Length space as a condition for the programme

[Anderson]

The code below reads the lines in the text file and converts into a body. That's understandable. But I can't stop at that. Condition to be fulfilled for the continuation of work - Exactly. Six lines in the file. How should it be:
6 lines - programme ongoing
* Less than six lines - the program gives a wrong window and finishes the job.

using (System.IO.StreamReader sr = new System.IO.StreamReader(f.FullName, Encoding.GetEncoding("windows-1251")))
 {
     string str = "";
     string s = "";
     while ((s = sr.ReadLine()) != null)
           {
               str += s + "@";//делим текст при помощи собаки на строки
           }
               string[] split = str.Split('@');
               int counter = split.Length;
               if (counter == 6)                   
               {
                   //действия
               }
               else
               {
                   //выход из приложения
               }

In my case, the program always ends with a mistake, even if I put a "slower or equal to six." Line in file six, checked. I don't know what I'm doing wrong. The text has been translated into a mass, Length is the nature of the mass, a whole number. What's the mistake?

[amarantha]

How hard is it? It's C#, it doesn't have to be tortured.

Do that.

var lines = File.GetLines(f.FullName, Encoding.GetEncoding("windows-1251"));
var first6lines = lines.Take(6).ToList();
if (first6lines.Count < 6)
{
    // ... то выдать сообщение об ошибке и свалить
}
// а дальше проходитесь циклом по строкам и делайте с ними, что хотите