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Web Forms in ASP.NET

In ASP.NET Web forms, it is essential to write all the server controls within the <form> tag and runat="server" attribute must be included in it. This attribute shows that the form is to be processed on the server. Also it indicates that the controls enclosed in the <form> tag can be accessed by server scripts.

 

For instance:

<form runat="server"> …HTML + server controls </form>

If you don't specify the action attribute, it will be ignored as the form is always submitted to the page itself. If method attribute is left out, by default it will be set to method="post". Other attributes like name and if are also automatically assigned by ASP.NET if you don't mention them. Important point to keep in mind here is that an .aspx page can have only one <form runat="server"> control.

If you leave out name, method, action and ID attribute, the .aspx page will look as follows if you view the source:

<form name="_ct10" method="post" action="page.aspx" id="_ct10"> // code </form>

Submitting a Form

A button is used mostly to submit a form. The format of the Button server control is as follows in ASP.NET:

<asp:Button id="id" text="label" OnClick="sub" runat="server"/>

Here the id attribute sets a unique name for the button and the text attribute assigns a label to the button. The onClick event handler sets a named subroutine to execute.

In the following example we declare a Button control in an .aspx file. A button click runs a subroutine which changes the text on the button:

<script runat="server">
Sub submit(Source As Object, e As EventArgs)
button1.Text="You clicked me!"
End Sub
</script>
<html>
<body>
<form id="Form1" runat="server">
<asp:Button id="button1" Text="Click me!" runat="server"
OnClick="submit" />
</form>
</body>
</html>

The above example will show the following page:

When you will click on this button, the output will be as follows:

Maintaining the ViewState

In Classic ASP, on the submission of the form all the values are cleared out. For instance, you submit a form with a mass of information and it comes back with an error. For this, you will need to correct the information in the form. As soon as you hit the "back" button you go back to the previous page where all the values would be cleared and you will have to write all the information again. In short, it means that Classis ASP didn't offer to maintain your ViewState.

In ASP.NET when you submit a form, the form will re-appear in the browser with the values that you filled-in. This is possible because your ViewState is mainted by ASP.NET. It indicates the status of the page when the form is submitted to the server. A hidden field on each page is placed with a <form runat="server"> control to define this status.

The code would look like as follows:

<form name="_ctl0" method="post" action="page.aspx" id="_ctl0">
<input type="hidden" name="__VIEWSTATE"
value="dDwtNTI0ODU5MDE1Ozs+ZBCF2ryjMpeVgUrY2eTj79HNl4Q=" />
\\ code
</form>

ASP.NET by default maintains the ViewState for the Web Forms. If you don't want to maintain it then include the directive <%@ Page EnableViewState="false" %> at the top of an .aspx page or add the attribute EnableViewState="false" to any control.

Following code shows the old method of doing it. As soon as you click on the submit button, the values on the forms will disappear.

<html>
<body>
<form action="demo_classicasp.aspx" method="post">
Your name: <input type="text" name="fname" size="20">
<input type="submit" value="Submit">
</form>
<%
Dim fname
fname = Request.Form("fname")
If fname <> "" Then
    Response.Write("Hello " & fname & "!")
End If
%>
</body>
</html>

Following screen will appear when you run the page:

I entered my name and clicked on the submit button, following is the output:

Following code demonstrates the new way adopted in ASP.NET. The form values don't disappear when you click on the submit button.

<script runat="server">
Sub submit(sender As Object, e As EventArgs)
lbl1.Text="Hello " & txt1.Text & "!"
End Sub
</script>
<html>
<body>
<form id="Form1" runat="server">
Your name: <asp:TextBox id="txt1" runat="server" />
<asp:Button ID="Button1" OnClick="submit" Text="Submit" runat="server" />
<p><asp:Label id="lbl1" runat="server" /></p>
</form>
</body>
</html>

You see the following output when you run this page:

I typed my name and clicked on the submit button, following is the output:

The TextBox Control

There are scenarios where you require taking input text from users, for this the TextBox control is used to create a text box. The following table shows the properties and description of TextBox control:

Properties

Description

AutoPostBack

A Boolean value that specifies whether the control is automatically posted back to the server when the contents change or not. Default is false

Columns

The width of the textbox

id

A unique id for the control

MaxLength

The maximum number of characters allowed in the textbox

OnTextChanged

The name of the function to be executed when the text in the textbox has changed

Rows

The height of the textbox (only used if TextMode="Multiline")

runat

Specifies that the control is a server control. Must be set to "server"

