Refactor1.cs

public class SignUp
{
    private readonly DbContext _dbContext;
    private readonly Email _email;

    public SignUp(DbContext dbContext, Email email)
    {
        _dbContext = dbContext;
        _email = email;
    }
    
    public async Task Handle(SignUpRequest request)
    {
        ValidateUsername(request);
        ValidateExistingUser(request);

        var user = new User(request.Username, request.Email);
        var channels = request.Channels;
        
        FilterAgeRestrictedChannels(request, channels);

        user.Channels.AddRange(channels);
        
        _dbContext.Users.Add(user);
        _dbContext.Save();
        
        await SendEmail(user);
    }

    private async Task SendEmail(User user)
    {
        var email = new MailMessage("derek@codeopinion.com", user.Email);
        email.Subject = "Welcome!";
        email.Body = "Activate your account [link goes here]";
        await _email.Send(email);
    }
    
    private void ValidateExistingUser(SignUpRequest request)
    {
        var existingUser = _dbContext.Users.SingleOrDefault(x => x.Username == request.Username);

        if (ExistingSignUpNotYetActivated(request, existingUser))
        {
            throw new InvalidOperationException("You've already registered. Please active your account.");
        }
        
        if (existingUser != null)
        {
            throw new InvalidOperationException("Username is already registered.");
        }
    }
    
    private void FilterAgeRestrictedChannels(SignUpRequest request, List<int> channels)
    {
        var now = DateTime.UtcNow;
        if (request.Birthday.AddYears(18) <= now)
        {
            var kidsAllowedChannels = _dbContext.Channels
                .Where(x => request.Channels.Contains(x.ChannelId) && x.Age18Required == false)
                .Select(x => x.ChannelId)
                .ToArray();
            
            channels.Clear();
            channels.AddRange(kidsAllowedChannels);
        }
    }
    
    private static void ValidateUsername(SignUpRequest request)
    {
        if (string.IsNullOrWhiteSpace(request.Username))
        {
            throw new InvalidOperationException("Username is required.");
        }
    }
    
    private static bool ExistingSignUpNotYetActivated(SignUpRequest request, User? existingUser)
    {
        return existingUser is { ActivationDate: null } && existingUser.Email.Equals(request.Email, StringComparison.InvariantCultureIgnoreCase);
    }
}