AuthRepository.cs

 public async Task Verify(string otp)
        {
            var account = _context.Users.SingleOrDefault(x => x.OTP == otp);

            if (account == null)
            {
                throw new BadHttpRequestException("Invalid verification OTP!");
            }

            if (account.VerifiedAt != DateTime.MinValue)
            {
                throw new BadHttpRequestException("Account already verified!");
            }

            account.VerifiedAt = DateTime.Now;
            account.IsVerified = true;

            account.OTP = null;
            account.OTPExpiresAt = DateTime.MinValue;

            await _context.SaveChangesAsync();
        }