如何使用AutoMapper按父对象条件映射子DTO的RunEt属性值

问题根因 #

1. 你现有配置的核心问题是CreateMap<Pair, QualifyingPairDto>的链式调用语法不完整，两个ForMember配置结束后没有续接后续的ForPath/AfterMap配置，且末尾缺少分号，导致附加配置未生效。
2. 子对象QualifyingRunDto的RunEt字段在Run类中没有同名对应属性，默认映射规则下自然赋值为null，需要显式配置赋值逻辑。
3. 你现有配置仅处理了左车道的RunEt，右车道没有对应配置。

解决方案 #

不需要自定义值解析器，以下两种常用实现方式均可满足需求：

方案1：使用AfterMap（最直观，适合复杂分支逻辑）

直接在Pair到QualifyingPairDto的映射中补全AfterMap配置，同时处理左右车道，增加空校验避免异常：

public MappingProfiles()
{
    CreateMap<Event, Event>();
    CreateMap<EventMaster, EventMaster>();
    CreateMap<Track, Track>();

    CreateMap<Event, EventDto>()
        .ForMember(d => d.Name, o => o.MapFrom(s => s.EventEm.Name))
        .ForMember(d => d.Track, o => o.MapFrom(s => s.EventEm.Track));

    CreateMap<Run, QualifyingRunDto>();

    CreateMap<Pair, QualifyingPairDto>()
        .ForMember(d => d.LeftLane, o => o.MapFrom(s => s.Runs.FirstOrDefault(r => r.RunLane == 1)))
        .ForMember(d => d.RightLane, o => o.MapFrom(s => s.Runs.FirstOrDefault(r => r.RunLane == 2)))
        .AfterMap((src, dest) =>
        {
            // 处理左车道属性
            if (dest.LeftLane != null)
            {
                var leftRun = src.Runs.FirstOrDefault(r => r.RunLane == 1);
                if (leftRun != null)
                {
                    dest.LeftLane.RunEt = src.PairFinish switch
                    {
                        1 => leftRun.RunEt1320,
                        2 => leftRun.RunEt1000,
                        3 => leftRun.RunEt660,
                        _ => null
                    };
                    // 可同步补充RunSp的赋值逻辑
                    dest.LeftLane.RunSp = src.PairFinish switch
                    {
                        1 => leftRun.RunSp1320,
                        2 => leftRun.RunSp1000,
                        3 => leftRun.RunSp660,
                        _ => null
                    };
                }
            }
            // 处理右车道属性
            if (dest.RightLane != null)
            {
                var rightRun = src.Runs.FirstOrDefault(r => r.RunLane == 2);
                if (rightRun != null)
                {
                    dest.RightLane.RunEt = src.PairFinish switch
                    {
                        1 => rightRun.RunEt1320,
                        2 => rightRun.RunEt1000,
                        3 => rightRun.RunEt660,
                        _ => null
                    };
                    dest.RightLane.RunSp = src.PairFinish switch
                    {
                        1 => rightRun.RunSp1320,
                        2 => rightRun.RunSp1000,
                        3 => rightRun.RunSp660,
                        _ => null
                    };
                }
            }
        });
}

方案2：使用ForPath（适合单字段简单赋值）

如果仅需要赋值RunEt字段，也可以用ForPath实现：

CreateMap<Pair, QualifyingPairDto>()
    .ForMember(d => d.LeftLane, o => o.MapFrom(s => s.Runs.FirstOrDefault(r => r.RunLane == 1)))
    .ForMember(d => d.RightLane, o => o.MapFrom(s => s.Runs.FirstOrDefault(r => r.RunLane == 2)))
    // 左车道RunEt
    .ForPath(d => d.LeftLane.RunEt, o => o.MapFrom((src, _) => 
    {
        var leftRun = src.Runs.FirstOrDefault(r => r.RunLane == 1);
        if(leftRun == null) return null;
        return src.PairFinish switch
        {
            1 => leftRun.RunEt1320,
            2 => leftRun.RunEt1000,
            3 => leftRun.RunEt660,
            _ => null
        };
    }))
    // 右车道RunEt
    .ForPath(d => d.RightLane.RunEt, o => o.MapFrom((src, _) => 
    {
        var rightRun = src.Runs.FirstOrDefault(r => r.RunLane == 2);
        if(rightRun == null) return null;
        return src.PairFinish switch
        {
            1 => rightRun.RunEt1320,
            2 => rightRun.RunEt1000,
            3 => rightRun.RunEt660,
            _ => null
        };
    }));

注意事项 #

不要直接使用First()方法，建议用FirstOrDefault()配合空校验，避免Runs集合中没有对应车道数据时抛出序列空异常。如果相同的RunEt赋值逻辑需要在多个映射场景复用，可以再考虑自定义值解析器，否则上述两种方案已经足够满足需求。

内容的提问来源于stack exchange，提问作者Adam