Tsiren Naimanov

string path = @"c:\temp\MyTest.txt";
        if (!File.Exists(path))
        {
            // Create a file to write to.
            using (StreamWriter sw = File.CreateText(path))
            {
                File.WriteAllLines(folder+"\\file.txt", file, Encoding.Default);
                sw.WriteLine(MyText2);
                sw.WriteLine(MyText3);
            }
        }

// Open the file to read from.
        int I = 0;
        using (StreamReader sr = File.OpenText(path))
        {
            string s = "";
            while ((s = sr.ReadLine()) != null)
            {
                I++;
            }
       label1.Text = Convert.ToString(I);
и всё в лебеле у тебя кол-во строк - непустых
        }