strings.cs

using System;
using System.IO;
using System.Linq;
using System.Text;

namespace ConsoleApplication7
{
    class Program
    {
        static void Main(string[] args)
        {
            var seps = new[] {'.', '?', '!'};
            var sentencesCount = 0;
            var maxSentences = 5;
            var sentences = new string[maxSentences];
            using (var fl = new StreamReader(@"text.txt"))
            {
                var text = new StringBuilder();

                while (fl.Peek() >= 0)
                {
                    var ch = Convert.ToChar(fl.Read());
                    if (seps.Contains(ch))
                    {
                        text.Append(ch);
                        sentences[sentencesCount] = text.ToString().Trim();
                        sentencesCount++;

                        if (sentencesCount >= maxSentences)
                        {
                            break;
                        }

                        text.Clear();
                    }
                    else
                    {
                        text.Append(ch);
                    }
                }
            }

            WriteLineReversed(sentences);
        }

        private static void WriteLineReversed(string[] sentences)
        {
            foreach (var sentence in sentences.Reverse())
            {
                Console.WriteLine(sentence);
            }
        }
    }
}