Handling reading large files in C#

                FileStream fs = new FileStream(filepath, FileMode.Open, FileAccess.Read);
                using (BinaryReader br = new BinaryReader(fs))
                {
                    byte[] bytes = new byte[0];
                    using (MemoryStream test = new MemoryStream())
                    {
                        fs.CopyTo(test);
                        bytes = test.ToArray();
                    }

                    byte[] searchBytes = Encoding.UTF8.GetBytes("test");
                    List<long> positions = new List<long>();

                    foreach(long pos in Extensions.SearchStringInBytes(bytes, searchBytes))
                    {
                        positions.Add(pos - 4);
                    }
                }