Validating User Input in ASP.NET Web Pages 

You can add input validation to ASP.NET Web pages using validation controls. Validation controls provide an easy-to-use mechanism for all common types of standard validation—for example, testing for valid dates or values within a range—plus ways to provide custom-written validation. In addition, validation controls allow you to customize how error information is displayed to the user.

Using Validation Controls

You enable validation of user input by adding validation controls to your page as you would add other server controls. There are controls for different types of validation, such as range checking or pattern matching. For a complete list of validation types, see Types of Validation for ASP.NET Server Controls. Each validation control references an input control (a server control) elsewhere on the page. When user input is being processed (for example, when a page is submitted), the validation control tests the user input and sets a property to indicate whether the entry passed the test. After all of the validation controls have been called, a property on the page is set indicating whether any validation check has failed.
Validation controls can be associated into validation groups so that validation controls belonging to a common group are validated together. You can use validation groups to selectively enable or disable validation for related controls on a page. Other validation operations, such as displaying a ValidationSummary control or calling the GetValidators method, can reference the validation group.
You can test the state of the page and of individual controls in your own code. For example, you would test the state of the validation controls before updating a data record with information entered by the user. If you detect an invalid state, you bypass the update. Typically, if any validation checks fail, you skip all of your own processing and return the page to the user. Validation controls that detect errors then produce an error message that appears on the page. You can display all validation errors in one place using a ValidationSummary control.

You can attach more than one validation control to an input control. For example, you might specify that a control is required and that it also contains a specific range of values.
A related control, the ValidationSummary control, does not perform validation, but is often used in conjunction with other validation controls to display the error messages from all the validation controls on the page together. For more information, see How to: Control Validation Error Message Display for ASP.NET Server Controls.

 When Validation Occurs

Validation controls perform input checking in server code. When the user submits a page to the server, the validation controls are invoked to check the user input, control by control. If a validation error is detected in any of the input controls, the page itself is set to an invalid state so you can test for validity before your code runs. Validation occurs after page initialization (that is, after view state and postback data have been processed) but before any change or click event handlers are called.
If the user is working with a browser that supports ECMAScript (Javascript), the validation controls can also perform validation using client script. This can improve response time in the page because errors are detected immediately and error messages are displayed as soon as the user leaves the control containing the error. If client-side validation is available, you have greater control over the layout of error messages and can display an error summary in a message box. For more information, see Client-Side Validation for ASP.NET Server Controls.
ASP.NET performs validation on the server even if the validation controls have already performed it on the client, so that you can test for validity within your server-based event handlers. In addition, re-testing on the server helps prevent users from being able to bypass validation by disabling or changing the client script check.
You can invoke validation in your own code by calling a validation control's Validate method. For more information, see How to: Validate Programmatically for ASP.NET Server Controls.
 Validating for Multiple Conditions
Each validation control typically performs one test. However, you might want to check for multiple conditions. For example, you might want to specify both that a user entry is required and that the user entry is limited to accepting dates within a specific range.
You can attach more than one validation control to an input control on a page. In that case, the tests performed by the controls are resolved using a logical AND operator, which means that the data entered by the user must pass all of the tests in order to be considered valid.
In some instances, entries in several different formats might be valid. For example, if you are prompting for a phone number, you might allow users to enter a local number, a long-distance number, or an international number. Using multiple validation controls would not work in this instance because the user input must pass all tests to be valid. To perform this type of test—a logical OR operation where only one test must pass—use the RegularExpressionValidator validation control and specify multiple valid patterns within the control. Alternatively, you can use the CustomValidator validation control and write your own validation code.
 Displaying Error Information
Validation controls are not normally visible in the rendered page. However, if the control detects an error, it displays the error message text that you specify. The error message can be displayed in a variety of ways, as listed in the following table.

If you use the in-place or summary display options, you can format the error message text using HTML.

Security Note

If you create custom error messages, make sure that you do not display information that might help a malicious user compromise your application. For more information, see How to: Display Safe Error Messages.

 Validation Object Model
You can interact with validation controls by using the object model that is exposed by individual validation controls and by the page. Each validation control exposes its own IsValid property that you can test to determine whether a validation test has passed or failed for that control. The page also exposes an IsValid property that summarizes the IsValid state of all the validation controls on the page. This property allows you to perform a single test to determine whether you can proceed with your own processing.
The page also exposes a Validators collection containing a list of all the validation controls on the page. You can loop through this collection to examine the state of individual validation controls.

Note

There are a few differences in the object model for client-side validation. For more information, see Client-Side Validation for ASP.NET Server Controls.

 Customizing Validation
You can customize the validation process in the following ways:

• You can specify the format, text, and location of error messages. In addition, you can specify whether the error messages appear individually or as a summary.
• You can create custom validation using CustomValidator control. The control calls your logic but otherwise functions like other validation controls in setting error state, displaying error messages, and so on. This provides an easy way to create custom validation logic while still using in the validation framework of the page.
• For client-side validation, you can intercept the validation call and substitute or add your own validation logic.

