Server-Side Controls

Now having web pages with button and checkboxes is nothing new. But what is new is how we can write code to interact with these controls. In classic ASP, writing code to manipulate them was very tedious. With ASP.NET we can access server-side controls simply as properties of the page. For example we can exclude a button on a page by setting the Visible property of the button to false. Furthermore, we can write new web server controls made up of standard HTML elements that we can re-use on a number of pages. The .NET Framework already ships with a number of pre-built server-side controls like that. Besides controls to handle all the standard HTML elements: check boxes, radio buttons, drop-down lists, etc, there are also more complex element like a input validators, a date picker, a grid control and a tree view, just to name a few.

Every interaction with a user element can generate an event which we can process with an event handler method. Clicking on a button, for example, generates an event which we can appropriately handle with a click event handler. Even though this sounds simple, keep in mind that under the covers is still HTTP and HTML. Since events occur in the client’s browser and the event handler method runs on the server each event generates an HTTP POST request that is sent  across network (think: slow transfer!) to the web server as illustrated by the following diagram.

Figure 17.3: Events from an ASP.NET page in a web browser are handled by the page on the server.

The web server processes the request and sends back another HTML page to display the results of the event. This can be a pretty lengthy operation, especially if your client sits in Zimbabwe on a 28.8k dial-up connection. Therefore you need to carefully choose when you want to employ server-side controls. Do not rely on server-controls for every task you need to perform, regular HTML controls are a lot more efficient if you can process an event on the client. For example, you probably do not want every click on a check box or a radio button to result in a trip to the web server. You can still design pages with web forms designer, even if you mix server-side controls and regular HTML elements. However, you do need to process a user event on the server when you have to update records in a database.