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Before formally defining .NET delegates, let s gain a bit of perspective. Historically, the Windows API made frequent use of C-style function pointers to create entities termed callback functions, or simply callbacks. Using callbacks, programmers were able to configure one function to report back to (call back) another function in the application. With this approach, Windows developers were able to handle button-clicking, mouse-moving, menu-selecting, and general bidirectional communications between two entities in memory. The problem with standard C-style callback functions is that they represent little more than a raw address in memory. Ideally, you should be able to configure callbacks to include additional type-safe information such as the number of (and types of) parameters and the return type (if any) of the method pointed to. Sadly, this is not the case in traditional callback functions and, as you may suspect, they can therefore be a frequent source of bugs, hard crashes, and other runtime disasters. Nevertheless, callbacks are useful entities.

ASP.NET 2.0 still provides the templated controls that were introduced with ASP.NET 1.0. These include the DataGrid, DataList, and Repeater. Most ASP.NET programmers won t use any of these controls any longer except for backward compatibility (in fact, they aren t discussed in this book). DataGrid: The DataGrid is completely replaced by the GridView, which provides the same set of features (and more) and simplifies the coding mode. By default, the DataGrid doesn t appear in the Visual Studio 2005 Toolbox. DataList: The DataList is mostly replaced by the GridView, which provides a similar set of templates and much simpler coding model. However, you could still use the DataList if you want to create a multicolumn table, where each cell is a separate record. The GridView doesn t support this unusual design, because it forces every record to occupy a separate row. Repeater: The Repeater still plays the same role as a bare-bones template-based control. Although it doesn t provide many features or frills, you might use it to create customized data displays. The Repeater doesn t add any built-in elements, so you aren t locked into a table-based format. However, getting the result you want takes a lot of work because the Repeater doesn t include higher-level features such as selection and editing. The chapter focuses exclusively on the new ASP.NET 2.0 data controls: the GridView, DetailsView, and FormView. If you are interested in using the older 1.x controls, refer to the previous edition of this book.

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In the .NET Framework, callbacks are still possible, and this functionality is accomplished in a much safer and more object-oriented manner using delegates. In essence, a delegate is a type-safe object that points to another method (or possibly a list of methods) in the application, which can be invoked at a later time. Specifically, a delegate maintains three important pieces of information: The address of the method on which it makes calls The parameters (if any) of this method The return type (if any) of this method

If you ve programmed with ASP.NET 1.x, you ve probably used the similar DataGrid control. Faced with the challenge of enhancing the DataGrid while preserving backward compatibility, the ASP .NET team decided to create an entirely new control to implement their improvements. This control is the GridView. The GridView is an extremely flexible grid control for showing data. It includes a wide range of hard-wired features, including selection, paging, and editing, and it is extensible through templates. The great advantage of the GridView over the DataGrid is its support for code-free scenarios. Using the GridView, you can accomplish many common tasks, such as paging and selection, without writing any code. With the DataGrid, you were forced to handle events to implement the same features.

Once a delegate object has been created and given the necessary information, it may dynamically invoke the method(s) it points to at runtime. Every delegate in the .NET Framework (including your custom delegates) is automatically endowed with the ability to call its methods synchronously or asynchronously. This fact greatly simplifies programming tasks, given that you can call a method on a secondary thread of execution without manually creating and managing a Thread object.

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