string s1 = "";
using (System.IO.TextReader tr = new System.IO.StreamReader("c:\\windows\\system32\\eula.txt"))
{
    s1 = tr.ReadToEnd().Split(new string[] { "\r\n" }, StringSplitOptions.None)[99];
}
Console.WriteLine(s1);

string s2 = "";
using (TextReader tr = new System.IO.StreamReader("c:\\windows\\system32\\eula.txt"))
{
    for (int i = 1; i < 100; i++) tr.ReadLine();
    s2 = tr.ReadLine();
}
Console.WriteLine(s2);

------解决方案--------------------

C# code

private void button1_Click(object sender, EventArgs e)
{
          string[] arr= File.ReadAllLines("d:\\a.txt",Encoding.Default);
          if(arr.Length>568)
          {
                MessageBox.Show(arr[568].ToString());
          }
}

------解决方案--------------------
下面是示例代码

using System;
using System.IO;

class Test  
{
   public static void Main()  
   {
       try  
       {
           // Create an instance of StreamReader to read from a file.
           // The using statement also closes the StreamReader.
           using (StreamReader sr = new StreamReader("TestFile.txt"))  
           {
               String line;
               // Read and display lines from the file until the end of  
               // the file is reached.
               while ((line = sr.ReadLine()) != null)  
               {
                   Console.WriteLine(line);
               }
           }
       }
       catch (Exception e)  
       {
           // Let the user know what went wrong.
           Console.WriteLine("The file could not be read:");
           Console.WriteLine(e.Message);
       }
   }
}