apply-action-filters.cs

namespace MyRestApi.Filters
{
    public class ValidateStateAttribute : ActionFilterAttribute, IActionFilter
    {
        public override void OnActionExecuting(HttpActionContext actionContext)
        {
            if (!actionContext.ModelState.IsValid)
            {
                var dict = new List<object>();
                //Loop through each "model"/"contract" attribute, and return the error with a 422 status with the error messages.
                foreach (var entry in actionContext.ModelState.Where(x => x.Value.Errors.Count > 0))
                {
                    dict.Add(new
                    {
                        PropertyName = Regex.Replace(entry.Key, @"^[^\.]+\.", ""),
                        ErrorMessage = entry.Value.Errors.FirstOrDefault().ErrorMessage
                    });
                }
                actionContext.Response = actionContext.Request.CreateResponse(
                    422, dict
                );
            }
        }
    }
}

namespace MyRestApi.Contracts
{
    public enum EAddressType
    {
        Billing,
        Shipping
    }

    //validations are defined directly on the objects, instead of in the controller
    public class dtoAddress
    {
        public EAddressType addressType { get; set; }

        [Required]
        public String firstName { get; set; }

        [Required]
        public String lastName { get; set; }

        [Required]
        public String line1 { get; set; }

        public String line2 { get; set; }

        [Required]
        public String city { get; set; }

        [RegularExpression(@"^[a-zA-Z]{2}$", ErrorMessage="State must use two-character code (e.g. NY)")]
        public String state { get; set; }

        [RegularExpression(@"^\d{5}(?:-\d{4}|)$", ErrorMessage="Please use 5 digit zip code or 9 digit zip code with hyphen")]
        public String zipCode { get; set; }

        [RegularExpression(@"^\d{10}$", ErrorMessage = "Phone number must be 10 digits")]
        public String phone { get; set; }

        [RegularExpression(@"^.+@.+\..+$", ErrorMessage = "Invalid Email Address")]
        public String email { get; set; }
    }
}


//FilterConfig.cs
namespace MyRestApi.Config
{
    public class FilterConfig
    {
        public static void RegisterGlobalFilters(HttpFilterCollection filters)
        {
            filters.Add(new Filters.ValidateStateAttribute());
        }
    }
}

//Register filter config globally
//Global.asax.cs
public class WebApiApplication : System.Web.HttpApplication
{
    protected void Application_Start()
    {
        AreaRegistration.RegisterAllAreas();

        WebApiConfig.Register(GlobalConfiguration.Configuration);
        FilterConfig.RegisterGlobalFilters(GlobalConfiguration.Configuration.Filters); //<-- filter registered here
        RouteConfig.RegisterRoutes(RouteTable.Routes);
        BundleConfig.RegisterBundles(BundleTable.Bundles);
    }
    //..
}


//AddressController.cs
namespace MyRestApi.Controllers
{
    public class AddressController : ApiController
    {
        public HttpResponseMessage Post(dtoAddress address)
        {
            //code here. ActionFilter will handle validations for city/state/zip/etc. before this method is called.
        }
    }
}