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ASP.NET also allows you to use assemblies with custom controls just as easily as you use assemblies with custom components. This allows you to bundle reusable user interface output and functionality into self-contained packages so that they can be used over and over again within the same or multiple applications. Part 4 has more information about this technique.

For example, you could split the earlier example into two pages. In the first page, define a list control that shows all the available cities: <asp:SqlDataSource ID="sourceEmployeeCities" runat="server" ProviderName="System.Data.SqlClient" ConnectionString="<%$ ConnectionStrings:Northwind %>" SelectCommand="SELECT DISTINCT City FROM Employees"> </asp:SqlDataSource> <asp:ListBox ID="lstCities" runat="server" DataSourceID="sourceEmployeeCities" DataTextField="City"></asp:ListBox><br /> Now, you ll need a little extra code to copy the selected city to the query string and redirect the page. Here s a button that does just that: Protected Sub cmdGo_Click(ByVal sender As Object, ByVal e As EventArgs) Response.Redirect("QueryParameter2.aspx city=" & lstCities.SelectedValue) End Sub Finally, the second page can bind the GridView according to the city value that s supplied in the query string: <asp:SqlDataSource ID="sourceEmployees" runat="server" ProviderName="System.Data.SqlClient" ConnectionString="<%$ ConnectionStrings:Northwind %>" SelectCommand="GetEmployeesByCity" SelectCommandType="StoredProcedure">

All the rich data controls include a DataKeyNames property This property indicates which field (or fields) are considered primary keys To use this feature, you must set the DataKeyNames property with a comma-separated list of one or more key fields Often, you ll have only one key field, as shown here: <asp:GridView ID="gridEmployees" runat="server" DataSourceID="sourceEmployees" DataKeyNames="EmployeeID" .. > Key fields are given special treatment For example, the GridView makes it possible to easily retrieve the key information for a given row Additionally, when you commit an update to a row, the data control passes the original value of the key fields, as well as the changed value (unless it s read-only) The original value is used in a WHERE clause to locate the row so you can perform the update To avoid confusion, the parameter that contains the original value is automatically given the prefix original_.

As explained earlier, the pipeline of application events isn t limited to requests for .aspx web forms. It also applies if you request web services or even create your own handlers to deal with custom file types. Why would you want to create your own handler For the most part, you won t. However, sometimes it s convenient to use a lower-level interface that still provides access to useful objects such as Response and Request but doesn t use the full control-based web form model. One example is if you want to create a web resource that dynamically renders a custom graphic (a technique demonstrated in 30). In this situation, you simply need to receive a request, check the URL parameters, and then return raw image data as a JPEG or GIF file. By avoiding the full web control model, you save some overhead, because ASP .NET does not need to go through as many steps (such as creating the web-page objects, persisting view state, and so on). ASP.NET makes scenarios like these remarkably easy through its pluggable architecture. You can snap in new handlers for specialized file types just by adding configuration settings. But first, you need to take a closer look at the HTTP pipeline.

<SelectParameters> <asp:QueryStringParameter Name="City" QueryStringField="city" /> </SelectParameters> </asp:SqlDataSource>

For example, the @EmployeeID parameter becomes @original_EmployeeID This difference can lead to a potential problem You may begin using a parameter name such as @EmployeeID and then set the DataKeyName property to get access to another feature, such as selection When you attempt to perform an update, you ll receive an error indicating the parameter @EmployeeID can t be found What makes this problem even more confusing is that you can take certain actions in Visual Studio such as refreshing the schema for your data control that will automatically set the DataKeyName property without warning you Now that you understand the problem, the solution is quite simple Most of the time you will use the DataKeyNames property, so you should modify your command to use the original value of any read-only primary keys.