Shouldn’t validating a checkbox be easier than this?


Well it's been a while since I have done a post but with changing jobs recently and life in general getting in the way I just couldn’t seem to find the time. However, now I have a spare five minutes I thought I would fire off a post :)

So what's the post about? Validating a checkbox web control. Now I know your reading this thinking that's easy all you need is a custom validator and a event handler in your code behind and job done? Well in most cases yes I would agree with you. However, my latest project (a legacy application) uses code generation to create the vast majority of the ASP.NET pages and user controls. Therefore it’s difficult for me to include anything other than the standard set of ASP.NET validators or something which inherits off the “BaseValidator” and get it working correctly in the XSLT templates generating the code. Because of this I decided to create my own validator for the checkbox web control.

So how do we get started? Well first off you need to inherit off the “BaseValidator” and then override the required "EvaluateIsValid" method. Once done you can then write some simple logic to ensure your checkbox has been checked. As shown below.

   1:  public class CheckBoxValidator : BaseValidator
   2:  {
   3:      protected override bool EvaluateIsValid()
   4:      {
   5:          var controlValue = this.GetControlValidationValue(this.ControlToValidate);
   6:          bool checkBoxValue;
   7:          var hasBeenParsed = bool.TryParse(controlValue, out checkBoxValue);
   9:          var result = hasBeenParsed && checkBoxValue;
  10:          return result;
  11:      }
  12:  }


So is that it all done? Well no because the out of the box checkbox control is missing an important thing. The "ValidationProperty" attribute. This attribute is used within the base validator to work out which property on the control being validated should to used when getting the control validation value. In order to solve this problem you will need to create your own version of the checkbox control by inheriting from it and adding the required attribute as shown below.

   1:  [ValidationProperty("Checked")]
   2:  public class ValidationCheckBox : CheckBox
   3:  {
   4:  }


Ok so now we have our own custom checkbox and checkbox validator we can finally validate those lovely checkboxes. However, if you recall from a bit earlier I said this is a legacy application and although it is to a large degree generated for me I'm not sure I really want to find all the places where a standard checkbox web control should be switched out for my super custom one. So what's the answer? Tag mapping.

Now tag mapping if you haven't heard of it before enables you to define within the web.config that tag "A" should instead be mapped to tag "B". You can see below how I used it to always return my custom checkbox whether a standard checkbox should be used.

  1: <pages controlRenderingCompatibilityVersion="3.5" clientIDMode="AutoID">
  2:   <tagMapping>
  3:     <add tagType="System.Web.UI.WebControls.CheckBox" mappedTagType="Demo.Web.Code.Controls.ValidationCheckBox" />
  4:   </tagMapping>
  5: </pages>
Now that’s done I don’t have to change a single line of code to get my new checkbox control in place and get my validations working. One finial point though or really a question is why haven’t Microsoft just created this type of validator and given it to me straight out of the box? Yes I know I could change the checkbox to some radio buttons to achieve a similar thing, but sometimes I don’t get the finial say in the design of a page and this type of validator would come in handy I think for a lot of people.
April 6 2011
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