body here. This From clause declares an iteration variable, c, to be used to iterate over the result of the expression, customers, that is, over the typed table we earlier created and loaded. A query body must include a Select or Group By clause, which may be preceded by a Where or Order By clause. Your Select clause was the most primitive possible:
Select _ c
and, like a SQL SELECT *, gets all columns, so the variable custs is implicitly typed to handle a collection of objects that contain all the fields in the Customers class.
CHAPTER 18 s INTRODUCING LINQ
s Note As we mentioned earlier in What Is LINQ VB translates query expressions into calls to SQO methods. In fact, VB can also translate parts of query expressions into extension methods, which enable you to code custom methods for use with LINQ objects. This is far beyond our scope, but it s a functional programming feature that you can use to great advantage.
Finally, you looped through the custs collection and displayed each customer. Except for the use of the var type in the For Each statement, this was just VB 2005:
' Display customers For Each c In custs Console.WriteLine( _ "{0} {1} {2} {3}", _ c.customerId, _ c.companyName, _ c.city, _ c.country _ )
Despite the new VB features and terminology, what you did should feel familiar. Once you get the hang of it, it s an appealing alternative for coding queries. You basically coded a query expression instead of SQL to populate a collection that you could iterate through with a For Each statement. However, you provided a connection string, but didn t explicitly open or close a connection. Further, no command, data reader, or indexer was required. You didn t even need the System.Data or System.Data.SqlClient namespace to access SQL Server. Pretty cool.
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