AutoMapper映射含列表与分页值的元组到ResponseModel报错如何解决

AutoMapper的ForMember方法仅支持指定目标类型的顶层单个成员，你之前的写法里直接在dest => dest.ExternalAPIResponse.Select(...)位置试图配置集合内部元素的映射规则，不符合AutoMapper的配置要求，所以触发了对应异常。

有两种常用的正确配置方式，可根据业务复用需求选择： #

方案1：拆分映射规则（推荐，可复用性更高） #

先单独配置Contact到ExternalAPIResponseModel的元素映射规则，再配置值元组到ResponseModel的顶层映射，AutoMapper会自动处理集合类型的转换。

// 第一步：配置单元素映射规则
CreateMap<Contact, ExternalAPIResponseModel>()
    .ForMember(dest => dest.Address, opt => opt.MapFrom(src => src.ContactAddress1))
    .ForMember(dest => dest.CompanyName, opt => opt.MapFrom(src => src.HqcompanyName))
    .ForMember(dest => dest.JobFunction, opt => opt.MapFrom(src => src.IdjobFunctionNavigation.JobFunction1))
    .ForMember(dest => dest.JobLevel, opt => opt.MapFrom(src => src.IdjobLevelNavigation.JobLevel1))
    .ForMember(dest => dest.Phone, opt => opt.MapFrom(src => src.ContactPhone))
    .ForMember(dest => dest.State, opt => opt.MapFrom(src => src.ContactState));
// FirstName、LastName、Email属性名两边一致，AutoMapper会自动映射，无需额外配置

// 第二步：配置值元组到ResponseModel的映射
CreateMap<(IEnumerable<Contact> Contacts, int pagesCount), ResponseModel>()
    .ForMember(dest => dest.ExternalAPIResponse, opt => opt.MapFrom(src => src.Contacts))
    .ForMember(dest => dest.TotalPageCount, opt => opt.MapFrom(src => src.pagesCount));

方案2：直接在集合映射时手动转换（适合一次性场景） #

如果不需要复用Contact到ExternalAPIResponseModel的映射规则，也可以直接在顶层属性的映射配置里完成集合转换：

CreateMap<(IEnumerable<Contact> Contacts, int pagesCount), ResponseModel>()
    .ForMember(dest => dest.ExternalAPIResponse, opt => opt.MapFrom(src => 
        src.Contacts.Select(c => new ExternalAPIResponseModel
        {
            Address = c.ContactAddress1,
            CompanyName = c.HqcompanyName,
            FirstName = c.FirstName,
            LastName = c.LastName,
            JobFunction = c.IdjobFunctionNavigation?.JobFunction1,
            JobLevel = c.IdjobLevelNavigation?.JobLevel1,
            Email = c.Email,
            Phone = c.ContactPhone,
            State = c.ContactState
        })))
    .ForMember(dest => dest.TotalPageCount, opt => opt.MapFrom(src => src.pagesCount));

控制器中的调用代码不需要修改，和原有写法保持一致即可：

var mappedResult = _mapper.Map<ResponseModel>(result);

内容的提问来源于stack exchange，提问作者Sargis