customfluentvalidator.cs

public class PurchasePlaystationGameDto
{
    public int ID { get; set; }
    public int UserID { get; set; }
    public decimal Price { get; set; }
    public bool PlaystationPlusMember { get; set; }
}

public class PurchasePlaystationGameDtoValidator : AbstractValidator
{
    public PurchasePlaystationGameDtoValidator()
    {
        RuleFor(x => x.ID).NotNull().GreaterThan(0).WithMessage("ID must be greater than 0.");
        RuleFor(x => x.UserID).NotNull().GreaterThan(0).WithMessage("UserID must be greater than 0.");
        RuleFor(x => x.Price).Must(BeGreaterThanZeroForNonMembers).WithMessage("Price must be greater than 0");
    }

    private bool BeGreaterThanZeroForNonMembers(PurchasePlaystationGameDto dtoInstance, decimal price)
    {
        // game is free for members :) 
        if (!dtoInstance.PlaystationPlusMember & price <= 0)
        {
            return false;
        }
        return true;
    }
}