Refactor2.cs

public class SignUp
{
    private readonly DbContext _dbContext;

    public SignUp(DbContext dbContext)
    {
        _dbContext = dbContext;
    }
    
    public Task Handle(SignUpRequest request)
    {
        if (string.IsNullOrWhiteSpace(request.Username))
        {
            throw new InvalidOperationException("Username is required.");
        }
        
        var existingUser = _dbContext.Users.SingleOrDefault(x => x.Username == request.Username);
        if (IsExistingUserAlreadyActivated(request, existingUser))
        {
            throw new InvalidOperationException("You've already registered. Please active your account.");
        }
        
        if (existingUser != null)
        {
            throw new InvalidOperationException("Username is already registered.");
        }
        
        var user = new User(request.Username, request.Email)
        {
            Channels = FilterAgeAppropriateChannels(request)
        };

        _dbContext.Users.Add(user);
        _dbContext.Outbox.Add(new UserSignedUp(user.Username, user.Email));
        _dbContext.Save();

        return Task.CompletedTask;
    }
    
    private static bool IsExistingUserAlreadyActivated(SignUpRequest request, User? existingUser)
    {
        return existingUser is { ActivationDate: null } && existingUser.Email.Equals(request.Email, StringComparison.InvariantCultureIgnoreCase);
    }

    public List<int> FilterAgeAppropriateChannels(SignUpRequest request)
    {
        var channels = new List<int>();
        
        var now = DateTime.UtcNow;
        if (request.Birthday.AddYears(18) > now)
        {
            channels.AddRange(request.Channels);
            return channels;
        }
        
        var kidsAllowedChannels = _dbContext.Channels
            .Where(x => request.Channels.Contains(x.ChannelId) && x.Age18Required == false)
            .Select(x => x.ChannelId)
            .ToArray();
            
        channels.AddRange(kidsAllowedChannels);

        return channels;
    }

}