Recipe 2.5 Requiring that a Drop-Down List Selection Be Made

     

2.5.1 Problem

You need to make sure a user selects an entry in a drop-down list.

2.5.2 Solution

Add a CustomValidator control to the drop-down list, along with some client-side JavaScript to validate the selection. Next, implement an event handler for the CustomValidator control's ServerValidate event. Finally, check the Page.IsValid property in the event handler for the control that completes the user's entry for the page.

In the .aspx file:

  1. Add a CustomValidator control for each drop-down list where you must verify that an item has been selected.

  2. Add JavaScript to validate the selection on the client side.

  3. Add Save and Cancel (or equivalently named) buttons .

  4. Set the Save button's CausesValidation attribute to True to have validation performed when the button is clicked (set it to False for the Cancel button).

In the code-behind class for the page, use the .NET language of your choice to:

  1. Add an event handler for the CustomValidator control's ServerValidate event whose purpose is to provide the server-side validation to ensure an item has been selected.

  2. Add code to the event handler for the Save button's click event to check the Page.IsValid property and verify that all validation was successful (see Recipe 2.1 for details).

Figure 2-9 shows a typical form with normal output prior to data entry. Figure 2-10 shows the form with validation errors. Example 2-7 through Example 2-9 show the .aspx and code-behind files for our application that implements the solution.

Figure 2-9. Form with selection validation output ”normal
figs/ancb_0209.gif

Figure 2-10. Form with selection validation output ”with error summary
figs/ancb_0210.gif

2.5.3 Discussion

This recipe involves using a CustomValidator control to verify that an item has been selected in a drop-down list. But the approach we advocate is a bit out of the ordinary for a couple of reasons. First, by implementing validation via client-side JavaScript, it allows errors to be detected on the client, thus avoiding unnecessary round trips to the server for server-side validation.

Besides using a CustomValidator control, this application also uses validation controls from all of the previous recipes combined, specifically RequiredFieldValidator , RangeValidator , CompareValidator , and RegularExpressionValidator . Combining validators of various types is typical when performing validation on a complex form.


Second, instead of displaying an error message at each control, this example shows how to use a ValidationSummary control to provide a list of all errors on the page in one place. This approach is very useful for a "busy" form.

To implement the solution, the ControlToValidate attribute must be set to the drop-down list you will be validating. In our case, it is set to ddCountry :

 <asp:CustomValidator id="valItemSelected" runat="server"  ControlToValidate="ddCountry"  ClientValidationFunction="isItemSelected"                      CssClass="AlertText"                       Display="Dynamic"                      EnableClientScript="True"                      ErrorMessage="Country Must Be Selected">     <img src="images/arrow_alert.gif" /> </asp:CustomValidator> 

The ClientValidationFunction attribute must be set to the name of the client JavaScript function that will perform the client-side validation, which is done to ensure that an item has been selected from the drop-down:

 <asp:CustomValidator id="valItemSelected" runat="server"                       ControlToValidate="ddCountry"  ClientValidationFunction="isItemSelected"  CssClass="AlertText"                       Display="Dynamic"                      EnableClientScript="True"                      ErrorMessage="Country Must Be Selected">     <img src="images/arrow_alert.gif" /> </asp:CustomValidator> 

A client script block must be added to the page containing the function named in the ClientValidationFunction attribute of the CustomValidator , as shown in Example 2-7. The function must have source and argument parameters. When the function is called, the source will be set to a reference to the validator that called the function. In this case, it is the " valItemSelected " validator.

The arguments parameter is a structure containing two elements: Value and IsValid . Value contains the current value of the control that is being validated . In this case, it is the value of the selected item in the drop-down list. In our example, three entries are added to the drop-down list, with the first entry being the " Select Country " instruction with a value of 0. All legitimate selections from the drop-down contain values greater than 0; therefore, if the value of arguments.Value is less than 1, no selection has been made, and the value of arguments.IsValid is set to False to indicate a validation failure. If arguments.Value is greater than 0, then a selection has been made, and arguments.IsValid is set to True to indicate the validation passed.

The EnableClientScript attribute is set to True or False according to how you want validation to be performed. Setting the attribute to True causes validation to be performed on the client and again on the server when the page is submitted. Setting the attribute to False causes validation to be performed only on the server when the form is submitted. In our example we are providing client-side JavaScript, so it must be set to True :

 <asp:CustomValidator id="valItemSelected" runat="server"                       ControlToValidate="ddCountry"                      ClientValidationFunction="isItemSelected"                      CssClass="AlertText"                       Display="Dynamic"  EnableClientScript="True"  ErrorMessage="Country Must Be Selected">     <img src="images/arrow_alert.gif" /> </asp:CustomValidator> 

