Within a subprogram, one of the primary ways in which Visual Basic code is packaged on a Web page, variously call a subroutine or procedure. A subprogram has following general structure:

A subprogram is identified by the keyword Sub at its beginning and the keywords End Sub at its end and a subprogram name is a programmer-supplied name that follows Visual Basic naming conventions. The name must be unique among the collection of subprograms on the page. Visual Basic commands, or statements, are enclosed inside the subprogram to carry out the processing for which the subprogram is created.

A subprogram may or may not require a signature and a signature is a list of variables, called arguments, that are supplied to the subprogram in order for it to carry out its processing. Various types of signatures are given below.

Inside <script> tags with the attribute runat="server",all subprograms must be contained.Inside a single pair of <script> tags,any number of subprograms can be contained.

Subprograms are activated to perform their processing in several ways and one of the ways is to activate the subprogram every time the Web page is accessed, and prior to sending the page to the client who made the request. This particular subprogram has the required name Page_Load and it does not have a signature, although a pair of empty parentheses can be coded to indicate its absence: Page_Load().

The Page_Load subprogram is not required unless there is processing to take place every time the page opens and it might, for instance, decide to display the current date on your page. In this case to retrieve the system date from the server,the Page_Load subprogram would be an appropriate place and place it in an output control on the page, prior to sending the page to the browser.

Other common uses for the Page_Load subprogram are to populate drop-down lists,checkboxes, and radio buttons that appear on the page, or to access a set of records from a database for display on the page.

Every time the page loads or is refreshed,statements that appear in the Page_Load subprogram are run. Often times, however, it is necessary to perform the processing only the first time the page loads and this is common practice under ASP.NET where page controls automatically retain their values between post-backs of the page (when the page is reloaded due to an action of the user). All controls take part in page's View State and their values are retained between page postings.

As it has illustrated in the above example,this need to run a script only the first time the page loads and since the system date is assigned to an <asp:Label> control, the value of the control will be displayed and retained if the page is posted back to the server through some user action. It is not necessary every time the page loads to get the system date.

For these situations, script statements are placed inside a Page.IsPostBack condition test and they are executed only the first time the page loads and a server control is populated with the information generated by the script and subsequently, the control retains its information without having to repopulate it.

In most cases,by server controls subprograms are called upon to do their work through explicit subprogram calls. Various controls have various ways of passing information to it and calling a subprogram.

A common way for a page control to call a subprogram is through its OnClick="subprogram" event handler and naming a subprogram to run when the user clicks the control.As for example, a button control,

displays a standard button on the page and when the user clicks the button, the named subprogram is run

Subprograms that are called from server controls have special signatures depending on which event handler makes the call and what type of control. These signatures are in the general format shown below.

The table below gives the argument type for the server controls that call a subprogram through their various event handlers.

As a simple example of the kinds of information available through a subprogram's signature and take the case of a standard button that calls a subprogram through its OnCommand event handler.