How to calculate linecost and cart total in C#

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C#

I have product with options and extras in cart that line cost is 200

Bass product cost -20 and option 1-50 and option2 -30 and extra1-30 and extra2 -70. I had applied 16 percentage discount for 200 pounds i.e 32 pounds

discount at the time linecost 168(200-32) and cart total 168(200-32).

Then finally i want to remove option or extras from cart one by one whatever

the want. For every option or extra remove cart recalculate line cost and

cart total but i got negative values in cart and please suggest whatver i had
done correct or not.

My linecost function and cart total function

public decimal CalculateLineCost(Product product, List<Option> options, List<Extra> extras, decimal? discount)
       {
           decimal Linesalesprice = 0.00M;

           if (!HasOptions(product.Id) && !HasExtrs(product.Id))
           {
               if (product.DiscountType == 0)
               {
                   product.SubTotal = Convert.ToDecimal((product.EatInSalesPrice - (discount ?? 0)) * product.Qty);
                   Linesalesprice = product.SubTotal;
               }
               else if(product.DiscountType > 0)
               {
                   product.SubAmountAfterDiscount = Convert.ToDecimal(product.SubTotal - product.DiscountSubTotal);
                   Linesalesprice = Convert.ToDecimal(product.SubAmountAfterDiscount);
               }

           }
           else
           {
               if (product.DiscountType == 0)
               {
                   decimal lineSubTotal = Convert.ToDecimal(product.EatInSalesPrice + options.SelectMany(option => option.SubProductOptions).Sum(s => s.EatInSalesPrice) + extras.Sum(e => e.EatInSalesPrice));
                   product.SubTotal = Convert.ToDecimal((lineSubTotal - (discount ?? 0)) * product.Qty);
                   Linesalesprice = product.SubTotal;
               }
               else if (product.DiscountType > 0)
               {
                   product.SubAmountAfterDiscount = Convert.ToDecimal(product.SubTotal - product.DiscountSubTotal);
                   Linesalesprice = Convert.ToDecimal(product.SubAmountAfterDiscount);
               }



           }

           return Linesalesprice;
       }

       public decimal CalculateCartTotal(List<Product> products,List<Option> options, List<Extra> extras, decimal? discount)
       {
           decimal carttotal = 0.00M;

           foreach (var item in products)
           {
               decimal linecost = CalculateLineCost(item,item.options, item.Extras,item.DiscountSubTotal);
               carttotal += linecost;
           }
            shoppingmodel.Total = carttotal;

            return shoppingmodel.Total;
       }

I had used when remove option or extra from cart

product.SubAmountAfterDiscount = Convert.ToDecimal(product.SubTotal - product.DiscountSubTotal);

.

Please give this procedure is correct or not and i dont want negative values in cart

What I have tried:

I had used when remove option or extra from cart

product.SubAmountAfterDiscount = Convert.ToDecimal(product.SubTotal - product.DiscountSubTotal);

.

Please give this procedure is correct or not and i dont want negative values