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.NET Programming: Forms & Controls. Christopher M. Pascucci. 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 .

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net programming forms controls

.NET Programming: Forms & Controls

Christopher M. Pascucci

web forms
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:
    • First is that of controls, which allows the user to enter information, make selections and indicate that data is to be transmitted.
    • 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: <input name=”txtName” type=”textbox”>
    • WFC Example: <asp:DropdownList id="lstProducts" runat="server">
web forms controls
Web Forms & Controls
  • An aspx page can only contain ONE <form runat="server"> control…!
  • All server controls must appear within a <form> tag, and the <form> 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.
    • <input name=”btnSubmit” type=”button” runat=“server”>
  • ***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
Client-Side Processing
  • 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.

<input type=“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.redirector an application.transfer.

<script language=vbscript>

    • Sub myClientProcess()
    • formName.submit()
    • end Sub

</script>

<Form name=”formName”>

<Input name=”btnClient” onclick=”myClientProcess” type=”button”>

OR

  • <script language=vbscript>
    • Sub btnClient_onclick()
    • formName.submit()
  • end Sub
  • </script>
  • <Form name=”formName”>
  • <Input name=”btnClient” type=”button”
client side processing1
Client-Side Processing
  • 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.
  • <script language=vbscript>
    • Sub checkForm()
    • IfMyForm.txtName.value = “” Then
    • msgbox(“Name cannot be blank!”)
    • Else
    • MyForm.submit()
    • End If
    • End Sub
  • </script>
  • <form name=”MyForm”>
  • <input name=”txtName” type=”textbox”>
  • <input name=”btnClient” onclick=”checkForm” type=”button”>
server side processing
Server-Side Processing
  • 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 <%= textVariable %>
  • 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
server side processing1
Server-Side Processing
  • Activating a function on the server-side from a button control with runat=“server.

With a CodeBehind

    • 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.
    • 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

    • Code in script tag
  • <script language="vbscript" runat="server">
    • Sub Answer_Click (ByVal sender As System.Object, ByVal e As System.EventArgs)
      • [Insert code of the Sub]
    • End Sub
  • </script>
  • <asp:Button id="btnAnswer" OnClick="Answer_Click" runat=”server”… </asp:Button>
more on html controls
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 & Controls

Demo

Code of ASPX page