example1.cs

public class FoodViewModel
{
  public string FavouriteFood { get; set; }
}

public ActionResult UpdateFood(FoodViewModel viewModel)
{
  if(ModelState.IsValid)
  {
    UpdateStats(viewModel.FavouriteFood);
    return RedirectToAction("Thanks");
  }
  return View(viewModel);
}

example2.cs

public ActionResult UpdateFood(FoodViewModel viewModel)
{
  if(viewModel.FavouriteFood.Contains("House"))
  {
    ModelState.AddModelError("","House is not a food");
  }

  if (!ModelState.IsValid)
  {
    return View(viewModel);
  }

  UpdateStats(viewModel.FavouriteFood);

  return RedirectToAction("Thanks");
}

example3.cs

public interface IValidatableObject
{
  IEnumerable<ValidationResult> Validate(ValidationContext validationContext)
}

example4.cs

public class FoodViewModel : IValidatableObject
{
  public string FavouriteFood { get; set; }

  public IEnumerable<ValidationResult> Validate(ValidationContext validationContext)
  {
    if (FavouriteFood.Contains("House"))
    {
      yield return new ValidationResult("Sorry, house is not a food");
    }
  }
}

example5.cs

var validationResults = new List<ValidationResult>();

var viewModel = Activator.CreateInstance(typeof (FoodViewModel)) as IValidatableObject;

if (viewModel != null)
{
  validationResults.AddRange(viewModel.Validate(null));
}

foreach (var validationResult in validationResults)
{
	Console.WriteLine(validationResult.ErrorMessage);
}