Table Web Server Control Overview 

The Table Web server control allows you to create server-programmable tables on ASP.NET pages. The TableRow and TableCell Web server controls provide a way to display the actual content for the Table control.
 Comparing the Table Web Server Control to Other Table Elements
Tables are typically used not only to present tabular information, but as a way to format information on a Web page. There are a number of ways to create tables on your Web Forms page:

• HTML table. You can add an HTML <table> element. If you are creating a static table (one in which you will not add or change content at run time), you should use an HTML table and not a Table control.
• HtmlTable control. This is a <table> HTML element that has been converted to an HTML server control by adding the runat=server attribute. You can program this control in server code. For details about HTML server controls, see ASP.NET Web Server Controls Overview.
• Table. A Web control that allows you to create and manipulate tables (for example, adding table rows and cells) using an object model that is consistent with other Web controls.

In general, you use a Table Web server control when you intend to add rows and cells (columns) to the table in code at run time. Although you can use it as a static table with predefined rows and columns, it is easier in this case to work with the HTML <table> element.
The Table Web server control can be easier to program than the HtmlTable control because it offers an object model with typed properties that is consistent with other Web server controls. (The model is also consistent between the Table, TableRow, and TableCell controls.)
 Comparing the Table Web Server Control to Other List Web Server Controls
Some of the functions you might accomplish with a Table Web server control can also be accomplished with the list Web server controls, specifically the Repeater, DataList, and GridView controls. All these controls are rendered (or have the option to be rendered) as HTML tables.
The differences between the list controls and the Table control are these:

• The list controls are data-bound. The list controls work only against a data source, whereas the Table control can display any combination of HTML text and controls, whether or not they are data-bound.
• The list controls use templates to specify the layout of their elements. The Table control supports the TableCell child control, which you can fill as you would any HTML <td> element.

 Table Web Server Control Object Model
The Table control acts as a parent control for TableRow controls. The table supports a property called Rows that is a collection of TableRow objects. By managing this collection — adding or deleting items in it — you specify the rows for the table. The TableRow control in turn supports a collection called Cells that contains TableCell objects.
The content to be displayed in the table is added to the TableCell control. The cell has a Text property that you set to any HTML text. Alternatively, you can display controls in the cell by creating instances of, and adding controls to, the cell's Controls collection.
The parent Table control supports properties to control the appearance of the entire table, such as Font, BackColor, and ForeColor. The TableRow and TableCell controls support these properties as well, so you can specify the look of individual rows or cells, overriding the parent table appearance.

Example Creating Table in Runtime

Private Sub Button3_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button3.Click
        Dim i, j As Long
        For i = 1 To 4
            Dim tRow As New TableRow
            For j = 1 To 5
                Dim tCell As New TableCell
                tCell.Text = "Row " & i & ", Cell " & j
                Dim bt As New WebControls.Button
                bt.Text = "Row " & i & ", Cell " & j
                'tCell.Controls.Add(bt)
                tCell.BackColor = Color.FromArgb(RGB(i * 20, j * 40, (i + j) * 10))
                ' Add new TableCell object to row.
                tRow.Cells.Add(tCell)
            Next
            ' Add new row to table.
            Table1.Rows.Add(tRow)
        Next

    End Sub

Read Text File

Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
        Dim f As IO.File
        Dim FILENAME As String = Server.MapPath("f.txt")

        'Get a StreamReader class that can be used to read the file
        Dim os As IO.StreamReader
        os = New IO.StreamReader(FILENAME)
        While os.Peek() >= 0
            Response.Write(os.ReadLine() & "<br>")

        End While

        os.Close()

    End Sub

Latihan

Terdapat text file dengan susunan seperti berikut :

tukul
jakarta
avatar1.jpg
ronaldo
manchester
avatar2.jpg
messi
barcelona
avatar3.jpg

Buatlah agar dibaca dan menapilkan bentuk table seperti berikut :

JAWABAN

Private Sub Button3_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button3.Click
        Table1.BorderStyle = BorderStyle.Double
        Table1.BorderColor = Color.Black

        Dim tr As TableRow
        tr = New TableRow
        Dim tc As TableCell
        tc = New TableCell
        tc.Text = "Nama"
        tr.Cells.Add(tc)
        tc = New TableCell
        tc.Text = "Alamat"
        tr.Cells.Add(tc)
        tc = New TableCell
        tc.Text = "Foto"
        tr.Cells.Add(tc)
        tr.BackColor = Color.Cyan

        Table1.Rows.Add(tr)
        Dim i As Long
        Dim filename As String = Server.MapPath("tes2.txt")
        Dim sr As IO.StreamReader = New IO.StreamReader(filename)
        While sr.Peek >= 0
            tr = New TableRow
            tc = New TableCell
            tc.Text = sr.ReadLine
            tr.Cells.Add(tc)
            tc = New TableCell
            tc.Text = sr.ReadLine
            tr.Cells.Add(tc)
            tc = New TableCell
            Dim im As New WebControls.Image
            im.ImageUrl = sr.ReadLine
            tc.Controls.Add(im)
            tr.Cells.Add(tc)

            tr.BorderStyle = BorderStyle.Inset
            Table1.Rows.Add(tr)
        End While
    End Sub