AutoMapper技术问题：如何通过构造函数映射Class<T>到Class<T>

问题核心 #

当前代码报错的根源是dest参数始终为null——在AutoMapper的ConvertUsing自定义转换逻辑中，框架不会预先实例化目标对象，因此无法通过dest.GetType()获取泛型参数。

修复方案 #

直接从源对象src的类型中提取泛型参数，而非依赖null的dest对象。同时优化反射操作的稳定性：

CreateMap(typeof(Request<>), typeof(Request<>))
    .ConvertUsing((src, dest, context) =>
    {
        // 从源对象类型提取泛型参数
        var srcType = src.GetType();
        var genericTypes = srcType.GetGenericArguments();
        
        // 提取UserId和RequestId，用nameof避免硬编码
        var userId = srcType.GetProperty(nameof(Request<object>.UserId))?.GetValue(src) as string 
            ?? throw new ArgumentException("UserId has no value");
        var requestId = srcType.GetProperty(nameof(Request<object>.RequestId))?.GetValue(src) as string 
            ?? throw new ArgumentException("RequestId has no value");

        // 创建带参数的目标实例
        var destinationType = typeof(Request<>).MakeGenericType(genericTypes);
        return Activator.CreateInstance(destinationType, new RequestContext(userId, requestId)) as object;
    });

更优替代方案 #

如果仅需控制目标对象的构造逻辑（无复杂转换逻辑），建议使用AutoMapper专门的ConstructUsing方法，更贴合场景：

CreateMap(typeof(Request<>), typeof(Request<>))
    .ConstructUsing((src, context) =>
    {
        var srcType = src.GetType();
        var genericTypes = srcType.GetGenericArguments();
        var userId = srcType.GetProperty(nameof(Request<object>.UserId))?.GetValue(src) as string 
            ?? throw new ArgumentException("UserId has no value");
        var requestId = srcType.GetProperty(nameof(Request<object>.RequestId))?.GetValue(src) as string 
            ?? throw new ArgumentException("RequestId has no value");
        
        var destinationType = typeof(Request<>).MakeGenericType(genericTypes);
        return Activator.CreateInstance(destinationType, new RequestContext(userId, requestId));
    });

额外优化建议 #

• 提前缓存Request<>的UserId和RequestId属性信息，避免每次转换重复反射，提升性能。
• 若Request<T>实现了通用接口（如IRequest），可直接通过接口访问属性，彻底消除反射：

  // 定义通用接口
  public interface IRequest
  {
      string UserId { get; }
      string RequestId { get; }
  }
  
  public class Request<T> : IRequest { ... }
  
  // 转换逻辑简化
  var request = src as IRequest;
  var userId = request?.UserId ?? throw new ArgumentException("UserId has no value");
  

内容的提问来源于stack exchange，提问作者Jacob Roberts