如何在AutoMapper中为数组对象映射值：父属性赋值给子数组元素

需要实现从ParentDto到Parent的映射，核心需求是把ParentDto的StockNumber属性值，赋值给Parent对象中Suppliers列表内每一个Supplier的StockNumber属性。

实体代码 #

public class Parent
{
    public int ParentId { get; set; }
    public long StockNumber { get; set; }
    public DateTimeOffset? EarliestPaymentDate { get; set; }
    public List<Supplier> Suppliers { get; set; }
}

public class Supplier
{
    public int SupplierId { get; set; }
    public long StockNumber { get; set; }
    public int ParentId { get; set; }
}

public class ParentDto
{
    public int ParentId { get; set; }
    public long StockNumber { get; set; }
    public DateTimeOffset? EarliestPaymentDate { get; set; }
    public List<Supplier> Suppliers { get; set; }
}

public class SupplierDto
{
    public int SupplierId { get; set; }
    public long StockNumber { get; set; }
    public int ParentId { get; set; }
}

错误尝试说明 #

你之前的写法无法实现需求，因为AutoMapper的ForMember无法直接绑定到集合元素的属性操作：

CreateMap<ParentDto, Parent>()
    .ForMember(dest => dest.EarliestPaymentDate, opt => opt.MapFrom(src => src.EarliestPaymentDateTime))
    // 错误写法：无法直接对数组元素的属性进行映射
    .ForMember(dest => dest.Suppliers.ForEach(x => x.StockNumber), opt => opt.MapFrom(src => src.StockNumber))
;

正确解决方案 #

方法1：使用AfterMap完成后续赋值

先完成Parent和Suppliers的基础映射，再在映射结束后遍历子集合统一设置StockNumber：

CreateMap<ParentDto, Parent>()
    // 基础属性映射
    .ForMember(dest => dest.EarliestPaymentDate, opt => opt.MapFrom(src => src.EarliestPaymentDateTime))
    // 自动映射Suppliers的同名属性
    .ForMember(dest => dest.Suppliers, opt => opt.MapFrom(src => src.Suppliers))
    // 映射完成后批量设置子元素的StockNumber
    .AfterMap((src, dest) => 
    {
        dest.Suppliers?.ForEach(supplier => supplier.StockNumber = src.StockNumber);
    });

方法2：结合子级映射与上下文传递（适用于ParentDto.Suppliers为SupplierDto的场景）

如果ParentDto中的Suppliers实际是List<SupplierDto>，可以通过自定义解析逻辑传递父级值：

1. 先定义子级映射，忽略原StockNumber：

CreateMap<SupplierDto, Supplier>()
    .ForMember(dest => dest.StockNumber, opt => opt.Ignore());

2. 在父级映射中解析生成带父级StockNumber的子集合：

CreateMap<ParentDto, Parent>()
    .ForMember(dest => dest.EarliestPaymentDate, opt => opt.MapFrom(src => src.EarliestPaymentDateTime))
    .ForMember(dest => dest.Suppliers, opt => opt.ResolveUsing((src, dest, member, context) =>
    {
        return src.Suppliers?.Select(dto => 
        {
            var supplier = context.Mapper.Map<Supplier>(dto);
            supplier.StockNumber = src.StockNumber;
            return supplier;
        }).ToList();
    }));

内容的提问来源于stack exchange，提问作者GThree