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List Validator

, 24 Jul 2007
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Validates a ListControl to ensure at least one RadioButton or CheckBox is checked.

Objective

This article has two goals.

  1. Implement a good ListValidator to validate whether at least one item in a CheckBoxList or RadioButtonList has been checked.
  2. Demonstrate a fully functional validator.

Common omissions / problems

Many validators seem to fall down in one area or another. This control is an attempt to demonstrate how a complete validator should look. As this is my first article, I'm sure I'll miss something or lots of things. Please let me know and I'll try to keep it up-to-date. Here are a few common problems. My example shows how to implement these features.

  • Validators that don't implement client script.
  • Validators that don't implement EnableClientScirpt="false".
  • Validator client scripts that don't work with multiple validators on a page.

Here's the code

/*

Author:        Murray Roke
Email:        murray@roke.co.nz

Features
 - ClientScript        works.
 - EnableClientScript    works.
 - Multiple Validators    works.

Change Log:
2006-02-17
Implemented fix for javascript validation 
of radio buttons.. Thanks to JCollum from TheCodeProject 

*/
using System;
using System.ComponentModel;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

namespace CompanyName.Web.Controls.Validators
{
    /// <summary>
    /// Validates if at least one item in a ListControl is checked.
    /// </summary>
    /// <remarks>
    /// Original javascript code came from this example, but 
    /// I've improved upon it. 
    /// to get multiple validators on a page working correctly.
    /// http://www.dotnetjunkies.com/
    /// Article/ECCCD6A6-B312-41CB-87A1-10BB5D641D20.dcik
    /// </remarks>
    [DefaultProperty("ErrorMessage")] // make the error message the default 
                                      // property to edit at design time 
                                      // for those using gui editor
    public class ListControlRequiredFieldValidator : BaseValidator 
    {
        /// <summary>
        /// Validator Requirement
        /// </summary>

        /// <returns>True if dependencies are valid.</returns>
        protected override bool ControlPropertiesValid()
        {
            Control controlToValidate = 
                FindControl(ControlToValidate) as ListControl;
            return (controlToValidate != null);
        }

        /// <summary>
        /// Validator Requirement
        /// </summary>
        /// <returns>true if ControlToValidate 
        /// has one item or more selected</returns>

        protected override bool EvaluateIsValid() 
        {
            return this.EvaluateIsChecked();
        }

        /// <summary>
        /// Return true if an item in the list is selected.
        /// </summary>
        /// <returns>true if ControlToValidate 
        ///      has one item or more selected</returns>
        protected bool EvaluateIsChecked() 
        {
            ListControl listToValidate = 
                ((ListControl) this.FindControl(this.ControlToValidate));

            foreach( ListItem li in listToValidate.Items ) 
            {
                if ( li.Selected == true ) 
                    return true;
            }
            return false;
        }

        /// <summary>

        /// Pre Render
        /// </summary>
        /// <param name="e"></param >
        protected override void OnPreRender( EventArgs e )
        {
            System.Web.HttpContext.Current.Trace.Write(
                                         "Override OnPreRender");
            if(this.DetermineRenderUplevel() && this.EnableClientScript)
            {
                Page.ClientScript.RegisterExpandoAttribute(this.ClientID, 
                    "evaluationfunction", "ListItemVerify");
                Page.ClientScript.RegisterExpandoAttribute(this.ClientID, 
                    "minimumNumberOfSelectedCheckBoxes", "1"); 
                    //TODO: imporove to allow variable number.
                this.RegisterClientScript();
            }
            else
            {
                this.Attributes.Remove("evaluationfunction");
            }
            base.OnPreRender( e );
        }

        /// <summary>
        /// Register the client script.
        /// </summary>

        protected void RegisterClientScript() 
        {
            string script = @"

            <script language=""javascript"">
            function ListItemVerify(val) 
            {
                var control = 
                    document.getElementById(val.controltovalidate);
                var minimumNumberOfSelectedCheckBoxes = 
                    parseInt(val.minimumNumberOfSelectedCheckBoxes);
                var selectedItemCount = 0;
                var liIndex = 0;
                var currentListItem = 
                    document.getElementById(control.id + 
                    '_' + liIndex.toString());
                while (currentListItem != null)
                {
                    if (currentListItem.checked) selectedItemCount++;
                    liIndex++;
                    currentListItem = 
                        document.getElementById(control.id + 
                        '_' + liIndex.toString());
                }
                return selectedItemCount >= 
                    minimumNumberOfSelectedCheckBoxes;
            }
            </script>
            ";

            this.Page.ClientScript.RegisterClientScriptBlock(
                typeof(ListControlRequiredFieldValidator), 
                "ListRequiredValidator_Script",script);
        }
    }
}

Example usage

Register the library.

