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Programming Microsoft ASP.NET 4 - Dino Esposito [131]

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are used to bind the list item’s Text and Value properties to columns in the data source. (I’ll cover data binding in Chapter 10.)

The HtmlTextArea Control

The HtmlTextArea control corresponds to the HTML element and allows you to programmatically create and configure a multiline text box. The HtmlTextArea class provides the Rows and Cols properties to control the number of rows and columns of the text box. The Value property can be used to assign some text to display in the control area.<p>The HtmlTextArea class also provides a ServerChange event that fires during a postback and allows you to validate on the server the data contained in the control. Note that the HtmlTextArea control does not fire the event itself and does not directly cause the page to post back. Rather, when the page posts back in response to a click on a link or submit button, the HtmlTextArea control intervenes in the server-side chain of events and gives the programmer a chance to run some code if the internal content of the control is changed between two successive postbacks.<p>All ASP.NET controls that, like HtmlTextArea, implement the IPostBackDataHandler interface can invoke user-defined code when the control’s internal state changes. As discussed in Chapter 5, controls can fire custom events by overriding the RaisePostDataChangedEvent method on the aforementioned interface. The following pseudocode shows what happens in the method’s implementation of HtmlTextArea:<p><p>void System.Web.UI.IPostBackDataHandler.RaisePostDataChangedEvent()<p>{<p>this.OnServerChange(EventArgs.Empty);<p>}<p>Finally, note that the control raises the event only if the state has changed between two successive posts. To determine whether that has happened, the control needs to track the content it had the time before. This value can be stored only in the view state. Of course, the ServerChange even won’t fire if you disable the view state for the host page or the control.<p><p><p>HTML Input Controls<p><br>In HTML, the <input> element has several variations and can be used to provide a submit button as well as a check box or text box. In ASP.NET, each possible instance of the <input> element is mapped to a specific class. All input classes derive from the HtmlInputControl class. HtmlInputControl is the abstract class that defines the common programming interface for all input controls. The class inherits from HtmlControl and simply adds three custom properties—Name, Type, and Value—to the inherited interface.<p>The Name property returns the name assigned to the control. In ASP.NET, this property is peculiar because, although it’s marked as read/write, it actually works as a read-only property. The get accessor returns the control’s UniqueID property, while the set accessor is just void. As a result, whatever value you assign to the property, either programmatically or declaratively, is just ignored and no exception or compile error is ever thrown.<p>The Type property mirrors the type attribute of the HTML input elements. The property is read-only. Finally, the Value property is read/write and represents the content of the input field.<p>Table 6-8 lists the HTML input controls defined in ASP.NET.<p><p>Table 6-8. HTML Input Controls<p><p>Class<p>Description<p><p>HtmlInputButton<p>Represents the various flavors of a command button supported by HTML. Feasible values for the Type attribute are button, submit, and reset.<p><p>HtmlInputCheckBox<p>Represents an HTML check box—that is, the < input> tag with a type equal to checkbox.<p><p>HtmlInputFile<p>Represents the file uploader—that is, the < input> tag with a type equal to file.<p><p>HtmlInputHidden<p>Represents a hidden buffer of text data—that is, the < input> tag with a type equal to hidden.<p><p>HtmlInputImage<p>Represents a graphic button—that is, the < input> tag with a type equal to image. Note that this tag is supported by all browsers.<p><p>HtmlInputPassword<p>Represents a protected text field—that is, the < input> tag with a type of password.<p><p>HtmlInputRadioButton<p>Represents a radio button—that is, the < input> tag with a type equal to</div> </div> <div id="pager" style="text-align: center;margin: auto;"> <a data-ajax="false" class="ui-btn ui-btn-inline ui-corner-all" data-position-to="window" href="/article/programming-microsoft-aspnet-4-dino-esposito-f20b3e.html">Return Main Page</a> <a data-ajax="false" class="ui-btn ui-btn-inline ui-corner-all" data-position-to="window" href="/read/programming-microsoft-aspnet-4-dino-esposito-f20b3e/130.html">Previous Page</a> <a data-ajax="false" class="ui-btn ui-btn-inline ui-corner-all" data-position-to="window" href="/read/programming-microsoft-aspnet-4-dino-esposito-f20b3e/132.html">Next Page</a> </div> <div data-role="footer" id="tail" style="margin-top: 30px;"> <h3>®Online Book Reader</h3> </div> </div> </body>  </html>