Rather than outputting an error message at each control, as was done in all the previous examples in this chapter, we've added a ValidationSummary control to the form in this example to provide in one place a summary of all the errors on the form. When a validation summary is being used, an error message is no longer required between the start and end tags of the validator element. Instead, the ErrorMessage attribute of the validator is set to the error message to display. To provide visual feedback of which control has an error, an arrow image is inserted between the start and end tags of the validator element in our example:

 <asp:CustomValidator id="valItemSelected" runat="server"                       ControlToValidate="ddCountry"                      ClientValidationFunction="isItemSelected"                      CssClass="AlertText"                       Display="Dynamic"                      EnableClientScript="True"                  ErrorMessage="Country Must Be Selected">  <img src="images/arrow_alert.gif" />  </asp:CustomValidator> 

The DisplayMode attribute of the asp:ValidationSummary control defines how the summary is displayed. Valid values are BulletList , List , and SingleParagraph . BulletList will generate a bulleted list of the errors, which is what we've chosen for our example. List will generate the same list as the BulletList setting but without the bullets. SingleParagraph generates a single HTML paragraph containing all of the error information. The HeaderText attribute is set to the title placed at the top of the list of errors.

 <asp:ValidationSummary id="vsErrors" Runat="server"                        CssClass="AlertText"  DisplayMode="BulletList"  EnableClientScript="True"                   HeaderText="Error Summary" /> 

The sample application's code-behind includes an event handler for the CustomValidator control's ServerValidate event, as shown in Example 2-8 (VB) and Example 2-9 (C#). This event handler provides the server-side validation to ensure a country has been selected using the same technique implemented in the client script.

The code-behind also includes an event handler for the Save button's click event. This event handler checks to make sure the page is valid (all validation passed) and then performs the processing of the form data.