Text

The contents of the textbox

TextMode

SingleLine creates a text box with only one line. MultiLine creates a text box with multiple lines. Password creates a one-line text box that masks the value entered by the user. Default is SingleLine. Legal values are:
   - SingleLine
   - MultiLine
   - Password

Wrap

A Boolean value that indicates whether the contents of the textbox should wrap or not

The example below demonstrates some of the attributes you may use with the TextBox control:

<html>
<body>
<form id="Form1" runat="server">
A basic TextBox:
<asp:TextBox id="tb1" runat="server" />
<br /><br />
A password TextBox:
<asp:TextBox id="tb2" TextMode="password" runat="server" />
<br /><br />
A TextBox with text:
<asp:TextBox id="tb4" Text="Hello World!" runat="server" />
<br /><br />
A multiline TextBox:
<asp:TextBox id="tb3" TextMode="multiline" runat="server" />
<br /><br />
A TextBox with height:
<asp:TextBox id="tb6" rows="5" TextMode="multiline"
runat="server" />
<br /><br />
A TextBox with width:
<asp:TextBox id="tb5" columns="30" runat="server" />
</form>
</body>
</html>

<html>
<body>
<form id="Form1" runat="server">
A basic TextBox:
<asp:TextBox id="tb1" runat="server" />
<br /><br />
A password TextBox:
<asp:TextBox id="tb2" TextMode="password" runat="server" />
<br /><br />
A TextBox with text:
<asp:TextBox id="tb4" Text="Hello World!" runat="server" />
<br /><br />
A multiline TextBox:
<asp:TextBox id="tb3" TextMode="multiline" runat="server" />
<br /><br />
A TextBox with height:
<asp:TextBox id="tb6" rows="5" TextMode="multiline"
runat="server" />
<br /><br />
A TextBox with width:
<asp:TextBox id="tb5" columns="30" runat="server" />
</form>
</body>
</html>

The above example has the following output:

To change the contents and settings of a TextBox control when a form is submitted, you may use server scripts. To submit a form you may make the user click the button or enter a value in the TextBox control.

Let us see the following example for a better clarification. In an .aspx file, one TextBox control, one button and one label is declared. When the user clicks the submit button, the submit subroutine is executed, which consequently writes some text on to the label control.

<script runat="server">
Sub submit(sender As Object, e As EventArgs)
lbl1.Text="Your name is " & txt1.Text
End Sub
</script>
<html>
<body>
<form id="Form1" runat="server">
Enter your name:
<asp:TextBox id="txt1" runat="server" />
<asp:Button ID="Button1" OnClick="submit" Text="Submit" runat="server" />
<p><asp:Label id="lbl1" runat="server" /></p>
</form>
</body>
</html>

The above example will show the following page, where I typed my name and clicked on the submit button and see the following output:

The Button Control

To use a push button in your .aspx pages, you may use the Button control. This push button can be a submit button or a command button. This control is a submit button by default.

When you click the submit button it will post back to the server. It doesn't have a command name. You can write an event handler to control the actions performed when it's clicked.

Where as a command button do have a command name. It permits to create multiple Button controls on a page. You can write an event handler to control the actions performed when it's clicked.

The following table shows the attributes and properties of the button control.

Properties

Description

CausesValidation

By default, a page is validated when a Button control is clicked. To prevent a page from being validated when clicking on a Button control, set this property to "false"

CommandArgument

Additional information about the command to perform

CommandName

The command associated with the Command event

id

A unique id for the control

OnClick

The name of the function to be executed when the button is clicked

runat

Specifies that the control is a server control. Must be set to "server"

Text

The text on the button

To elaborate a bit more, let’s have a look at the following example.

<html>
<body>
<form id="Form1" runat="server">
<asp:Button id="b1" Text="Submit" runat="server" />
</form>
</body>
</html>

To submit a form you click on a button. In the example below, one TextBox, one Button control and one Label control is declared in an .aspx file. When the submit button is clicked, the submit routine will be executed, which will write some text on the Label control.

<script runat="server">
Sub submit(sender As Object, e As EventArgs)
lbl1.Text="Your name is " & txt1.Text
End Sub
</script>
<html>
<body>
<form id="Form1" runat="server">
Enter your name:
<asp:TextBox id="txt1" runat="server" />
<asp:Button ID="Button1" OnClick="submit" Text="Submit" runat="server" />
<p><asp:Label id="lbl1" runat="server" /></p>
</form>
</body>
</html>

The above example will show the following page, where I typed my name and clicked on the submit button and see the following output:


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