string filename = @"c:\test.txt";
int numberoflines = 10;
StreamReader reader = new StreamReader(); //pick appropriate Encoding
reader.BaseStream.Seek(0, SeekOrigin.End);
int count = 0;
while ((count < numberoflines) && (reader.BaseStream.Position > 0))
{
    reader.BaseStream.Position--;
    int c = reader.BaseStream.ReadByte();
    if (reader.BaseStream.Position > 0)
        reader.BaseStream.Position--;
    if (c == Convert.ToInt32('\n'))
    {
        ++count;
    }
}
string str = reader.ReadToEnd();
string[] arr = str.Replace("\r", "").Split('\n');
reader.Close();