2.5.4 See Also

Recipe 2.1; Recipe 2.2; Recipe 2.3; Recipe 2.4

Example 2-7. Form with selection validation (.aspx)
 <%@ Page Language="vb" AutoEventWireup="false"           Codebehind="CH02CustomSelectionValidationVB.aspx.vb"           Inherits="ASPNetCookbook.VBExamples.CH02CustomSelectionValidationVB" %> <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <html>   <head>     <title>Custom Selection Validator</title>     <link rel="stylesheet" href="css/ASPNetCookbook.css">  <script language="javascript">   <!--   function isItemSelected(source, arguments)   {   if (arguments.Value < 1)   {   arguments.IsValid = false;   }   else   {   arguments.IsValid = true;   }   }   //-->   </script>  </head>   <body leftmargin="0" marginheight="0" marginwidth="0" topmargin="0">     <form id="frmValidation" method="post" runat="server">       <table width="100%" cellpadding="0" cellspacing="0" border="0">         <tr>           <td align="center">             <img src="images/ASPNETCookbookHeading_blue.gif">           </td>         </tr>         <tr>           <td class="dividerLine">             <img src="images/spacer.gif" height="6" border="0"></td>         </tr>       </table>       <table width="90%" align="center" border="0">         <tr>           <td><img src="images/spacer.gif" height="10" border="0"></td>         </tr>         <tr>           <td align="center" class="PageHeading">             Custom Validation (VB)           </td>         </tr>         <tr>           <td><img src="images/spacer.gif" height="10" border="0"></td>         </tr>         <tr>           <td align="center">             <table border="0">               <tr>                 <td colspan="2" align="left">                   <asp:ValidationSummary id="vsErrors" Runat="server"                                         CssClass="AlertText"                                          DisplayMode="BulletList"                                         EnableClientScript="True"                                         HeaderText="Error Summary" />                 </td>               </tr>               <tr>                 <td class="LabelText">First Name: </td>                 <td>                   <asp:TextBox id="txtFirstName" Runat="server"                                 Columns="30" CssClass="LabelText" />                   <asp:RequiredFieldValidator id="rfvFirstName"                         Runat="server"                        ControlToValidate="txtFirstName"                        CssClass="AlertText"                        Display="Dynamic"                        EnableClientScript="True"                        ErrorMessage="First Name Is Required">                      <img src="images/arrow_alert.gif">                   </asp:RequiredFieldValidator>                 </td>               </tr>               <tr>                 <td class="LabelText">Last Name: </td>                 <td>                   <asp:TextBox id="txtLastName" Runat="server"                                 Columns="30" CssClass="LabelText" />                   <asp:RequiredFieldValidator id="rfvLastName"                         Runat="server"                        ControlToValidate="txtLastName"                        CssClass="AlertText"                        Display="Dynamic"                        EnableClientScript="True"                        ErrorMessage="Last Name Is Required">                      <img src="images/arrow_alert.gif">                   </asp:RequiredFieldValidator>                 </td>               </tr>               <tr>                 <td class="LabelText">Age: </td>                 <td>                   <asp:TextBox id="txtAge" Runat="server"                                 Columns="30" CssClass="LabelText" />                   <asp:RequiredFieldValidator id="Requiredfieldvalidator1"                         Runat="server"                        ControlToValidate="txtAge"                        CssClass="AlertText"                        Display="Dynamic"                        EnableClientScript="True"                        ErrorMessage="Age Is Required">                      <img src="images/arrow_alert.gif">                   </asp:RequiredFieldValidator>                   <asp:RangeValidator id="rvAge" Runat="server"                        ControlToValidate="txtAge"                        CssClass="AlertText"                        Display="Dynamic"                        EnableClientScript="True"                        MinimumValue="18"                         MaximumValue="99"                         Type="Integer"                        ErrorMessage="Age Must Be Between 18 and 99">                      <img src="images/arrow_alert.gif">                   </asp:RangeValidator>                  </td>               </tr>               <tr>                 <td class="LabelText">Country: </td>                 <td>                   <asp:DropDownList id="ddCountry" Runat="server" >                     <asp:ListItem Selected="True"                           Value="0">----- Select Country -----</asp:ListItem>                     <asp:ListItem Value="1">Canada</asp:ListItem>                     <asp:ListItem Value="2">United States</asp:ListItem>                   </asp:DropDownList>  <asp:CustomValidator id="valItemSelected" runat="server"   ControlToValidate="ddCountry"   ClientValidationFunction="isItemSelected"   CssClass="AlertText"   Display="Dynamic"   EnableClientScript="True"   ErrorMessage="Country Must Be Selected">   <img src="images/arrow_alert.gif" />   </asp:CustomValidator>  </td>               </tr>               <tr>                 <td class="LabelText">Email Address: </td>                 <td>                   <asp:TextBox id="txtEmailAddress" Runat="server"                                 Columns="30" CssClass="LabelText" />                   <asp:requiredfieldvalidator id="rfvEmailAddress"                         runat="server"                        controltovalidate="txtEmailAddress"                        cssclass="AlertText"                        display="Dynamic"                        enableclientscript="True">                      <img src="images/arrow_alert.gif">                   </asp:requiredfieldvalidator>                   <asp:RegularExpressionValidator id="revEmailAddress"                        Runat="server"                        ControlToValidate="txtEmailAddress"                        CssClass="AlertText"                        Display="Dynamic"                        EnableClientScript="True"            ValidationExpression="\w+([-+.]\w+)*@\w+([-.]\w+)*\.\w+([-.]\w+)*"                        ErrorMessage="Invalid Email Address">                      <img src="images/arrow_alert.gif">                   </asp:RegularExpressionValidator>                 </td>               </tr>               <tr>                 <td class="LabelText">Password: </td>                 <td>                   <asp:TextBox id="txtPassword1" Runat="server"                                 TextMode="Password"                                 Columns="30" CssClass="LabelText" />                   <asp:RequiredFieldValidator id="rfvPassword1"                         Runat="server"                        ControlToValidate="txtPassword1"                        CssClass="AlertText"                        Display="Dynamic"                        EnableClientScript="True"                        ErrorMessage="Password Is Required">                      <img src="images/arrow_alert.gif">                   </asp:RequiredFieldValidator>                 </td>               </tr>               <tr>                 <td class="LabelText">Re-enter Password: </td>                 <td>                   <asp:TextBox id="txtPassword2" Runat="server"                                 TextMode="Password"                                 Columns="30" CssClass="LabelText" />                   <asp:RequiredFieldValidator id="rvPassword2"                         Runat="server"                        ControlToValidate="txtPassword2"                        CssClass="AlertText"                        Display="Dynamic"                        EnableClientScript="True"                        ErrorMessage="Re-Entered Password Is Required">                      <img src="images/arrow_alert.gif">                   </asp:RequiredFieldValidator>                   <asp:CompareValidator ID="cvPassword2" runat="server"                        ControlToValidate="txtPassword2"                        ControlToCompare="txtPassword1"                        CssClass="AlertText"                        Display="Dynamic"                        EnableClientScript="True"                        ErrorMessage="Both Passwords Must Match">                      <img src="images/arrow_alert.gif">                   </asp:CompareValidator>                 </td>               </tr>               <tr>                 <td colspan="2">                   <br>                   <table align="center" width="50%">                     <tr>                       <td align="center">                         <asp:ImageButton id="btnSave" Runat="server"                              CausesValidation="True"                              ImageUrl="images/buttons/button_save.gif" />                       </td>                       <td align="center">                         <asp:ImageButton id="btnCancel" Runat="server"                              CausesValidation="False"                              ImageUrl="images/buttons/button_cancel.gif" />                       </td>                     </tr>                   </table>                 </td>               </tr>             </table>           </td>         </tr>       </table>     </form>   </body> </html> 

