我觉得 NullSubstitute 选项就可以了。

I think NullSubstitute option will do the trick

.ForMember(d => d.BusinessGroup_Id, o => o.MapFrom(s => (int?)s.BusinessGroup));
.ForMember(d => d.BusinessGroup_Id, o => o.NullSubstitute(0));

BTW，你可以写你的条件映射动作：

BTW you can write your conditions in mapping action:

.ForMember(d => d.BusinessGroup_Id,
           o => o.MapFrom(s => s.BusinessGroup == null ? 0 : (int)s.BusinessGroup));   

的更新的，如果你不能一些默认值分配给你的财产，你可以忽视它，只映射不是空值：

UPDATE if you cannot assign some default value to your property, you can just ignore it and map only not nulls:

.ForMember(d => d.BusinessGroup_Id, o => o.Ignore())
.AfterMap((s, d) =>
    {
        if (s.BusinessGroup != null)
            d.BusinessGroup_Id = (int)s.BusinessGroup;
    });