Pricing.cs

namespace AboutComments
{
    using System;
    static class Product
    {
        public const int CAFE_PRIX = 1;
        public const int ASSIETTE_PRIX = 15;
        public const int SANDWICH_PRIX = 10;
        public const int DESSERT_NORMAL_PRIX = 2;
        public const int DESSERT_SPECIAL_PRIX = 4;
        public const int PETIT_PRIX = 2;
        public const int MOYEN_PRIX = 3;
        public const int GRAND_PRIX = 4;
    }
    static class Size
    {
        public const string PETIT = "petit";
        public const string MOYEN = "moyen";
        public const string GRAND = "grand";

    }

    static class Formule
    {
        public const int ASSIETTE_NORMAL_SIZE = 18;
        public const int ASSIETTE_MAX_SIZE = 21;
        public const int SANDWICH_NORMAL_SIZE = 13;
        public const int SANDWICH_MAX_SIZE = 16;
    }

    static class Constante
    {
        public const int EMPTY_DATA_SET = -1;
        public const int ASSIETTE = 1;
        public const int SANDWICH = 2;
        
    }

    public class App
    {
        private int total = 0;
        private string type;
        private string name;
        private string size;
        private string dsize;
        private string coffee;

        public App(string ptype, string pname, string psize, string pdsize, string pcoffee)
        {
            type = ptype;
            name = pname;
            size = psize;
            dsize = pdsize;
            coffee = pcoffee;
        }

        public int Compute()
        {
            if(isDataEmpty())
                return Constante.EMPTY_DATA_SET;

            if(type=="assiette")
            {
                total+= Product.ASSIETTE_PRIX;
                AddToMenu(Constante.ASSIETTE);
            } 
            else if(type == "sandwich") {
                total+=Product.SANDWICH_PRIX;
                AddToMenu(Constante.SANDWICH);
            }
            AddCoffe();
            return total;
        }

        public void AddCoffe(){
            if(type=="assiette" && size==Size.MOYEN && dsize=="normal" && coffee=="yes")
            {
                Console.Write(" avec café offert!");
            } else {
                total+=Product.CAFE_PRIX;
            }
        }

        public void AddToMenu(int name){
            switch(size)
            {
                case Size.PETIT: 
                    total+=Product.PETIT_PRIX;
                    if(dsize=="normal")
                    {
                        total+=Product.DESSERT_NORMAL_PRIX;
                    } else {
                        total+=Product.DESSERT_SPECIAL_PRIX;
                    }
                    break;
                case Size.MOYEN: 
                    total+=Product.MOYEN_PRIX;
                    if(dsize!="normal")
                    {
                        total+=Product.DESSERT_SPECIAL_PRIX;
                    }
                    break;
                case Size.GRAND: 
                    total+=Product.GRAND_PRIX;
                    if(dsize=="normal")
                    {
                        total+=Product.DESSERT_NORMAL_PRIX;
                    } else {
                        Console.Write("Prix Formule Max appliquée ");
                        if(name == Constante.SANDWICH)
                            total=Formule.SANDWICH_MAX_SIZE;
                        else if(name == Constante.ASSIETTE)
                            total=Formule.ASSIETTE_MAX_SIZE;
                    }
                    break;
            }
        }
        public bool isDataEmpty(){
            if(string.IsNullOrEmpty(type+name+size+dsize+coffee))
                return true;
            return false;
        }
    }   
}

run.cs

using System;
using AboutComments;

namespace Clean_code
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Start");
            var pricing = new App("assiette","Lol","normal","normal", "no");
            Console.WriteLine(pricing.Compute());
            Console.WriteLine("Ended");


        }
    }

}