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    <p><em>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.</em></p> <p>There are three steps to handle this:</p> <ol> <li>Create a custom attribute adapter which adds that validation parameter</li> <li>Register your adapter as an adapter factory</li> <li>Override the jQuery Validation function to allow empty strings when that attribute is present</li> </ol> <p><strong>Step 1: The custom attribute adapter</strong></p> <p>I modified the RequiredAttributeAdapter to add in that logic:</p> <pre><code>using System; using System.Collections.Generic; using System.ComponentModel.DataAnnotations; using System.Web.Mvc; namespace CustomAttributes { /// &lt;summary&gt;Provides an adapter for the &lt;see cref="T:System.Runtime.CompilerServices.RequiredAttributeAttribute" /&gt; attribute.&lt;/summary&gt; public class RequiredAttributeAdapter : DataAnnotationsModelValidator&lt;RequiredAttribute&gt; { /// &lt;summary&gt;Initializes a new instance of the &lt;see cref="T:System.Runtime.CompilerServices.RequiredAttributeAttribute" /&gt; class.&lt;/summary&gt; /// &lt;param name="metadata"&gt;The model metadata.&lt;/param&gt; /// &lt;param name="context"&gt;The controller context.&lt;/param&gt; /// &lt;param name="attribute"&gt;The required attribute.&lt;/param&gt; public RequiredAttributeAdapter(ModelMetadata metadata, ControllerContext context, RequiredAttribute attribute) : base(metadata, context, attribute) { } /// &lt;summary&gt;Gets a list of required-value client validation rules.&lt;/summary&gt; /// &lt;returns&gt;A list of required-value client validation rules.&lt;/returns&gt; public override IEnumerable&lt;ModelClientValidationRule&gt; 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 }; } } } </code></pre> <p><strong>Step 2. Register this in your global.asax / Application_Start()</strong></p> <pre><code> protected void Application_Start() { AreaRegistration.RegisterAllAreas(); DataAnnotationsModelValidatorProvider.RegisterAdapterFactory(typeof(RequiredAttribute), (metadata, controllerContext, attribute) =&gt; new CustomAttributes.RequiredAttributeAdapter(metadata, controllerContext, (RequiredAttribute)attribute)); RegisterGlobalFilters(GlobalFilters.Filters); RegisterRoutes(RouteTable.Routes); } </code></pre> <p><strong>Step 3. Override the jQuery "required" validation function</strong></p> <p>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 <a href="http://blogs.teamb.com/craigstuntz/2009/01/15/37923/" rel="noreferrer">here</a>. 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:</p> <pre><code>&lt;script&gt; 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 ); &lt;/script&gt; </code></pre>
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    1. PORequiredAttribute with AllowEmptyString=true in ASP.NET MVC 3 unobtrusive validation
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    1. USJon Egerton
    1. USJon Galloway
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    1. COThanks for your efforts, Jon! Unfortunately it's not working for me, i still see "field is required" message :( I double checked that your code is executed with VS and browser debuggers and i don't have any custom scripts or validation related code that might interfere. I use jQuery 1.4.1. Can this be the problem?
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    2. COAnd yes, you're right that i need the feature outside of web scenarios (my application service layer performs similar validation checks to be DRY).
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    3. CO+1 This seems to be a nice solution, I wonder why it does not work. But maybe you could simplify it, if you return no validation rule (empty array) in the adapter in case of AllowEmptyStrings. You could then spare the client side magic. You could then also easily check if the required attribute is correctly omitted on your input. If it still does not work, you should debug the adapter code and see if it is called appropriately (if the registration is ok).
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