C# 自定义 DynamicObject 强制转换为派生对象

Posted 2023-03-31

【中文标题】C# 自定义 DynamicObject 强制转换为派生对象

【英文标题】：C# Custom DynamicObject cast to derived object

【发布时间】：2021-11-01 11:55:40

【问题描述】：

我有一个类 DocumentObject，它扩展 DynamicObject 以允许动态成员属性。

public class DocumentObject : DynamicObject
    
        /// <summary>
        /// Inner dictionary that holds the dynamic members of the object
        /// </summary>
        Dictionary<string, object> dictionary = new Dictionary<string, object>();

        /// <summary>
        /// Try to get the member that is not defined in the class (additional dynamic members) from inner dictionary
        /// </summary>
        /// <param name="binder"></param>
        /// <param name="result"></param>
        /// <returns></returns>
        public override bool TryGetMember(GetMemberBinder binder, out object result)
        
            // Converting the property name to lowercase
            // so that property names become case-insensitive.
            string name = binder.Name.ToLower();

            // If the property name is found in a dictionary,
            // set the result parameter to the property value and return true.
            // Otherwise, return false.
            return dictionary.TryGetValue(name, out result);
        

        /// <summary>
        /// Try to set the member that is not defined in the class (additional dynamic members) to inner dictionary
        /// </summary>
        /// <param name="binder"></param>
        /// <param name="value"></param>
        /// <returns></returns>
        public override bool TrySetMember(SetMemberBinder binder, object value)
        
            // Converting the property name to lowercase
            // so that property names become case-insensitive.
            dictionary[binder.Name.ToLower()] = value;

            // You can always add a value to a dictionary,
            // so this method always returns true.
            return true;
        

        /// <summary>
        /// Get the names of all the dynamic members
        /// </summary>
        /// <returns></returns>
        public override IEnumerable<string> GetDynamicMemberNames()
        
            return dictionary.Keys;
        

    

我有一个继承 DocumentObject 的基本 Person 类

public class PersonDto : DocumentObject
    
        [JsonProperty("id")]
        public string Id  get; set; 
    

另一个继承 PersonDto 的子 OfficePersonDto 类

public class OfficePersonDto : PersonDto 
    
        [JsonProperty("name")]
        public string Name  get; set; 
    

在我的函数中，我收到的 JSON 对象必须至少是 PersonDto 对象，但如果它是 OfficePersonDto 类型，我希望能够将 PersonDto 转换为 OfficePersonDto。 IE。 JSON = "Id":1, "Name": "Orchard"，在 PersonDto 中，name 属性将使用 DocumentObject 的字典保存，而在转换为 OfficePersonDto 时，Id 和 name 都是该类的属性。

如何从 PersonDto 转换为子类，例如OfficePersonDto?

PersonDto personDto = ...
OfficePersonDto off = personDto as OfficePersonDto  // results in null or Name is null

【参考方案1】：

Automapper 在此类转换问题上非常有用。 我将向您展示一个快速工作的示例。但它仍然会开放，当然可以进行更多重构。

PersonDto personDto = ...
// Do not use the following conversion, instead get help from automapper
// OfficePersonDto off = personDto as OfficePersonDto
    
var config = new MapperConfiguration(cfg =>

    cfg.CreateMap<PersonDto, OfficePersonDto>()
        .ForMember(d => d.Name, opt => opt.MapFrom(new OfficePersonNameResolver()));
);
    
var mapper = config.CreateMapper();
var off = mapper.Map<OfficePersonDto>(personDto); // off is created with correct values

OfficePersonNameResolver 是这样的：

public class OfficePersonNameResolver : IValueResolver<PersonDto, OfficePersonDto, string>

    public string Resolve(PersonDto source, OfficePersonDto destination, string destMember, ResolutionContext context)
    
        return (source as dynamic).Name;
    

如果您对此有任何疑问，欢迎提问。

编辑：将解析器泛化为IValueResolver

像这样更改配置的创建：

var config = new MapperConfiguration(cfg =>

      cfg.CreateMap<PersonDto, OfficePersonDto>()
          .ForMember(d => d.Name, opt => opt.MapFrom(new DynamicObjectValueResolver<PersonDto, OfficePersonDto, string>("name")));
);

而值解析器现在应该是这样的：

public class DynamicObjectValueResolver<TSource, TDestination, TDestinationMember> : IValueResolver<TSource, TDestination, TDestinationMember>
   where TDestinationMember : class

    private readonly string _propertyName;

    public DynamicObjectValueResolver(string propertyName)
    
       _propertyName = propertyName;
    

    public TDestinationMember Resolve(TSource source, TDestination destination, TDestinationMember destMember, ResolutionContext context)
    
        dynamic eo = JsonConvert.DeserializeObject<ExpandoObject>(JsonConvert.SerializeObject(source));
        IDictionary<string, object> dictionary = eo;
        return dictionary[_propertyName] as TDestinationMember;
     

工作示例：https://dotnetfiddle.net/KS91To

【讨论】：

谢谢@Koray Elbek，请问是否可以将解析器泛化为 IValueResolver，其中 S 和 D 属于 DocumentObject 类型？这样我就不必为 DocumentObject 的每个基类创建解析器。

看到您的更新，如果我也可以概括映射器配置，是否有可能？ cfg.CreateMap()?我试过了。但是从 DocumentObject 到 OfficePersonDto 获取 invalidCastException。

老实说，我认为配置的泛化也是不可能的。即使有可能，也可能需要大量的脚手架。这将导致严重的过度设计。