Beverage.cs

namespace DesignPatterns
{
    public abstract class Beverage
    {
        public string description = "Unknown beverage";

        public virtual string GetDescription()
        {
            return description;
        }

        public abstract double Cost();
        
    }
}

CondimentDecorator.cs

namespace DesignPatterns
{
    public abstract class CondimentDecorator : Beverage
    {
        public abstract new string GetDescription();
    }
}

Espresso.cs

namespace DesignPatterns
{
    public class Espresso : Beverage
    {
        public Espresso()
        {
            description = "Espresso";
        }

        public override double Cost()
        {
            return 1.99;
        }
    }
}

Mocha.cs

namespace DesignPatterns
{
    public class Mocha : CondimentDecorator
    {
        Beverage beverage;

        public Mocha(Beverage beverage)
        {
            this.beverage = beverage;
        }

        public override string GetDescription()
        {
            return beverage.GetDescription() + " Mocha";
        }

        public override double Cost()
        {
            return 0.20 + beverage.Cost();
        }
    }
}

Program.cs

using System;

namespace DesignPatterns
{
    class Program
    {
        static void Main(string[] args)
        {
            Beverage espresso = new Espresso();
            Console.WriteLine($"{espresso.description}");
            Console.WriteLine(espresso.GetType());
            Console.WriteLine($"{espresso.GetDescription()} and {espresso.Cost()}");

            espresso = new Mocha(espresso);
            Console.WriteLine($"{espresso.description}");
            Console.WriteLine(espresso.GetType());
            Console.WriteLine($"{espresso.GetDescription()} and {espresso.Cost()}");
        }
    }
}