Just check for null
before running the rules that depend on them, using a When
condition.
this.CascadeMode = CascadeMode.StopOnFirstFailure;
RuleFor(x => x.TechnicalHeader).NotNull().WithMessage("Header cannot be null");
// Ensure TechnicalHeader is provided
When(x => x.TechnicalHeader != null, () => {
RuleFor(x => x.TechnicalHeader.Userid).NotEmpty().WithMessage("Userid cannot be null or an empty string");
RuleFor(x => x.TechnicalHeader.CabCode).GreaterThan(0).WithMessage("CabCode cannot be or less than 0");
RuleFor(x => x.TechnicalHeader.Ndg).NotEmpty().WithMessage("Ndg cannot be null or an empty string");
});