Validator access to database

Question

My question is how do I need to validate fields. I have this class (request dto):

public class CompleteGoogleRegistrationRequest
{
    public string IdToken { get; set; } = string.Empty;
    public string Username { get; set; } = string.Empty;
}

And I need to check Username existance. On which layer should this be checked? On this request validator`s layer or when I create UserEntity and apply entity`s validator to user instance. Maybe neither and check uniqueness only in business logic?

Validator logic:

.MustAsync(async (username, cancellation) =>
{
    return await userRepository.GetByUsernameReadonlyAsync(username, cancellation) == null;
})
.WithMessage("This username is already taken.");

Handler logic (business logic):

var existingUser = await userRepository.GetByUsernameReadonlyAsync(userRequest.Username, cancellationToken);
if (existingUser != null)
{
    var errorMessage = localization.GetString("Auth.Register.UsernameAlreadyExists");

    var errors = new ValidationResult(new[]
    {
        new ValidationFailure(nameof(userRequest.Username), errorMessage)
    });
    return Result.Failure<RegisterResponse, ValidationResult>(errors);
}

chaghy · Accepted Answer · 2025-11-15 17:58:39Z

public class CompleteGoogleRegistrationRequestValidator 
    : AbstractValidator<CompleteGoogleRegistrationRequest>
{
    public CompleteGoogleRegistrationRequestValidator()
    {
        RuleFor(x => x.IdToken).NotEmpty();
        RuleFor(x => x.Username)
            .NotEmpty()
            .MinimumLength(3)
            .MaximumLength(30);
    }
}   
``

public async Task<Result<RegisterResponse>> Handle(
    CompleteGoogleRegistrationRequest request,
    CancellationToken cancellationToken)
{
    var existingUser = await userRepository
        .GetByUsernameReadonlyAsync(request.Username, cancellationToken);

    if (existingUser != null)
    {
        var errorMessage = localization.GetString("Auth.Register.UsernameAlreadyExists");

        var errors = new ValidationResult(new[]
        {
            new ValidationFailure(nameof(request.Username), errorMessage)
        });

        return Result.Failure<RegisterResponse>(errors);
    }

    // Continue registration...









try this
}

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