C#

A
B
C
D
E
F

static void  Main(string[] args)
{
    string filePath = @"C:\File1.txt";

    StreamReader reader = new StreamReader(filePath);
    long position = 0;

    while (!reader.EndOfStream)
    {
        string line = reader.ReadLine();

        if (line == "B")
        {
            // Next line in file is C
            // Store location in file for C
            position = reader.BaseStream.Position;  // Store location. This will always store 16
        }

        Console.WriteLine(line);
    }

    reader.BaseStream.Seek(position, SeekOrigin.Begin);  // Seek to position of C
            

    Console.WriteLine(reader.ReadLine());   // Expect it to print C, it prints nothing

    reader.Close();
}