A simple ASP.NET MVC FilterAttribute that validates the model in the request pipeline

ValidateModelAttribute.cs

namespace Mvc.Examples
{
    internal class ValidateModelAttribute : ActionFilterAttribute
    {
        public override void OnActionExecuting(ActionExecutingContext actionContext)
        {
            if (actionContext.ModelState.IsValid == false)
            {
                actionContext.Result = new BadRequestObjectResult(
                    actionContext.ModelState.Values
                        .SelectMany(e => e.Errors)
                        .Select(e => e.ErrorMessage));
            }
        }
    }
}