Validation Filter to send a JSON response with validation errors when an HTTP Request contains an invalid data model. Uses JSON.net.

Error Validation - CustomGlobalConfig.cs

using Newtonsoft.Json.Serialization;
using System.Web.Http;
using System.Web.Http.Validation.Providers;
using MyApplication.Web.Filters;

namespace MyApplication.Web
{
    public static class CustomGlobalConfig
    {
        public static void Customize(HttpConfiguration config)
        {
            config.Filters.Add(new ValidationActionFilter());
        }
    }
}

Error Validation - Global.asax.cs

using System.Web.Http;
using System.Web.Mvc;
using System.Web.Optimization;
using System.Web.Routing;

namespace MyApplication.Web
{
    public class MvcApplication : System.Web.HttpApplication
    {
        protected void Application_Start()
        {
            AreaRegistration.RegisterAllAreas();

            WebApiConfig.Register(GlobalConfiguration.Configuration);
            FilterConfig.RegisterGlobalFilters(GlobalFilters.Filters);
            RouteConfig.RegisterRoutes(RouteTable.Routes);
            BundleConfig.RegisterBundles(BundleTable.Bundles);
            AuthConfig.RegisterAuth();

            CustomGlobalConfig.Customize(GlobalConfiguration.Configuration);
        }
    }
}

Error Validation - showValidationError.js

var showError = function(error) { // error is the JSON error message sent from the validation filter
  
  if (error.responseText) {
      var
          msg = JSON.parse(error.responseText),
          keys = _.keys(msg),
          txt = '';

      _.each(keys, function(key) {
          txt += msg[key] + ' ';
      });
      displayErrorMessage(txt);  // However this is defined for the system
  }
};

Error Validation - ValidationActionFilter.cs

using Newtonsoft.Json.Linq;
using System.Linq;
using System.Net;
using System.Net.Http;
using System.Web.Http.Controllers;
using System.Web.Http.Filters;

namespace MyApplication.Web
{
    public class ValidationActionFilter : ActionFilterAttribute
    {
        public override void OnActionExecuting(HttpActionContext context)
        {
            var modelState = context.ModelState;
            if (!modelState.IsValid)
            {
                var errors = new JObject();
                foreach (var key in modelState.Keys)
                {
                    var state = modelState[key];
                    if (state.Errors.Any())
                    {
                        errors[key] = state.Errors.First().ErrorMessage;
                    }
                }

                context.Response = context.Request.
                    CreateResponse<JObject>(HttpStatusCode.BadRequest, errors);
            }
        }
    }
}