Gits for Medium Article - RegisterNewUserValidator

RegisterNewUserValidator.cs

    public class RegisterNewUserValidator : AbstractValidator<RegisterNewUserCommand>
    {
        public RegisterNewUserValidator()
        {
            RuleFor(u => u.Dto.UserName)
                .MustAsync(NotContainUserAlready)
                .WithMessage((u,userName) => $"{userName} {Constants.Validation.PartMessages.UserAlreadyExists}");
        }

        async Task<bool> NotContainUserAlready(RegisterNewUserCommand dto, string userName, CancellationToken cancellation = new CancellationToken())
        {
            // Normally we would hit a database and retrieve the User here.
            // For demo purposes, I will enforce an assumption that a user with UserName "DaveRogers" already exists.

            return await Task.FromResult(!userName.Equals("DaveRogers"));
        }
    }