lang__problem_details

public static class ErrorCodes
{
    public static class Books
    {
        public const string TitleRequired = "ERR_BOOK_TITLE_REQUIRED";
        public const string NotFound = "ERR_BOOK_NOT_FOUND";
        // ... more codes
    }
}

if (string.IsNullOrWhiteSpace(command.Title))
{
    Log.Books.TitleRequired(logger, command.Id);
    return Result.Failure(
        Error.Validation(
            ErrorCodes.Books.TitleRequired,
            "Book title is required"
        )).ToProblemDetails();
}

var book = await session.LoadAsync<BookDetails>(id);
if (book is null)
{
    Log.Books.BookNotFound(logger, id);
    return Result.Failure(
        Error.NotFound(
            ErrorCodes.Books.BookNotFound,
            "Book not found"
        )).ToProblemDetails();
}

if (aggregate.IsDeleted)
{
    return Result.Failure(
        Error.Conflict(
            ErrorCodes.Books.AlreadyDeleted,
            "Cannot update a deleted book"
        )).ToProblemDetails();
}

if (userId is null)
{
    return Result.Failure(
        Error.Unauthorized(
            ErrorCodes.Auth.InvalidToken,
            "User not authenticated"
        )).ToProblemDetails();
}

var eventResult = BookAggregate.CreateEvent(/* ... */);
if (eventResult.IsFailure)
{
    return eventResult.ToProblemDetails();
}

// Use the event
session.Events.StartStream<BookAggregate>(id, eventResult.Value);

catch (Exception ex)
{
    Log.Books.UnexpectedError(logger, ex);
    return Result.Failure(
        Error.InternalServerError(
            ErrorCodes.Books.InternalError,
            $"An unexpected error occurred: {ex.Message}"
        )).ToProblemDetails();
}

{
  "type": "https://tools.ietf.org/html/rfc7231#section-6.5.1",
  "title": "Bad Request",
  "status": 400,
  "detail": "Book title is required",
  "error": "ERR_BOOK_TITLE_REQUIRED"
}

// Check culture codes
if (!CultureValidator.IsValidCultureCode(command.Language))
{
    return Result.Failure(
        Error.Validation(
            ErrorCodes.Books.LanguageInvalid,
            "Invalid language code"
        )).ToProblemDetails();
}

// Check translations
if (!CultureValidator.ValidateTranslations(command.Translations, out var invalidCodes))
{
    return Result.Failure(
        Error.Validation(
            ErrorCodes.Books.TranslationLanguageInvalid,
            $"Invalid language codes: {string.Join(", ", invalidCodes)}"
        )).ToProblemDetails();
}

// Let aggregate handle business rules
var eventResult = aggregate.UpdateEvent(command.Name);
if (eventResult.IsFailure)
{
    // Aggregate already created appropriate Error type
    return eventResult.ToProblemDetails();
}

session.Events.Append(id, eventResult.Value);

var expectedVersion = ETagHelper.ParseETag(command.ETag);
if (expectedVersion.HasValue && aggregate.Version != expectedVersion.Value)
{
    return ETagHelper.PreconditionFailed(); // Returns 412 Precondition Failed
}