Read from a file starting at the end, similar to tail

The code below uses a random-access FileStream to seed a StreamReader at an offset near the end of the file, discarding the first read line since it is most likely only partial.

FileStream stream = new FileStream(@"c:\temp\build.txt", 
    FileMode.Open, FileAccess.Read);

stream.Seek(-1024, SeekOrigin.End);     // rewind enough for > 1 line

StreamReader reader = new StreamReader(stream);
reader.ReadLine();      // discard partial line

while (!reader.EndOfStream)
{
    string nextLine = reader.ReadLine();
    Console.WriteLine(nextLine);
}

Take a look at this related question's answer to read a text file in reverse. There is a lot of complexity to reading a file backward correctly because of stuff like encoding.

Maybe something like this will work for you:

using (var fs = File.OpenRead(filePath))
{
    fs.Seek(0, SeekOrigin.End);

    int newLines = 0;
    while (newLines < 3)
    {
        fs.Seek(-1, SeekOrigin.Current);
        newLines += fs.ReadByte() == 13 ? 1 : 0; // look for \r
        fs.Seek(-1, SeekOrigin.Current);
    }

    byte[] data = new byte[fs.Length - fs.Position];
    fs.Read(data, 0, data.Length);
}

Take note that this assumes \r\n.

To read the last 1024 bytes:

using (var reader = new StreamReader("foo.txt"))
{
    if (reader.BaseStream.Length > 1024)
    {
        reader.BaseStream.Seek(-1024, SeekOrigin.End);
    }
    string line;
    while ((line = reader.ReadLine()) != null)
    {
        Console.WriteLine(line);
    }
}