AutoMapper自定义类型转换器无法编译及使用问题

错误原因分析 #

1. 泛型类型不匹配：你的转换器实现的是ITypeConverter<TPWorkPlanDTO, IEnumerable<WorkingTime>>，但映射配置的目标类型是List<WorkingTime>。AutoMapper要求自定义转换器的目标类型必须与映射配置的目标类型严格匹配，二者不存在隐式转换关系，因此触发编译报错。
2. 类型转换逻辑错误：原转换器中直接将TPWorkingTimeDTO集合强制转换为WorkingTime集合，这两个类型无继承关系，强制转换会导致运行时异常，必须通过AutoMapper完成单个DTO到模型的映射。

解决方案 #

步骤1：配置单个DTO到模型的映射 #

首先定义TPWorkingTimeDTO到WorkingTime的属性映射，处理命名差异：

CreateMap<TPWorkPlanDTO.TPWorkingTimeDTO, WorkingTime>()
    .ForMember(dest => dest.Id, opt => opt.MapFrom(src => src.worktime_id))
    .ForMember(dest => dest.StartTime, opt => opt.MapFrom(src => src.start_time))
    .ForMember(dest => dest.EndTime, opt => opt.MapFrom(src => src.end_time));

方案A：调整转换器适配List目标类型 #

修改转换器，使其实现ITypeConverter<TPWorkPlanDTO, List<WorkingTime>>，并注入IMapper完成单个对象映射：

public class WorkPlanToWorkingTimeListConverter : ITypeConverter<TPWorkPlanDTO, List<WorkingTime>>
{
    private readonly IMapper _mapper;

    public WorkPlanToWorkingTimeListConverter(IMapper mapper)
    {
        _mapper = mapper;
    }

    public List<WorkingTime> Convert(TPWorkPlanDTO source, List<WorkingTime> destination, ResolutionContext context)
    {
        if (source == null)
        {
            return new List<WorkingTime>();
        }

        // 空值安全遍历并映射每个TPWorkingTimeDTO到WorkingTime
        return source.Weeks?
            .SelectMany(week => week.Days?
                .SelectMany(day => day.Shifts?
                    .Select(shift => _mapper.Map<WorkingTime>(shift))))
            .ToList() ?? new List<WorkingTime>();
    }
}

对应的映射配置：

CreateMap<TPWorkPlanDTO, List<WorkingTime>>()
    .ConvertUsing<WorkPlanToWorkingTimeListConverter>();

方案B：保持转换器返回IEnumerable #

如果希望转换器更通用，可保持其返回IEnumerable<WorkingTime>，映射配置目标类型同步调整：

public class WorkPlanToWorkingTimeListConverter : ITypeConverter<TPWorkPlanDTO, IEnumerable<WorkingTime>>
{
    private readonly IMapper _mapper;

    public WorkPlanToWorkingTimeListConverter(IMapper mapper)
    {
        _mapper = mapper;
    }

    public IEnumerable<WorkingTime> Convert(TPWorkPlanDTO source, IEnumerable<WorkingTime> destination, ResolutionContext context)
    {
        if (source == null)
        {
            return Enumerable.Empty<WorkingTime>();
        }

        return source.Weeks?
            .SelectMany(week => week.Days?
                .SelectMany(day => day.Shifts?
                    .Select(shift => _mapper.Map<WorkingTime>(shift)))) ?? Enumerable.Empty<WorkingTime>();
    }
}

映射配置：

CreateMap<TPWorkPlanDTO, IEnumerable<WorkingTime>>()
    .ConvertUsing<WorkPlanToWorkingTimeListConverter>();

使用时若需要List，直接调用ToList()：

var workingTimeList = _mapper.Map<IEnumerable<WorkingTime>>(workPlanDto).ToList();

内容的提问来源于stack exchange，提问作者Matt Baech