Writing text to the middle of a file

open master file for reading.
count := 0
while not EOF do
    read line from master file into buffer
    write line to output file
    count := count + 1
    if count = 5 then
       write added line to output file
    fi
od
rename output file to replace input file

public class Class1
{
    static void Main()
    {
        var beatles = new LinkedList<string>();

        beatles.AddFirst("John");
        LinkedListNode<string> nextBeatles = beatles.AddAfter(beatles.First, "Paul");
        nextBeatles = beatles.AddAfter(nextBeatles, "George");
        nextBeatles = beatles.AddAfter(nextBeatles, "Ringo");

        // change the 1 to your 5th line
        LinkedListNode<string> paulsNode = beatles.NodeAt(1);
        LinkedListNode<string> recentHindrance = beatles.AddBefore(paulsNode, "Yoko");
        recentHindrance = beatles.AddBefore(recentHindrance, "Aunt Mimi");
        beatles.AddBefore(recentHindrance, "Father Jim");


        Console.WriteLine("{0}", string.Join("n", beatles.ToArray()));

        Console.ReadLine();
    }
}

public static class Helper
{
    public static LinkedListNode<T> NodeAt<T>(this LinkedList<T> l, int index)
    {
        LinkedListNode<T> x = l.First;

        while ((index--) > 0) x = x.Next;

        return x;
    }
}