FailIf accumulates; Bind short-circuits

Now the idiom you'll write most. To validate, you chain guards with FailIf, then terminate with OrSuccess. The crucial, easy-to-miss detail: FailIf accumulates. It does not stop at the first failure. If three rules are broken, the Result comes back with all three errors, so the user fixes everything at once instead of playing whack-a-mole.

public static Result<FlightNoteVo> Create(string? ejdpUniqueFlightKey, string? body)
{
    var trimmedBody = (body ?? string.Empty).Trim();

    return Result<FlightNoteVo>
        .FailIf(string.IsNullOrWhiteSpace(ejdpUniqueFlightKey), MissingFlightKey)
        .FailIf(trimmedBody.Length > MaxBodyLength,
            new EjError(nameof(Body), $"A note body must be at most {MaxBodyLength} characters."))
        .OrSuccess(() => new FlightNoteVo(ejdpUniqueFlightKey!.Trim(), trimmedBody));
}

OrSuccess takes either a value or a factory Func<T>. Use the factory form (() => new ...) when building the value would be wrong or would throw if the guards had failed, since the factory only runs on the success path. Notice ejdpUniqueFlightKey! inside it: the null-forgiving ! is safe precisely because the FailIf above already proved it's non-null on this path.

Sometimes you need the opposite: step B depends on step A's value, so if A fails there's nothing to feed B. That's Bind. It short-circuits: on failure it returns the failure untouched; on success it runs the next step with the value. Use FailIf to gather independent rules, Bind to sequence dependent steps.

DisruptionDelayVo.Create(isCancelled, delayMinutes)
    .Bind(delay => Eu261Vo.Create(isExtraordinary, delay, pax, routeClaimRate, weighting));