Program.cs

using System;
using System.Collections.Generic;

namespace strategy
{
    class Context
    {
 
        private IStrategy _strategy;

        public Context()
        { }

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

        public void SetStrategy(IStrategy strategy)
        {
            _strategy = strategy;
        }

        // Delegate work

        public void DoSomeBusinessLogic()
        {
            Console.WriteLine("Context: Sorting data upon context");
            var result = _strategy.DoAlgorithm(new List<string> { "a", "b", "c", "d", "e" });

            string resultStr = string.Empty;
            foreach (var element in result as List<string>)
            {
                resultStr += element + ",";
            }

            Console.WriteLine(resultStr);
        }
    }

    public interface IStrategy
    {
        object DoAlgorithm(object data);
    }

    // normal sort
    class ConcreteStrategyA : IStrategy
    {
        public object DoAlgorithm(object data)
        {
            var list = data as List<string>;
            list.Sort();

            return list;
        }
    }
    // reverse sort
    class ConcreteStrategyB : IStrategy
    {
        public object DoAlgorithm(object data)
        {
            var list = data as List<string>;
            list.Sort();
            list.Reverse();

            return list;
        }
    }

    class Program
    {
        static void Main(string[] args)
        {

            var context = new Context();

            Console.WriteLine("BLL: Strategy is set to normal sorting.");
            context.SetStrategy(new ConcreteStrategyA());
            context.DoSomeBusinessLogic();

            Console.WriteLine();

            Console.WriteLine("BLL: Strategy is set to reverse sorting.");
            context.SetStrategy(new ConcreteStrategyB());
            context.DoSomeBusinessLogic();
        }
    }
}