WebApiConfig.cs

using FluentValidation.WebApi;
using System.Net;
using System.Net.Http;
using System.Web.Http;
using System.Web.Http.Controllers;
using System.Web.Http.Filters;
using Microsoft.Owin.Security.OAuth;

namespace Preveld
{
    public class ValidateModelStateFilter : ActionFilterAttribute
    {
        public override void OnActionExecuting(HttpActionContext actionContext)
        {
            if (!actionContext.ModelState.IsValid)
            {
                actionContext.Response = actionContext.Request.CreateErrorResponse(HttpStatusCode.BadRequest, actionContext.ModelState);
            }
        }
    }

    public static class WebApiConfig
    {
        public static void Register(HttpConfiguration config)
        {
            config.EnableCors();

            config.SuppressDefaultHostAuthentication();
            config.Filters.Add(new HostAuthenticationFilter(OAuthDefaults.AuthenticationType));

            config.MapHttpAttributeRoutes();

            config.Filters.Add(new ValidateModelStateFilter());
            FluentValidationModelValidatorProvider.Configure(config);

            config.Routes.MapHttpRoute(
                name: "ClientApi",
                routeTemplate: "api/client/{name}",
                defaults: new { controller = "Client", action = "Index" }
            );

            config.Routes.MapHttpRoute(
                name: "DefaultApi",
                routeTemplate: "api/{controller}/{action}/{id}",
                defaults: new { id = RouteParameter.Optional }
            );
        }
    }
}