Csharp AutoMapper 如何为两个不同的用例设置单个映射

我遇到了一种情况，我从API获取了一些有关旅行的数据.我正在使用AutoMapper将此"API模型"映射到EF Core实体，以存储在我的SQL服务器数据库中.

大多数映射都很简单，但我遇到了一种情况:TripModel有一个名为ReasonForTravel属性，它是一个int?值-如果它有一个值，且该值是> 0，那么我想使用自定义ValueConverter将int转换为我需要的字符串-但如果ReasonForTravel根本没有值，或者它是<= 0，那么我想从CustomTravelReason到TripModel上取值.

如何在AutoMapper CreateMap中表达这一点，以便我可以一步映射我的TripModel到Trip实体？

这是我的代码-模型、实体和AutoMapper Profile:

public class TripModel
{
    public string Firstname { get; set; }
    public string Lastname { get; set; }
    public int? ReasonForTravel { get; set; }
    public string CustomTravelReason { get; set; }
    // more stuff, but irrelevant here
}

public class Trip
{
    public string Firstname { get; set; }
    public string Lastname { get; set; }
    public string ReasonForTrip { get; set; }
}

public class TripProfile : Profile
{
    public TripProfile()
    {
        CreateMap<TripModel, Trip>
            // I know these two aren't strictly necessary - just for illustration of what I'm doing
            .ForMember(dest => dest.Firstname, opt => opt.MapFrom(src => src.Firstname))
            .ForMember(dest => dest.Lastname, opt => opt.MapFrom(src => src.Lastname))
            
            // but how do I specify this now?? This is for the case of ReasonForTravel > 0
            .ForMember(dest => dest.ReasonForTrip, opt => opt.ConvertUsing(new TravelReasonConverter(), src => src.Lastname))

            // and this is for the case of ReasonForTravel <= 0 or ReasonForTravel = null
            .ForMember(dest => dest.ReasonForTrip, opt => opt.MapFrom(src => src.CustomTravelReason));
    }
}