using System;
using System.Linq;
 
public class Test
{
	        static void Main(string[] args)
        {
            Console.WriteLine("Введи мне строку со словами..в любом регистре");
            string str = "ИНИМУМ  МИНИМУМ".ToUpper();
            char[] separator = { ' ' };
            string[] strArr = str.Split(separator,StringSplitOptions.RemoveEmptyEntries);
            Console.WriteLine("Все слова из предложения");
            foreach (var val in strArr)
            {
                Console.WriteLine(val);
                str = str.Replace(val, val.First() + String.Join("", val.Skip(1).ToArray()).Replace(val.First(), '.'));
            }
            Console.WriteLine("Результат замены: \n{0}",str);
            Console.ReadKey();
        }
}

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