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The following example puts it all together It defines two GridView controls The first shows a list of categories The second shows the products that fall into the currently selected category (or, if no category has been selected, this GridView doesn t appear at all) Here s the page markup for this example: Categories:<br /> <asp:GridView ID="gridCategories" runat="server" DataSourceID="sourceCategories" DataKeyNames="CategoryID"> <Columns> <asp:CommandField ShowSelectButton="True" /> </Columns> <SelectedRowStyle BackColor="#FFCC66" Font-Bold="True" ForeColor="#663399" /> </asp:GridView> <asp:SqlDataSource ID="sourceCategories" runat="server" ConnectionString="<%$ ConnectionStrings:Northwind %>" SelectCommand="SELECT * FROM Categories"></asp:SqlDataSource> <br /> Products in this category:<br /> <asp:GridView ID="gridProducts" runat="server" DataSourceID="sourceProducts"> <SelectedRowStyle BackColor="#FFCC66" Font-Bold="True" ForeColor="#663399" /> </asp:GridView> <asp:SqlDataSource ID="sourceProducts" runat="server" ConnectionString="<%$ ConnectionStrings:Northwind %>" SelectCommand="SELECT ProductID, ProductName, UnitPrice FROM Products WHERE CategoryID=@CategoryID"> <SelectParameters> <asp:ControlParameter Name="CategoryID" ControlID="gridCategories" PropertyName="SelectedDataKeyValue" /> </SelectParameters> </asp:SqlDataSource> As you can see, you need two data sources, one for each GridView.

When you deal with an outside resource such as a database, you need to protect your code with a basic amount of error-handling logic. Even if you ve avoided every possible coding mistake, you still need to defend against factors outside your control for example, if the database server isn t running or the network connection is broken.

The second data source uses a ControlParameter that links it to the SelectedDataKey property of the first GridView Best of all, you still don t need to write any code or handle the SelectedIndexChanged event on your own Figure 16-7 shows this example in action..

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Exception is the base class for all exceptions. You can extend from Exception to define custom exceptions. ( 13 discusses some of the extended, built-in exceptions that are available.) Exception s constructor optionally takes an error message and an error code. The error message is self-explanatory, but error codes deserve some explanation. By providing an error code, you can later decide pragmatically what to do in the event of an exception. By inspecting the code returned, you can numerically map exceptions without needing to make your programming dependent on the error strings, which may be changed from time to time. This technique is demonstrated in the Error Coding section later in this chapter.

You can count on the SqlDataSource to properly release any resources (such as connections) if an error occurs. However, the underlying exception won t be handled. Instead, it will bubble up to the page and derail your processing. As with any other unhandled exception, the user will receive a cryptic error message or an error page. This design is unavoidable if the SqlDataSource suppressed exceptions, it could hide potential problems and make debugging extremely difficult. However, it s a good idea to handle the problem in your web page and show a more suitable error message. To do this, you need to handle the data source event that occurs immediately after the error. If you re performing a query, that s the Selected event. If you re performing an update, a delete, or an insert operation, you would handle the Updated, Deleted, or Inserted event instead. (If you don t want to offer customized error messages, you could handle all these events with the same event handler.) In the event handler, you can access the exception object through the SqlDataSourceStatusEventArgs.Exception property. If you want to prevent the error from spreading any further, simply set the SqlDataSourceStatusEventArgs.ExceptionHandled property to True. Then, make sure you show an appropriate error message on your web page to inform the user that the command was not completed. Here s an example: Protected Sub sourceProducts_Selected(ByVal sender As Object, _ ByVal e As SqlDataSourceStatusEventArgs) Handles sourceProducts.Selected If e.Exception IsNot Nothing Then lblError.Text = "An exception occurred performing the query." ' Consider the error handled. e.ExceptionHandled = True End If End Sub

The GridView provides support for editing that s almost as convenient as its support for selection. To switch a row into select mode, you simply set the SelectedIndex property to the corresponding row number. To switch a row into edit mode, you set the EditIndex property in the same way. Of course, both of these tasks can take place automatically if you use specialized button types. For selection, you use a CommandField column with the ShowSelectButton property set to true. To add edit controls, you follow almost the same step once again, you use the CommandField column, but now you set ShowEditButton to true. Here s an example of a GridView that supports editing: <asp:GridView ID="gridProducts" runat="server" DataSourceID="sourceProducts" AutoGenerateColumns="False" DataKeyNames="ProductID"> <Columns> <asp:BoundField DataField="ProductID" HeaderText="ID" ReadOnly="True" />

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