Secure String Helper Class

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 void Dispose()
        {
            if(!_disposed)
            {
                Destroy();
                _disposed = true;
            }
            GC.SuppressFinalize(this);
        }
        public static string ToString(SecureString secureStr)
        {
            var Str = new NetworkCredential(string.Empty, secureStr).Password;
            return Str;
        }
        public static SecureString ToSecureString(string password)
        {
            if(password == null)
                throw new Exception("Error: Password cannot be null.");
            var securePassword = new SecureString();
            foreach(var c in password)
                securePassword.AppendChar(c);
            securePassword.MakeReadOnly();
            return securePassword;
        }
        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;
            }
        }
    }

public static class SecureStringEx
    {
        /// <summary>
        ///     Converts a standard string to a secure string
        /// </summary>
        public static SecureString ToSecureString(this string password)
        {
            if(password == null)
                throw new Exception("Error: Password cannot be null.");
            var securePassword = new SecureString();
            foreach(var c in password)
                securePassword.AppendChar(c);
            securePassword.MakeReadOnly();
            return securePassword;
        }
    }