问ASP.NET MVC:使用HtmlHelper.TextBox和自定义模型绑定器时出现空引用异常

EN

public object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
{
   string modelName = bindingContext.ModelName;
   string monthKey = modelName + ".Month";
   string dayKey = modelName + ".Day";
   string yearKey = modelName + ".Year";

   //get values submitted on form
   string year = bindingContext.ValueProvider[yearKey].AttemptedValue;
   string month = bindingContext.ValueProvider[monthKey].AttemptedValue;
   string day = bindingContext.ValueProvider[dayKey].AttemptedValue;

   DateTime parsedDate;
   if (DateTime.TryParse(string.Format(DateFormat, year, month, day), out parsedDate))
        return parsedDate;

   //could not parse date time report error, return current date
   bindingContext.ModelState.AddModelError(yearKey, ValidationErrorMessages.DateInvalid);

   //added this after reading similar problems, does not fix!
   bindingContext.ModelState.SetModelValue(yearKey, bindingContext.ValueProvider[modelName]);
   return DateTime.Today;
}