RequiredAttribute with AllowEmptyString=true in ASP.NET MVC 3 unobtrusive validation - validation

Question

If i have [Required(AllowEmptyStrings = true)] declaration in my view model the validation is always triggered on empty inputs. I found the article which explains why it happens. Do you know if there is a fix available? If not, how do you handle it?

Answer 1

Note: I'm assuming you have AllowEmptyStrings = true because you're also using your view model outside of a web scenario; otherwise it doesn't seem like there's much of a point to having a Required attribute that allows empty strings in a web scenario.
There are three steps to handle this:
Create a custom attribute adapter which adds that validation parameter
Register your adapter as an adapter factory
Override the jQuery Validation function to allow empty strings when that attribute is present
Step 1: The custom attribute adapter
I modified the RequiredAttributeAdapter to add in that logic:
using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.Web.Mvc;
namespace CustomAttributes
{
/// <summary>Provides an adapter for the <see cref="T:System.Runtime.CompilerServices.RequiredAttributeAttribute" /> attribute.</summary>
public class RequiredAttributeAdapter : DataAnnotationsModelValidator<RequiredAttribute>
{
/// <summary>Initializes a new instance of the <see cref="T:System.Runtime.CompilerServices.RequiredAttributeAttribute" /> class.</summary>
/// <param name="metadata">The model metadata.</param>
/// <param name="context">The controller context.</param>
/// <param name="attribute">The required attribute.</param>
public RequiredAttributeAdapter(ModelMetadata metadata, ControllerContext context, RequiredAttribute attribute)
: base(metadata, context, attribute)
{
}
/// <summary>Gets a list of required-value client validation rules.</summary>
/// <returns>A list of required-value client validation rules.</returns>
public override IEnumerable<ModelClientValidationRule> GetClientValidationRules()
{
var rule = new ModelClientValidationRequiredRule(base.ErrorMessage);
if (base.Attribute.AllowEmptyStrings)
{
//setting "true" rather than bool true which is serialized as "True"
rule.ValidationParameters["allowempty"] = "true";
}
return new ModelClientValidationRequiredRule[] { rule };
}
}
}
Step 2. Register this in your global.asax / Application_Start()
protected void Application_Start()
{
AreaRegistration.RegisterAllAreas();
DataAnnotationsModelValidatorProvider.RegisterAdapterFactory(typeof(RequiredAttribute),
(metadata, controllerContext, attribute) => new CustomAttributes.RequiredAttributeAdapter(metadata,
controllerContext, (RequiredAttribute)attribute));
RegisterGlobalFilters(GlobalFilters.Filters);
RegisterRoutes(RouteTable.Routes);
}
Step 3. Override the jQuery "required" validation function
This is done using the jQuery.validator.addMethod() call, adding our custom logic and then calling the original function - you can read more about this approach here. If you are using this throughout your site, perhaps in a script file referenced from your _Layout.cshtml. Here's a sample script block you can drop in a page to test:
<script>
jQuery.validator.methods.oldRequired = jQuery.validator.methods.required;
jQuery.validator.addMethod("required", function (value, element, param) {
if ($(element).attr('data-val-required-allowempty') == 'true') {
return true;
}
return jQuery.validator.methods.oldRequired.call(this, value, element, param);
},
jQuery.validator.messages.required // use default message
);
</script>

Answer 2

Rather than decorating the value with the 'Required' attribute, I use the following. I find it to be the simplest solution to this issue.
[DisplayFormat(ConvertEmptyStringToNull=false)]