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Here s the complete code: Protected Sub sourceEmployees_Selecting(ByVal sender As Object, ByVal e As SqlDataSourceSelectingEventArgs) If CStr(e.Command.Parameters("@City").Value) = "(Choose a City)" Then ' Do nothing. e.Cancel = True Else If CStr(e.Command.Parameters("@City").Value) = "(All Cities)" Then ' Manually change the command. e.Command.CommandText = "SELECT * FROM Employees" End If End Sub This brute-force approach changing the command using a hard-coded query is ugly. Another approach is to cancel the operation, call another method that returns the appropriate data, and bind that. However, that forces you to do a fair bit of work manually, and mixing manual and automatic data binding can quickly get confusing. Unfortunately, no perfect solution exists.

I want to make one more comment on generic methods and constraints as this chapter draws to a close. When you create generic methods, it might come as a surprise to you that it causes a compiler error if you apply any C# operators (+, -, *, ==, etc.) on the type parameters. For example, imagine the usefulness of a class that can Add(), Subtract(), Multiply(), and Divide() generic types: // Compiler error! Cannot apply // operators to type parameters! public class BasicMath<T> { public T Add(T arg1, T arg2) { return arg1 + arg2; } public T Subtract(T arg1, T arg2) { return arg1 - arg2; } public T Multiply(T arg1, T arg2) { return arg1 * arg2; } public T Divide(T arg1, T arg2) { return arg1 / arg2; } }

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The object data source handles the problem better, because it gives you the option to reroute the command to another method. If you find that a full list of employees is required, you can remove the City parameter altogether and use a no-parameter method for retrieving all the employees. Here s how it works: Protected Sub sourceEmployees_Selecting(ByVal sender As Object, ByVal e As ObjectDataSourceSelectingEventArgs) If e.InputParameters("employeeID") Is Nothing Then e.Cancel = True End If If CStr(e.InputParameters("City").Value) = "(Choose a City)" Then ' Do nothing. e.Cancel = True ElseIf CStr(e.InputParameters("City").Value) = "(All Cities)" Then ' Manually change the method. sourceEmployees.SelectMethod = "GetAllEmployees" e.InputParameters.Remove("City") End If End Sub This solution isn t possible with the SqlDataSource, because the command logic is embedded into the data source control. Still, this approach can easily be abused and lead to code that is difficult to maintain. For example, you won t receive any warning if you rename, remove, or modify the parameters for the GetAllEmployees() method. In this case, you ll receive an error only when you run the page and click the (All Cities) option.

Unfortunately, the preceding BasicMath class will not compile. While this might seem like a major restriction, you need to remember that generics are generic. Of course, the numerical data can work just fine with the binary operators of C#. However, for the sake of argument, if <T> were a custom class or structure type, the compiler could assume the class supports the +, -, *, and / operators. Ideally, C# would allow a generic type to be constrained by supported operators, as in this example: // Illustrative code only! public class BasicMath<T> where T : operator +, operator -, operator *, operator / { public T Add(T arg1, T arg2) { return arg1 + arg2; } public T Subtract(T arg1, T arg2) { return arg1 - arg2; } public T Multiply(T arg1, T arg2) { return arg1 * arg2; } public T Divide(T arg1, T arg2) { return arg1 / arg2; } } Alas, operator constraints are not supported under the current version of C#. However, it is possible (albeit it requires a bit more work) to achieve the desired effect by defining an interface that supports these operators (C# interfaces can define operators!) and then specify an interface constraint of the generic class. In any case, this wraps up this book s initial look at building custom generic types. In the next chapter, you will pick up the topic of generics once again in the course of examining the .NET delegate type.

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