How do I enforce a method to run when creating an object when I am already using validators from data annotation?

Question

I have a class product and I want to validate the promotion field and change the price based on the value. I am already using data annotation What is the best way to do it following solid principles?

` using System; using System.ComponentModel.DataAnnotations;

namespace RefactoringTest.ProductService.Entities { public class Product : BaseEntity { [DisplayFormat(ConvertEmptyStringToNull = false)] [Required(AllowEmptyStrings = false, ErrorMessage = "ProductName is required.")] public string ProductName { get; set; }

    [Required(AllowEmptyStrings = false, ErrorMessage = "ProductDescription is required.")]
    [DisplayFormat(ConvertEmptyStringToNull = false)]
    public string ProductDescription { get; set; }

    [Range(1, int.MaxValue, ErrorMessage = "Please enter a value bigger than 0")]
    public int Quantity { get; set; }

    [Range(1, int.MaxValue, ErrorMessage = "Please enter a value bigger than 0")]
    public decimal Price { get; set; }

    [Required(AllowEmptyStrings = false, ErrorMessage = "BrandName is required.")]
    [DisplayFormat(ConvertEmptyStringToNull = false)]
    public string BrandName { get; set; }

    public string Promotion { get; set; }

    public ProductBrand ProductBrand { get; set; }

    public int ProductBrandId { get; set; }


    public void CalculatePriceAfterDiscount()
    {
  switch (Promotion)
        {
            case "5PERCENTOFF":
                Price = Price - Price * 0.05m;
            break;
            case "10PERCENTOFF":
                  Price = Price - Price * 0.1m;
            break;
            case "20PERCENTOFF":
                 Price = Price - Price * 0.2m;
            break;
            default:
                if (!string.IsNullOrEmpty(Promotion))
                throw new ArgumentException("Invalid promotion specified");
            break;
        }
        }
    }      
}

}`

Also is it ok to have the calculate price method in the class or should be in the service?