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When you select a city, the second data source retrieves all the employees in that city. Here s the definition for the second data source: <asp:SqlDataSource ID="sourceEmployees" runat="server" ProviderName="System.Data.SqlClient" ConnectionString="<%$ ConnectionStrings:Northwind %>" SelectCommand="SELECT EmployeeID, FirstName, LastName, Title, City FROM Employees WHERE City=@City"> <SelectParameters> <asp:ControlParameter ControlID="lstCities" Name="City" PropertyName="SelectedValue" /> </SelectParameters> </asp:SqlDataSource> The trick here is the query is written using a parameter. Parameters are always indicated with an @ symbol, as in @City. You can define as many symbols as you want, but you must map each provider to another value. In this example, the value for the @City parameter is taken from the lstCities.SelectedValue property. However, you could just as easily modify the ControlParameter tag to bind to another property or control. Now when you run the page, you can view the employees in a specific city (see Figure 9-8).

Currently, no graphical tool exists for encrypting and decrypting configuration file settings. However, if you don t want to write code, you can use the aspnet_regiis.exe command-line utility, which is found in the directory c:\[WinDir]\Microsoft.NET\Framework\[Version]. To use this tool, you must have already created a virtual directory to set your application up in IIS (see 18 for more about that process). When using aspnet_regiis to protect a portion of a configuration file, you need to specify these command-line arguments: The -pe switch specifies the configuration section to encrypt. The -app switch specifies your web application s virtual path. The -prov switch specifies the provider name.

If you profile your database, you ll discover that by binding two controls to the same data source, you cause the query to be executed twice On the other hand, if you bind the page manually, you have the ability to bind the same object to two different controls, which means you need to execute the query only once Clearly, the SqlDataSource imposes a bit of unnecessary extra overhead here, but if you re aware of it you can design accordingly First, you should consider caching, which the SqlDataSource supports natively (see 11 for a full discussion) Second, realize that most of the time you won t be binding more than one control to a data source That s because the rich data controls the GridVew, DetailsView, and FormsView have the ability to present multiple pieces of data in a flexible layout.

Figure 9-8. Selecting records based on control selection It s important to understand the benefits and limitations of this example. First, when you create a parameterized command in a SqlDataSource tag, the parameters are properly encoded and SQL injection attacks aren t a problem (as discussed in 7). Second, all the data-bound controls you create rebind themselves after every postback. This means that when you select a city, the page will be posted back and both queries will be executed. This is probably extra database work you don t require, assuming the list of cities does not change frequently. Once again, this is a good place to consider caching (see 11 for details).

Here s the command line that duplicates the earlier example for an application at http://localhost/TestApp: aspnet_regiis -pe "appSettings" -app "/TestApp" -prov "DataProtectionConfigurationProvider"

If you use these controls, you ll need to bind only one control, which allows you to steer clear of this limitation It s also important to note that data binding is performed at the end of your web-page processing, just before the page is rendered That means the PageLoad event will fire, followed by any control events, followed by the PagePreRender event, and only then will the data binding take place The data binding is performed on every postback (unless you redirect to another page) If you need to write code that springs into action after the data binding is complete, you can override the PageOnPreRenderComplete() method This method is called immediately after the PreRender stage but just before the view state is serialized and the actual HTML is rendered..

A well-designed web application written for ASPNET will include separate components that may be organized into distinct data and business tiers Once you ve created these components, you can use them from any ASPNET web page or web service seamlessly You can create a component in two ways: Create a new cs file in the App_Code subdirectory: ASPNET automatically compiles any code files in this directory and makes the classes they contain available to the rest of your web application When you add a new class in Visual Studio, you ll be prompted to create the App_Code directory (if it doesn t exist yet) and place the file there Create a new class library project in Visual Studio: All the classes in this project will be compiled into a DLL assembly.