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CHAPTER 19 s USING LINQ
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A typed table is a collection (of type System.Data.Linq.Table(Of T) whose elements are of a specific type. The GetTable method of the DataContext class tells the data context to access the results and indicates where to put them. Here, you get all the rows (but only four columns) from the Customers table, and the data context creates an object for each row in the customers typed table. You initialize 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. You use 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 you earlier created and loaded. A query body must include a select or groupby clause that may be preceded by where or orderby clauses. Your select clause is the most primitive possible: Select c
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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. Finally, you loop through the custs collection and display each customer. Except for the use of the var type, which is a new data type in Visual Basic 2008, in the For Each statement, this was just Visual Basic 2005. 'display customers For Each c In custs Console.WriteLine("{0}, {1}, {2}, {3}", _ c.customerId, c.companyName, c.city, c.counTry) Next Despite the new Visual Basic 2008 features and terminology, this should feel familiar. Once you get the hang of it, it s an appealing alternative for coding queries. You basically code a query expression instead of SQL to populate a collection that you can iterate through with a For Each statement. However, you provide a connection string, but don t explicitly open or close a connection. Further, no command, data reader, or indexer is required. You don t even need the System.Data or System.Data.SqlClient namespaces to access SQL Server. Pretty cool, isn t it
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Try It Out: Using the where Clause
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Here, you ll modify LinqToSql to retrieve only customers in the United States. 1. Add the following two bold lines to LinqToSql.vb: 'query database Dim custs = From c In Customers _ where c.country = "USA" select c ;
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CHAPTER 19 s USING LINQ
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2. Rerun the program by pressing Ctrl+F5, and you should see the results shown in Figure 19-6.
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Figure 19-6. Retrieving only U.S. customers with a where clause
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You simply use a Visual Basic 2008 Where clause to limit the rows selected: Where c.country = "USA" It is just like a SQL WHERE clause, except for using "USA" instead of 'USA', since you code using Visual Basic 2008 here, not T-SQL.
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Using LINQ to XML
LINQ to XML provides an in-memory XML programming API that integrates XML querying capabilities into Visual Basic 2008 to take advantage of the LINQ framework and add query extensions specific to XML. LINQ to XML provides the query and transformation power of XQuery and XPath integrated into .NET. From another perspective, you can also think of LINQ to XML as a full-featured XML API comparable to a modernized, redesigned System.Xml API plus a few key features from XPath and XSLT. LINQ to XML provides facilities to edit XML documents and element trees in memory, as well as streaming facilities.
Try It Out: Coding a Simple LINQ to XML Query
In this exercise, you ll use LINQ to XML to retrieve element values from an XML document. 1. Navigate to Solution Explorer, right-click the 19 solution, and select Add New Project. From the provided list of Visual Studio installed templates, choose Console Application and name the newly added project LinqToXml. Click OK. 2. Rename Module1.vb to LinqToXml.vb. Replace the code in LinqToXml.vb with the code in Listing 19-3.
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