gistfile1.cs

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

            context.Response = new HttpResponseMessage<JsonValue>(errors, HttpStatusCode.BadRequest);
        }
    }
}