.NET 4.x Не запускается библиотека: "no suitable method found to override" - C#

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace Library_for_lab_2
{
    public abstract class Product
    {
        public virtual double Price()
        {
            return Topaz.Price + Gold.Price + Saphire.Price;
        }
        public virtual double Volume()
        {
            return Topaz.Volume + Saphire.Volume;
        }
 
        public Topaz Topaz { get; set; }
        public Gold Gold { get; set; }
        public Saphire Saphire { get; set; }
 
        protected abstract string Type { get; }
 
        public override string ToString()
        {
            return string.Format("Стоимость: {0}\nКолличество карат: {1}\nТип: {2}\nЗолото: {3}\nТопаз: {4}\nСапфир: {5}\n",
                Price(), Volume(), Type, Gold, Topaz, Saphire);
        }
    }
 
    public abstract class GoldProduct : Product 
    {
    }
 
    public class Rings : GoldProduct
    {
        protected override string Type
        {
            get { return "Создание кольца"; }
        }
        
        public Diamond Diamond { get; set; }
        
        public override double Price()
        {
            return base.Price() + Diamond.Price;
        }
        public override double Volume()
        {
            return base.Volume() + Diamond.VolumeTime;
        }
 
        public override string ToString()
        {
            return base.ToString() + string.Format("Алмаз для кольца: {0}\n", Diamond);
        }
    }
 
    public class Earrings : GoldProduct
    {
        protected override string Type
        {
            get { return "Создание сережек"; }
        }
  
    }
 
    public class Necklace : GoldProduct
    {
        public Saphire Saphire2 { get; set; }
 
        protected override string Type
        {
            get { return "Создание ожерелья"; }
        }
 
        public override double Price()
        {
            return base.Price() + Saphire2.Price;
        }
        public override double Volume()
        {
            return base.Volume() + Saphire2.Volume;
        }
 
        public override string ToString()
        {
            return base.ToString() + string.Format("Дополнительные сапфиры для ожерелья: {0}\n", Saphire2);
        }
    }
 
    public class SilverProduct : Product 
    {
        public Silver Silver { get; set; }
 
        protected override string Type
        {
            get { return "Изделия с серебром"; }
        }
 
        public override double Price()
        {
            return base.Price() + Silver.Price;
        }
       
        public override string ToString()
        {
            return base.ToString() + string.Format("Серебро: {0}\n", 
                Silver == null ? "Отсуствует" : "Присуствует");
        }
    }
 
    public class Topaz 
    {
        public double Price { get; set; }
        public int Volume { get; set; }
 
        public override string ToString()
        {
            return string.Format("Вес камня {0} карат", Volume);
        }
    }
 
    public class Gold
    {
        public double Price { get; set; }
        public double Congestion { get; set; }
 
        public override string ToString()
        {
            return string.Format("Проба {0} ", Congestion);
        }
    }
 
    public class Saphire
    {
        public double  Price { get; set; }
        public int Volume { get; set; }
 
        public override string ToString()
        {
            return string.Format("Вес камня {0} карат", Volume);
        }
    }
 
    public class Silver
    {
        public double Price { get; set; }
    }
 
    public class Diamond
    {
        public double Price { get; set; }
        public int VolumeTime { get; set; }
 
        public override string ToString()
        {
            return string.Format("Колличество карат {0} ", VolumeTime);
        }
    }
}

namespace Library_for_lab_2
{
    partial class UserControl1
    {
        /// <summary>
        /// Designer variable used to keep track of non-visual components.
        /// </summary>
        private System.ComponentModel.IContainer components = null;
        
        /// <summary>
        /// Disposes resources used by the form.
        /// </summary>
        /// <param name="disposing">true if managed resources should be disposed; otherwise, false.</param>
        protected override void Dispose(bool disposing)
        {
            if (disposing) {
                if (components != null) {
                    components.Dispose();
                }
            }
            base.Dispose(disposing);
        }
        
        /// <summary>
        /// This method is required for Windows Forms designer support.
        /// Do not change the method contents inside the source code editor. The Forms designer might
        /// not be able to load this method if it was changed manually.
        /// </summary>
        private void InitializeComponent()
        {
            // 
            // UserControl1
            // 
            this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
            this.Name = "UserControl1";
        }
    }
}