using System;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        var strs = new [] {"Dear Elaine Jasper, Thank you for coming", "Dear Muhammad Ali Yusuf, Thank you for coming", "Dear Yusuf, Thank you for coming" };

​

        foreach(var item in strs)

            Console.WriteLine(HideName(item));

    }

    public static string HideName(string str){

        //First split by comma

        var splittedByComma = str.Split(',');

​

        //Then get separate words from splitted array's first part

        var words = splittedByComma[0].Split(' ');

​

        //Get names before last name

        var name = words.Skip(1).Take(words.Length - 2);

​

        //Replace all chars from first names with '*'

        var hiddenPart = string.Join(" ", name.Select(s => new string(s.Select(ch => '*').ToArray())));

​

        //Compose result

        var result = string.Format("Dear {0} {1}, {2}", hiddenPart, words.Last(), splittedByComma[1]);

        return result;

    }

}