public void AddBrand(Brand brand)
 {
     brand.AddSneaker(this);
     Brand = brand;
 }
 public Sneaker(string name, string color, double price, DateTime releaseDate, Brand brand = null) : this()
 {
     Name        = name;
     Price       = price;
     Color       = color;
     Brand       = brand;
     ReleaseDate = releaseDate;
 }