C# Secure String Helper Class ToByteArray

SecureStringHelper.cs

public class SecureStringHelper : IDisposable
    {
        private readonly Encoding _encoding;
        private readonly SecureString _secureString;
        private byte[] _bytes;
        private bool _disposed;
        public SecureStringHelper(SecureString secureString)
            : this(secureString, Encoding.Default)
        {
        }
        public SecureStringHelper(SecureString secureString, Encoding encoding)
        {
            if (secureString == null)
                throw new Exception(nameof(secureString));
            _encoding = encoding ?? Encoding.Default;
            _secureString = secureString;
        }
        public static SecureString ToSecureString(string str)
        {
            if (str == null)
                throw new Exception("Error: String cannot be null.");
            var securestr = new SecureString();
            foreach (var c in str)
                securestr.AppendChar(c);
            securestr.MakeReadOnly();
            return securestr;
        }
        public void Dispose()
        {
            if (!_disposed)
            {
                Destroy();
                _disposed = true;
            }

            GC.SuppressFinalize(this);
        }
        internal unsafe byte[] ToByteArray()
        {
            if (_bytes != null)
            {
                _bytes.Fill(0);
                _bytes = null;
            }

            var maxLength = _encoding.GetMaxByteCount(_secureString.Length);
            var bytes = IntPtr.Zero;
            var str = IntPtr.Zero;
            try
            {
                bytes = Marshal.AllocHGlobal(maxLength);
                str = Marshal.SecureStringToBSTR(_secureString);
                var chars = (char*) str.ToPointer();
                var bptr = (byte*) bytes.ToPointer();
                var len = _encoding.GetBytes(chars, _secureString.Length, bptr, maxLength);
                _bytes = new byte[len];
                for (var i = 0; i < len; ++i)
                {
                    _bytes[i] = *bptr;
                    bptr++;
                }

                return _bytes;
            }
            finally
            {
                if (bytes != IntPtr.Zero)
                    Marshal.FreeHGlobal(bytes);
                if (str != IntPtr.Zero)
                    Marshal.ZeroFreeBSTR(str);
            }
        }
        private void Destroy()
        {
            if (_bytes != null)
            {
                _bytes.Fill(0);
                _bytes = null;
            }
        }
        ~SecureStringHelper()
        {
            Dispose();
        }
    }