List Controls

Many day-to-day tasks involve the listing of data in a Web page. In general, this data will be drawn from a data store of some type, perhaps a relational database. The aim of the ASP+ List Controls is to make building these kinds of pages easier. This involves encapsulating the functionality required within a single control or set of controls, saving development time and making the task easier.

The server-side ASP+ controls that generate the user interface can also manage tasks such as paging the list, sorting the contents, filtering the list, and selecting individual items. Finally, they can use the new server-side data binding features in ASP+ to automatically populate the lists with data. The three standard controls that are used to create lists are the Repeater, DataList, and DataGrid controls.

The Repeater control is the simplest, and is used simply to render the output a repeated number of times. The developer defines templates that are used to apply styling information to the header, item, footer and separator parts of the output that is created by the control. To create a table, for example, the header and footer information is used (as you would expect) for the header and footer of the table (the <thead> and <tfoot> parts in HTML terms). The item template content is applied to every row of the table that is generated from the repeated values – probably the records from a data store of some type. There is also the facility to use an alternatingItem template to apply different styles to alternate rows. The separator information defines the HTML output that will be generated after each row and before the next one.

The DataList control differs from the Repeater control in that it provides some intrinsic formatting of the repeated data as well as accepting the same kinds of template values as the Repeater control. The DataList control renders additional HTML (outside of that defined in its templates) to better control the layout and format of the rendered list, providing features such as vertical/horizontal flow and style support. However, unlike the Repeater control, it can also be used to edit the values in the elements on demand, and detect changes made by the user. 

The richest of the list controls is the DataGrid control. The output is an HTML table, and the developer can define templates that are used to apply styling information to the various parts of the table. As well as being able to edit the values in the table, the user can also sort and filter the contents, and the appearance is similar to that of using a spreadsheet. However, as the output from the control is simply HTML, it will work in all the popular browsers.

There are two other more specialized types of list control included with ASP+. These are the RadioButtonList and CheckboxList controls. In effect, they simply render on the client a list of HTML radio button or checkbox elements, with captions applied using span elements. However, you can easily specify the layout, i.e. if the items should appear listed horizontally across the page or vertically down it, or within an HTML table. You can also control the alignment of the text labels, and arrange for the list to automatically post back the selected values.