Slice/StoreServer/OrderPartial.cs

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

namespace StoreServer
{
    partial class Order
    {
        public string ProductInOrderString
        {
            get
            {
                string Product = "";
                foreach (var item in ProductInOrder)
                {
                    Product += $"{item.ProductInStore.Product.NameProduct} {item.AmountProduct.ToString("F2")} руб. * {item.AmountProduct}\n";
                }
                return Product;
            }
        }

        public decimal CostOfOrder
        {
            get
            {
                decimal Cost = 0;
                foreach (var item in ProductInOrder)
                {
                    Cost += item.ProductInStore.Product.PriceOfOne * item.AmountProduct;
                }
                return Cost;
            }
        }
    }
}