ListControl SelectedItem Validator

Introduction

As you all know the ASP.NET RequiredFieldValidator control checks that the validated control to see if it contains a value. If it doesn't it displays an error message. The problem is that RequiredFieldValidator does not work for ListControl derived controls (except ListBox) such as

• DropdownList
• CheckBoxList
• RadioButtonList

And actually it throws an exception in the CheckControlValidationProperty(String name,String propertyName) function if you try to set the ControlToValidate property of the RequiredFieldValidator control to one of the above controls. So I decided to write a server side validation control to do this.

Basically I have derived from BaseValidator which provides the base implementation for all validation controls. It is not an abstract class. I have overridden only two functions which are

• ControlPropertiesValid()
• EvaluateIsValid()

The first one checks the validity of the ControlToValidate property and if you return false in this function , the validation framework raises an exception. EvaluateIsValid is the main function which checks if there is a selected item in the ListControl. If the function doesn't return false then this means validation fails. 

An interesting Point is since the DropDownList control can not have a SelectedIndex property with a value of –1 , I checked against 0 so you may want to insert an empty item in the beginning.

So enough with the talk lets take a look at the code.

//
// 
//
using System;
using System.Web.UI;
using System.Web.UI.WebControls;
 
namespace MyValidations
{
    [DefaultProperty("ErrorMessage")]
    public class RequiredSelectedItemValidator : System.Web.UI.BaseValidator 
    {
        private ListControl _listctrl;
 
        protected override bool ControlPropertiesValid()
        {
           Control ctrl = FindControl(ControlToValidate);
       
           If (ctrl != null)
           {
               _listctrl = ctrl as ListControl;
               return (_listctrl != null);     
           }
           else 
              return false;  // raise exception
       }
 
       protected override bool EvaluateIsValid()
       {      
            return (_listctrl is DropDownList) ? (_lisctrl.SelectedIndex != 0) : 
                                                 ( _lisctrl.SelectedIndex != -1);
       }
   }
}