sentences.cs

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

namespace ConsoleApplication10
{
    internal class Program
    {
        public static void Main(string[] args)
        {
            var seps = new[] {'.', '?', '!'};
            var sepsIn = new[] {',', '-', ';', '(', ')'};

            var savedSentence = string.Empty;
            var savedSentenceSepsCount = 0;

            using (var fl = new StreamReader(@"text.txt"))
            {
                var text = new StringBuilder();

                while (fl.Peek() >= 0) // '\0'
                {
                    var ch = Convert.ToChar(fl.Read());

                    if (seps.Contains(ch))
                    {
                        text.Append(ch);

                        var currentSentence = text.ToString().Trim();
                        var currentSentenceSepsCount =
                            currentSentence.Split(sepsIn, StringSplitOptions.RemoveEmptyEntries).Length;

                        if (savedSentence == string.Empty)
                        {
                            savedSentenceSepsCount = currentSentenceSepsCount;
                            savedSentence = currentSentence;
                        }

                        if (currentSentenceSepsCount > savedSentenceSepsCount)
                        {
                            savedSentenceSepsCount = currentSentenceSepsCount;
                            savedSentence = currentSentence;
                        }

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

            Console.WriteLine(savedSentence);
        }
    }
}