ModelStateValidationActionFilterAttribute.cs

public class ModelStateValidationActionFilterAttribute : ActionFilterAttribute
{
    public override void OnActionExecuting(HttpActionContext actionContext)
    {
        var modelState = actionContext.ModelState;

        if (!modelState.IsValid)
            actionContext.Response = actionContext.Request
                 .CreateErrorResponse(HttpStatusCode.BadRequest, modelState);
    }
}