Split by Word Casing

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace SplitWordCas_24._05._2016
{
    class Program
    {

        static void Main(string[] args)
        {
            char[] separators = { ',', ';', ':', '.', '!', ' ' };

            List<string> words = Console.ReadLine().Split(separators, StringSplitOptions.RemoveEmptyEntries).ToList();

            List<string> lowerCaseWords = IsLowerCase(separators, words);
            List<string> upperCaseWords = IsUpperCase(separators, words);
            List<string> mixedCaseWords = IsMixedCase(separators, words);

            Console.WriteLine("Lower-case: {0}", string.Join(", ", lowerCaseWords));
            Console.WriteLine("Mixed-case: {0}", string.Join(", ", mixedCaseWords));
            Console.WriteLine("Upper-case: {0}", string.Join(", ", upperCaseWords));
        }

        static List<string> IsMixedCase(char[] separators, List<string> words)
        {
            List<string> mixedCaseWords = new List<string>();

            foreach (string word in words)
            {
                int upperCaseLetters = 0;
                int lowerCaseLetters = 0;
                foreach (char letter in word)
                {
                    if (char.IsUpper(letter))
                    {
                        upperCaseLetters++;
                    }
                    else if (char.IsLower(letter))
                    {
                        lowerCaseLetters++;
                    }
                }
                if (upperCaseLetters != word.Length && lowerCaseLetters != word.Length)
                {
                    mixedCaseWords.Add(word);
                }
            }
            return mixedCaseWords;
        }
        static List<string> IsUpperCase(char[] separators, List<string> words)
        {
            List<string> upperCaseWords = new List<string>();

            foreach(string word in words)
            {
                int upperCaseLetters = 0;
                foreach (char letter in word)
                {
                    if (char.IsUpper(letter))
                    {
                        upperCaseLetters++;
                    }
                    else if (char.IsLower(letter))
                    {
                        break;
                    }
                }
                if (upperCaseLetters == word.Length)
                {
                    upperCaseWords.Add(word);
                }
            }
            return upperCaseWords;

        }
        static List<string> IsLowerCase(char[] separators, List<string> words)
        {
            List<string> lowerCaseWords = new List<string>();

            foreach (string word in words)
            {
                int lowerCaseLetters = 0;
                foreach (char letter in word)
                {
                    if (char.IsLower(letter))
                    {
                        lowerCaseLetters++;
                    }
                    else if (char.IsUpper(letter))
                    {
                        break;
                    }
                }
                if (lowerCaseLetters== word.Length)
                {
                    lowerCaseWords.Add(word);
                }
            }
            return lowerCaseWords;
        }
    }
}