// このクラスをシリアル化可能に
[Serializable]
public class SelectedItem
{
    public int Id { set; get; }
    public string Name { set; get; }
    public int Price { set; get; }
    public int Quantity { set; get; }

    public SelectedItem(int id, string name, int price, int quantity)
    {
        this.Id = id;
        this.Name = name;
        this.Price = price;
        this.Quantity = quantity;
    }

    public override string ToString()
    {
        return "[" + Id + "]" + Name + "(" + Price + "円) " + Quantity + "個";
    }
}