AutoMapper 5 自定义值解析器“无法将表达式类型 X 转换为返回类型 Y”

Question

我从旧版本的 AutoMapper 升级并转换了我的自定义解析器，但我遇到了困难。

public class ProductMappingProfile : Profile
{
    public ProductMappingProfile()
    {
        CreateMap<Product, ProductViewModel>()
            .ForMember(
                dest => dest.Model,
                opt => opt.ResolveUsing<ModelNameResolver>(src => src.ModelId));
        // won't compile
    }
}

其中 Product 具有 int? ModelId 属性，而 ProductViewModel 具有 string Name 属性。

自定义解析器

public class ModelNameResolver : IValueResolver<short?, string, string>
{
    private readonly InventoryService _inventoryService;
    public ModelNameResolver(InventoryService inventoryService)
    {
        _inventoryService = inventoryService;
    }

    public string Resolve(short? source, string destination, string destMember, ResolutionContext context)
    {
        if (!source.HasValue)
            return "n/a";

        return _inventoryService.GetModel(source.Value)
            .Name;
    }
}

编译错误：

The type 'MyNamespace.Web.Resolvers.ModelCodeResolver' cannot be used as type parameter 'TValueResolver' in the generic type or method `'AutoMapper.IMemberConfigurationExpression<TSource,TDestination,TMember>.ResolveUsing<TValueResolver>()'`.

There is no implicit reference conversion from `'MyNamespace.Web.Resolvers.ModelCodeResolver'` to `'AutoMapper.IValueResolver<Data.Models.Product,Web.ViewModels.ProductViewModel,string>'`.

我做错了什么？我怀疑我误解了新的自定义解析器界面。

Answer 1

IValueResolver 接口应该用源对象类型、目标对象类型和目标成员类型参数化（应该是Resolve方法的类型结果）。在您的情况下，参数应该是

 IValueResolver<Product, ProductViewModel, string>

但是您正在创建参数化的解析器

 IValueResolver<short?, string, string>

short? 不是您的源对象类型

string 不是您的目标对象类型

你应该使用类似的东西：

public class ModelNameResolver : IValueResolver<Product, ProductViewModel, string>
{
    private readonly InventoryService _inventoryService;
    public ModelNameResolver(InventoryService inventoryService)
    {
        _inventoryService = inventoryService;
    }

    public string Resolve(Product source, ProductViewModel destination,
       string destMember, ResolutionContext context)
    {
        var modelId = source.ModelId;
        if (!modelId.HasValue)
            return "n/a";

        return _inventoryService.GetModel(modelId.Value).Name;
    }
}

你的映射应该是这样的

var inventoryService = new InventoryService();
var modelNameResolver = new ModelNameResolver(inventoryService);

Mapper.Initialize(c =>
{
    c.CreateMap<Product, ProductViewModel>()
        .ForMember(dest => dest.Model, opt => opt.ResolveUsing(modelNameResolver));
});

当然，您可以向您的 IoC 容器询问模型名称解析器的实例。

---

更新：如果您希望解析器可在不同的源数据类型和目标数据类型之间重用，那么您有两种选择：

如果你的解析器可以有无参数构造函数，那么你可以使用IMemberValueResolver:

public class ModelNameResolver : IMemberValueResolver<object, object, int?, string>
{
    // create or assign _inventoryService
    // also note objects as source and destination
    public string Resolve(object source, object destination,
       int? sourceMember, string destMember,
       ResolutionContext context)
    {
         if (!sourceMember.HasValue)
            return "n/a";

         return _inventoryService.GetModel(sourceMember.Value).Name;
    }
}

用法

.ForMember(dest => dest.Model, 
           opt => opt.ResolveUsing<ModelNameResolver, int?>(src => src.ModelId)

第二个选项 - 不要使用解析器。只需将 MapFrom 与您的自定义类一起使用即可：

.ForMember(dest => dest.Model, 
           opt => opt.MapFrom(src => someClass.GetModelName(src.ModelId)));     

并且 someSlass 应该包含通过 id 获取模型名称的方法

public string GetModelName(int? modelId)
{
    if (!modelId.HasValue)
        return "n/a";

    return _inventoryService.GetModel(modelId.Value).Name;
}