Example 2-8. Form with selection validation code-behind (.vb)
 Option Explicit On  Option Strict On '----------------------------------------------------------------------------- ' '   Module Name: CH02CustomSelectionValidationVB.aspx.vb ' '   Description: This module provides the code behind for '                CH02CustomSelectionValidationVB.aspx ' '***************************************************************************** Imports System.Web.UI.WebControls Namespace ASPNetCookbook.VBExamples   Public Class CH02CustomSelectionValidationVB     Inherits System.Web.UI.Page     'controls on form     Protected ddCountry As DropDownList     Protected WithEvents valItemSelected As CustomValidator     Protected WithEvents btnSave As ImageButton     '*************************************************************************     '     '   ROUTINE: Page_Load     '     '   DESCRIPTION: This routine provides the event handler for the page load     '                event.  It is responsible for initializing the controls     '                on the page.     '-------------------------------------------------------------------------     Private Sub Page_Load(ByVal sender As System.Object, _                           ByVal e As System.EventArgs) _             Handles MyBase.Load       If (Not Page.IsPostBack) Then         'Put user code to initialize the page here       End If     End Sub  'Page_Load     '*************************************************************************     '     '   ROUTINE: valItemSelected_ServerValidate     '     '   DESCRIPTION: This routine provides the event handler for the     '                valItemSelected server validate event.  It is responsible     '                for validating that a country has been selected     '-------------------------------------------------------------------------  Private Sub valItemSelected_ServerValidate(ByVal source As Object, _   ByVal args As System.Web.UI.WebControls.ServerValidateEventArgs) _   Handles valItemSelected.ServerValidate   If (ddCountry.SelectedIndex < 1) Then   args.IsValid = False   Else   args.IsValid = True   End If   End Sub  'valItemSelected_ServerValidate  '***************************************************************************     '     '   ROUTINE: btnSave_Click     '     '   DESCRIPTION: This routine provides the event handler for the save      '                button click event.  It is responsible for processing the     '                form data.     '---------------------------------------------------------------------------  Private Sub btnSave_Click(ByVal sender As Object, _   ByVal e As System.Web.UI.ImageClickEventArgs) _   Handles btnSave.Click   If (Page.IsValid) Then   'process form data and save as required for application   End If   End Sub  'btnSave_Click  End Class  'CH02CustomSelectionValidationVB End Namespace 

Example 2-9. Form with selection validation code-behind (.cs)
 //---------------------------------------------------------------------------- // //   Module Name: CH02CustomSelectionValidationCS.aspx.cs // //   Description: This module provides the code behind for //                CH02CustomSelectionValidationCS.aspx // //**************************************************************************** using System; using System.Web.UI; using System.Web.UI.WebControls; namespace ASPNetCookbook.CSExamples {   public class CH02CustomSelectionValidationCS : System.Web.UI.Page   {     // controls on form     protected DropDownList ddCountry;     protected CustomValidator valItemSelected;     protected ImageButton btnSave;     //************************************************************************     //     //   ROUTINE: Page_Load     //     //   DESCRIPTION: This routine provides the event handler for the page     //                load event.  It is responsible for initializing the     //                controls on the page.     //------------------------------------------------------------------------     private void Page_Load(object sender, System.EventArgs e)     {       // wire the save button click and the item selected validation events       this.btnSave.Click += new ImageClickEventHandler(this.btnSave_Click);       this.valItemSelected.ServerValidate +=         new ServerValidateEventHandler(this.valItemSelected_ServerValidate);       if (!Page.IsPostBack)       {         //Put user code to initialize the page here       }     }  // Page_Load     //************************************************************************     //     //   ROUTINE: valItemSelected_ServerValidate     //     //   DESCRIPTION: This routine provides the event handler for the     //                valItemSelected server validate event.  It is     //                responsible for validating that a country has been     //                selected     //------------------------------------------------------------------------  private void valItemSelected_ServerValidate(Object source,   System.Web.UI.WebControls.ServerValidateEventArgs args)   {   if (ddCountry.SelectedIndex < 1)   {   args.IsValid = false;   }   else   {   args.IsValid = true;   }   }  // valItemSelected_ServerValidate  //************************************************************************     //     //   ROUTINE: btnSave_Click     //     //   DESCRIPTION: This routine provides the event handler for the save     //                button click event.  It is responsible for processing     //                the form data.     //------------------------------------------------------------------------  private void btnSave_Click(Object sender,   System.Web.UI.ImageClickEventArgs e)   {   if (Page.IsValid)   {   // process form data and save as required for application   }   }  // btnSave_Click  }  // CH02CustomSelectionValidationCS } 



ASP. NET Cookbook
ASP.Net 2.0 Cookbook (Cookbooks (OReilly))
ISBN: 0596100647
EAN: 2147483647
Year: 2006
Pages: 179

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