Implementing Strategy pattern in C#

I am implementing strategy pattern. I have defined a strategy as interfaces and concrete classes to implement the strategy. Based on user selection/configuration, the algorithm to apply the discount changes.

  public interface IStrategy
    {
        double calculate(double x, double y);
    }

concrete classes implementing the gold strategy is listed below -

public class clsGoldDiscountStrategy : IStrategy
    {
        public double calculate(double x, double y)
        {
            return (x * y) * 0.8;
        }
    }
}

concrete classes implementing the platinumstrategy is listed below -

public class clsPlatinumDiscountStrategy : IStrategy
    {
        public double calculate(double x, double y)
        {
            return (x * y) * 0.7;
        }
    }

The business logic to apply

public class clsBL
    {
        public double costPrice { get; set; }
        public double qty { get; set; }
        IStrategy _strategy;

        public clsBL(IStrategy strategy)
        {
            _strategy = strategy;
        }

        public double GetfinalPrice(double cp, double qty)
        {
            return _strategy.calculate(cp, qty); 
        }

    }

//Main method

static void Main(string[] args)
        {
            Console.WriteLine("Enter the discount Plan (Gold/Platinum)");
            string filter = Console.ReadLine().ToUpper();
            double result = 0;

            if (filter.Length > 0)
            {
                switch (filter)
                {
                    case "GOLD":
                        //Gold 
                        clsBL blgold = new clsBL(new clsGoldDiscountStrategy());
                        blgold.costPrice = 5;
                        blgold.qty = 10;

                        result = blgold.GetfinalPrice(blgold.costPrice, blgold.qty);
                        break;

                    case "PLATINUM":
                        //Platinum
                        clsBL blplatinum = new clsBL(new clsPlatinumDiscountStrategy());
                        blplatinum.costPrice = 10;
                        blplatinum.qty = 8;

                        result = blplatinum.GetfinalPrice(blplatinum.costPrice, blplatinum.qty);
                        break;
                    default:
                        Console.WriteLine("Enter the discount value as either  gold or platinum");
                        break;
                }

                Console.WriteLine("The result for " + filter + " is " + result);

            }
            else
            {
                Console.WriteLine("Enter the discount value");
                return;
            }


            Console.ReadLine();

        }