ProductBlock.cs

public class ProductBlock
{
    private List<Product> _products;

    public List<Product> Products
    {
        get { return _products; }
        set
        {
            _products = value;

            foreach (Product product in _products)
            {
                this.BlockSum += product.Price;
            }
        }
    }

    public int BlockSum { get; internal set; }
}