User controls provide a way to encapsulate site functionality in ASP.NET. Because user controls are self-contained in standalone files, you can think of them as a "black box." Variables are explicitly passed to user controls by using public properties. Similarly, for user control methods to be accessed outside the scope of the user control, they must be declared as public. If the user control contains HTML or Web Form information, then it will be produced inline, where the user control was included in the calling form. In this way, a user control is like any other server control.
User controls are self-contained entities, and they support fragment caching. By using a user control and fragment caching, it is possible to cache the output of a single piece of functionality on a page. For instance, if a page in an application contains a menu and highly personalized user content, it would be difficult to send the entire page to the output cache because of the high number of potential permutations of the personalized content. For instance, if a page welcomes a user by name, it would be difficult to cache the entire page since a separate copy of the page would need to be stored in memory for each user name. Encapsulating certain pieces of the page, such as the menu, into a user control and then turning on fragment caching for the user control increases the overall performance of the page.
Because user controls can be programmatically manipulated, several developers can use user controls to work on the design of an ASP.NET application simultaneously. Because each user control is self-contained, each developer can work on his or her part of the puzzle and not worry too much about interfering with someone else's code.
Visual Studio and the Code Behind Method
Currently, there are two major paradigms for designing ASP.NET Web Forms:
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One method is to insert server-side code on a page that contains all the HTML and client-side code. This method of building pages should be familiar to ASP developers.
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Because ASP.NET Web Forms are compiled, you can place presentation logic such as HTML and any Web controls in one file and any server-side code (such as C# or Visual Basic.NET) in a second file. At compile time, the two pages are compiled as one entity. This method of designing ASP.NET Web Forms is called code behind.
Although many users might be reluctant to use this method, the code behind method offers some benefits. Presentation code such as HTML is neatly separated from any page logic being performed. This makes it easy for a development group in charge of graphics and layout to work independently of a group in charge of back-end development. Visual Studio.NET uses this code behind method of development. In fact, it doesn't support the traditional in-line code methodology at all.
If you have done significant development in previous versions of ASP, you might find code behind difficult to work with at first, especially when combined with all the other new features in ASP.NET. It's definitely worth learning to use, however, Visual Studio.NET impressively shortens development time.
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