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Unless you need to build some extremely type safe custom collections, you might never need to use the where keyword in your C# projects. Regardless, the following handful of (partial) code examples illustrate how to work with the where keyword.

Instead of using this property, you ll bind the control when the page first loads This gives you the chance to insert the items immediately, before any other data binding actions take place: Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs) If (Not PageIsPostBack) Then ' Trigger the sourceEmployeeCities query, and bind the results lstCitiesDataSource = sourceEmployeeCitiesSelect(DataSourceSelectArgumentsEmpty) lstCitiesDataBind() ' Add the two new items, and select the first lstCitiesItemsInsert(0, "(Choose a City)") lstCitiesItemsInsert(1, "(All Cities)") lstCitiesSelectedIndex = 0 End If End Sub In this example, the data binding for the list control is performed only once, when the page is requested for the first time After that, the values in view state are used instead This code is identical for the SqlDataSource and the ObjectDataSource That s not true for the remainder of the example..

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Begin by assuming that you have created a custom generic class, and you want to ensure that the type parameter has a default constructor. This could be useful when the custom generic class needs to create instances of the T because the default constructor is the only constructor that is potentially common to all types. Also, constraining T in this way lets you get compile-time checking; if T is a reference type, then programmer remembered to redefine the default in the class definition (you might recall that the default constructor is removed in classes when you define your own): // MyGenericClass derives from object, while // contained items must have a default ctor. public class MyGenericClass<T> where T : new() { ... } Notice that the where clause specifies which type parameter is being constrained, followed by a colon operator. After the colon operator, you list each possible constraint (in this case, a default constructor). Here is another example: // MyGenericClass derives from object, while // contained items must be a class implementing IDrawable // and must support a default ctor. public class MyGenericClass<T> where T : class, IDrawable, new() { ... } In this case, T has three requirements. It must be a reference type (not a structure), as marked with the class token. Second, T must implement the IDrawable interface. Third, it must also have a default constructor. Multiple constraints are listed in a comma-delimited list; however, you should be aware that the new() constraint must always be listed last! Thus, the following code will not compile:

The next challenge is to intercept clicks on either of the first two items. You can accomplish this by handling the data source Selecting event, which occurs just before the query is executed. You can then check the parameters that are about to be supplied and cancel the operation if needed.

} You will also create an interface and its implementation for this layer. Unlike with the DAO layer, for the service layer, you will use one package for both files, as shown in Figure 9-12.

// Error! new() constraint must be listed last! public class MyGenericClass<T> where T : new(), class, IDrawable { ... } If you ever create a custom generic collection class that specifies multiple type parameters, you can specify a unique set of constraints for each, using separate where clauses: // <K> must extend SomeBaseClass and have a default ctor, // while <T> must be a structure and implement the // generic IComparable interface. public class MyGenericClass<K, T> where K : SomeBaseClass, new() where T : struct, IComparable<T> { ... } You will rarely encounter cases where you need to build a complete custom generic collection class; however, you can use the where keyword on generic methods, as well. For example, if you want to specify that your generic Swap<T>() method can only operate on structures, you would update the method like this: // This method will swap any structure, but not classes. static void Swap<T>(ref T a, ref T b) where T : struct { ... } Note that if you were to constrain the Swap() method in this manner, you would no longer be able to swap string objects (as is shown in the sample code) because string is a reference type.

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