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public EmployeeDetails GetEmployee(int employeeID) { ... } The test page provides a list with all the employee IDs. This list control uses the GetEmployees() method through an Object data source: <asp:ObjectDataSource ID="sourceEmployeesList" runat="server" SelectMethod="GetEmployees" TypeName="DatabaseComponent.EmployeeDB"/> <asp:ListBox ID="lstEmployees" runat="server" DataSourceID="sourceEmployeesList" DataTextField="EmployeeID" AutoPostBack="True"/> When you choose an ID, the page posts back and uses a second data source to call GetEmployee(). The employeeID value is taken from the selected item in the list: <asp:ObjectDataSource ID="sourceEmployee" runat="server" SelectMethod="GetEmployee" TypeName="DatabaseComponent.EmployeeDB"/> <SelectParameters> <asp:ControlParameter ControlID="lstEmployees" Name="employeeID" PropertyName="SelectedValue" /> </SelectParameters> </asp:ObjectDataSource> The name you define for the parameter must match the parameter name you use in the method exactly. When the ObjectDataSource calls the method, it uses reflection to examine the method, and it examines the parameter names to determine the order of arguments. This system allows you to use overloaded methods, because the ObjectDataSource is able to correctly identify the overload you want based on the number of parameters you define and their names.

Earlier, you saw how you could use the SqlDataSource to execute parameterized commands. The same feat is possible with the ObjectDataSource, if you provide a suitable select method that accepts one or more parameters. You can then map each parameter to a control value, query string argument, and so on. To try this, you can use the EmployeeDB.GetEmployee() method, which retrieves a single employee by ID number. Here s the method declaration: Public Function GetEmployee(ByVal employeeID As Integer) As EmployeeDetails ... End Function The test page provides a list with all the employee IDs. This list control uses the GetEmployees() method through an Object data source: <asp:ObjectDataSource ID="sourceEmployeesList" runat="server" SelectMethod="GetEmployees" TypeName="DatabaseComponent.EmployeeDB"/> <asp:ListBox ID="lstEmployees" runat="server" DataSourceID="sourceEmployeesList" DataTextField="EmployeeID" AutoPostBack="True"/> When you choose an ID, the page posts back and uses a second data source to call GetEmployee(). The employeeID value is taken from the selected item in the list: <asp:ObjectDataSource ID="sourceEmployee" runat="server" SelectMethod="GetEmployee" TypeName="DatabaseComponent.EmployeeDB"/> <SelectParameters> <asp:ControlParameter ControlID="lstEmployees" Name="employeeID" PropertyName="SelectedValue" /> </SelectParameters> </asp:ObjectDataSource> The name you define for the parameter must match the parameter name you use in the method exactly. When the ObjectDataSource calls the method, it uses reflection to examine the method, and it examines the parameter names to determine the order of arguments. This system allows you to use overloaded methods, because the ObjectDataSource is able to correctly identify the overload you want based on the number of parameters you define and their names.

All expression builders must derive from the base ExpressionBuilder class (which is found in the SystemWebCompilation namespace) Here s how you might declare an expression builder for random number generation: public class RandomNumberExpressionBuilder : ExpressionBuilder { .. }.

The data types are not used in the matching process instead, the ObjectDataSource will attempt to convert the parameter value into the data type of the matching parameter using the appropriate type converter for that data type. If this process fails, an exception is raised.

To simplify life, it helps to create a static method that performs the task you need (in this case, random number generation): public static string GetRandomNumber(int lowerLimit, int upperLimit) { Random rand = new Random(); int randValue = randNext(lowerLimit, upperLimit + 1); return randValueToString(); } The advantage of this approach is that when you use CodeDOM, you simply generate the single line of code needed to call the GetRandomNumber() method (rather than the code needed to generate the random number) Now, you need to override the GetCodeExpression() method This is the method that ASPNET calls when it finds an expression that s mapped to your expression builder (while compiling the page) At this point, you need to examine the expression, verify no errors are present, and then generate the code for calculating the expression result (using a CodeExpression object from the SystemCodeDom namespace).

The data types are not used in the matching process instead, the ObjectDataSource will attempt to convert the parameter value into the data type of the matching parameter using the appropriate type converter for that data type. If this process fails, an exception is raised.