<%@ Register TagPrefix="CompanyName" 
    NameSpace="CompanyName.Web.Controls.Validators"
    Assembly="CompanyName.Web" %>

Create the validator. Imagine that the CheckBoxList you wish to validate is called MyCheckList.

<CompanyName:ListControlRequiredFieldValidator
    ControlToValidate="MyCheckList" 
    display="Dynamic" 
    ErrorMessage="Select at least one item" 
    EnableClientScript="true" 
    runat="Server">
    Tick at least one box
</CompanyName:ListControlRequiredFieldValidator>

History

  • 12 April, 2005 -- Original version posted
  • 17 February, 2006 -- Updated
  • 24 July, 2007 -- Updated

License

This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

A list of licenses authors might use can be found here

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About the Author

Murray Roke
Web Developer
New Zealand New Zealand
I live in Auckland, New Zealand.
 
I've been developing ASP.net applications in C# since the .net framework was in beta.
 

Before that I developed in ASP 3.0 for several years, with a smattering of Java servlets here and there.

Comments and Discussions

 
GeneralNicely Done Pinmembermerlin98125-Jul-07 5:35 
Questionassembly File PinmemberAdhoom13-Dec-06 7:04 
AnswerRe: assembly File PinmemberMurray Roke13-Dec-06 10:57 
QuestionHow can i get the assembly File PinmemberAdhoom13-Dec-06 7:04 
GeneralAn idea. Pinmemberturbochris7-Apr-06 3:51 
GeneralRe: An idea. PinmemberMurray Roke21-Mar-08 1:23 
QuestionCan I use CheckBoxList in a Dialog page? PinmemberAlpha Davis22-Feb-06 14:00 
GeneralRe: Can I use CheckBoxList in a Dialog page? PinmemberMurray Roke22-Feb-06 14:40 
Generalgreat but needs a tweak PinmemberJCollum16-Feb-06 13:21 
GeneralRe: great but needs a tweak PinmemberJCollum16-Feb-06 14:56 
GeneralRe: great but needs a tweak PinmemberMurray Roke16-Feb-06 15:40 
Jokehttp://buy-medications-online.ceroline.info/ Pinsusshttp://buy-medications-online.ceroline.info/4-Dec-07 6:44 
GeneralRe: great but needs a tweak Pinmemberaprenot17-Feb-06 10:58 
GeneralRe: great but needs a tweak PinmemberMurray Roke11-Sep-06 18:08 
GeneralRe: great but needs a tweak PinmemberJCollum22-Jul-07 8:29 
GeneralRe: great but needs a tweak PinmemberMurray Roke22-Jul-07 13:48 
GeneralRe: great but needs a tweak PinmemberJCollum23-Jul-07 9:20 
GeneralRe: great but needs a tweak PinmemberJCollum23-Jul-07 12:00 
AnswerRe: great but needs a tweak PinmemberJCollum24-Jul-07 7:38 
GeneralRe: great but needs a tweak PinmemberMiguel Hasse de Oliveira20-Mar-08 2:11 
GeneralRe: great but needs a tweak PinmemberMurray Roke21-Mar-08 1:38 
GeneralValidate a max count checked Pinmembertbaseflug8-Dec-05 9:00 
GeneralRe: Validate a max count checked PinmemberMurray Roke8-Dec-05 10:11 
GeneralRe: C# Code to Validate a max/min count checked Pinmemberdfg fd g26-Sep-07 14:52 
GeneralRe: C# Code to Validate a max/min count checked Pinmemberdfg fd g26-Sep-07 14:54 

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