using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    static void AddBirthdayToDictionary(Dictionary<DateTime, List<string>> dict, int month, int day, string name)

    {

      var bdate = new DateTime(DateTime.MinValue.Year, month, day);

      List<string> val;

      if(!dict.TryGetValue(bdate, out val))

      {

        val = new List<string>();

        dict.Add(bdate, val);

      }

      val.Add(name);

    }

    static List<string> GetBirthdays(Dictionary<DateTime, List<string>> dict, int month, int day)

    {

      var bdate = new DateTime(DateTime.MinValue.Year, month, day);

      List<string> val;

      if(dict.TryGetValue(bdate, out val))

      {

        return val;

      }

      return new List<string>();

    }

​

    public static void Main()

    {

        var birthdayDictionary = new Dictionary<DateTime, List<string>>();

        // add possible birthdays    

        AddBirthdayToDictionary(birthdayDictionary, 3, 26, "Person 1");

        AddBirthdayToDictionary(birthdayDictionary, 3, 26, "Person 2");

​

        var today = DateTime.Today;

            // get birthdays today

        var bdayList = GetBirthdays(birthdayDictionary, today.Month, today.Day);

        if(bdayList.Any())

        {

            var count = bdayList.Count();

            Console.WriteLine(string.Format("Hello, today is {0} and {1} {2} celebrating {3} birthday!", today, string.Join(", ", bdayList), (count>1) ? "are":"is", (count>1) ? "their" : "its")); 

        }       

    }

}