Christopher M. Pascucci.NET Programming: Forms & Controls

Web Forms  Forms are the means by which a user can transmit data to the server from the browser.  A form is populated with one or more elements that are called controls.  Examples are the textbox, dropdown, radio button, checkbox, and buttons.  Transmitting data to the server from the client/browser involves 2 concepts: 1.First is that of controls, which allows the user to enter information, make selections and indicate that data is to be transmitted. 2.Second are events, which occur during some action (clicking a button) and allow a particular control to connect itself to a function, either on the client-side or server-side.  ASP.NET introduces a different set of form controls, called Web Form Controls (WFC). The original html form elements are called HTML Form Controls (HFC).  HFC Example:  WFC Example:

Web Forms & Controls  An aspx page can only contain ONE control…!  All server controls must appear within a tag, and the tag must contain the runat="server" attribute.  The runat="server" attribute indicates that the form should be processed on the server.  It also indicates that the enclosed controls can be accessed by server scripts.  An HFC can be converted to exhibit the behavior of a WFC by adding the runat="server" attribute to its HTML control tag.   ***Only HFCs are subject to being intercepted on the client-side for script processing, such as one would want to do for validation.  WFCs or an HFC that has been converted like the above will submit the form and by pass any possibility of doing any client-side processing.  The client side script can access the content of either an HFC or WFC control.

Client-Side Processing 1.Intercepting a button for function processing.  To intercept a form button with a client script function, the button must be an HTML form control (HFC). It can be done in either of the following ways:  You can use a button with type=“button” attribute of the Input tag and call a client-side script function or the click event of the button.  The form action can be invoked within the client script function by the statement formName.submit(), as shown in the code snippet.  If you wish to change the action, you can do it in the function with the statement formName.action = “newURL”, but only if the URL refers to a page that is.htm or.asp.  Transferring to an aspx page from the client script will result in a viewstate error. Aspx transfers can be made in the client script function using the statement window.location.href =”newURL”. Alternatively, aspx transfers can be made on the server-side using either a response.redirect or an application.transfer. Sub myClientProcess() … formName.submit() end Sub OR Sub btnClient_onclick() … formName.submit() end Sub <Input name=”btnClient” type=”button”

Client-Side Processing 2.Referencing the controls from client-side script.  Either type of control (HFC or WFC) can be referenced for processing within a client script function. This is the means by which one can validate data contained in the control.  The control is referenced as formName.controlName.value. Sub checkForm() If MyForm.txtName.value = “” Then msgbox(“Name cannot be blank!”) Else MyForm.submit() End If End Sub

Server-Side Processing 1.Accessing data from an HFC.  To access an html form control on the server side, use request("controlName") in the aspx page.  To write data back to a web page use response.write(textVariable) or the shorthand format 2.Accessing data from a WFC or HFC with runat=“server”.  To reference any runat="server" control on the server side, whether it is an HFC or WFC, simply use its object name with an appropriate property.  An HFC control is referenced as ctlName.value, and a WFC control is referenced as controlName.text.  The following table summarizes the above cases for access of controls on both the client and server sides, the method is given for ctlName.method SideWeb Form ControlHTML Form Control (runat=“server”) HTML Form Control (run on client) Client.value Server.text.valueRequest(“ControlName”)

Server-Side Processing 3.Activating a function on the server-side from a button control with runat=“server. With a CodeBehind a)You can double click on the button control in design mode. It will automatically create a Private function in the Code Behind named controlName_Click with phrase Handles controlName.Click afterward. b)You can place an attribute OnClick="functionName()" in the button's tag, it will call a function Public functionName() in the codeBehind, when the button is clicked. You do not need the Handles btnResetTable.Click phrase. Without a CodeBehind c)Code in script tag Sub Answer_Click (ByVal sender As System.Object, ByVal e As System.EventArgs) [Insert code of the Sub] End Sub

More on HTML Controls  The expression Request("controlName") accesses the data in the control named controlName on the server-side from the form that was submitted on the client side.  This can be used in the ASPX page as script or in the Code Behind.  If no control with the name controlName has been submitted, then "" (blank string) is returned. This can happen in the following circumstances:  No such name exists on the form.  If all checkboxes or radio buttons with that name are unchecked.  If a textbox with that name has "“ (empty string) in it.  For more info on Form controls refer to the class website page on Forms & ControlsForms & Controls Demo Code of ASPX page