模拟多态/通用枚举 - C# [英] Simulating Polymorphic/Generic Enumerations - C#

问题描述

我知道这是一个奇怪的标题，而且我知道枚举不可能继承，所以让我解释一下.

I know it's a bizarre title, and i know inheritance is not possible with enums, so let me explain.

如果我有以下课程:

• 水果(摘要)
  • 苹果(混凝土 - 源自水果)
  • 橙色(混凝土 - 源自水果)

  我有以下方法，用泛型实现:

  And i have the following method, implemented with generics:

  public ICollection<T> FindFruit<T>() where T : Fruit
  {
     return _fruitRepository
               .Fruits
               .OfType<T>()
               .ToList();
  }
  

  我是这样使用的:

  var apples = _fruitServices.FindFruit<Apple>();
  

  一切正常.

  现在 - 我目前有以下枚举:

  Now - i currently have the following enum:

  public enum FruitAssociations
  {
     Color,
     Manufacturers
  }
  

  基本上，一个水果"可以有很多关联，它指定要包含在我的存储库结果中的关联.

  Basically a "Fruit" can have many associations, which designates what associations to include in the result from my repository.

  所以上面的FindFruit方法其实是这样的:

  So the FindFruit method above is actually like this:

  public ICollection<T> FindFruit<T>(FruitAssociations[] assocs) where T : Fruit
  {
     return _fruitRepository
               .Fruits
               .OfType<T>()
               .IncludeAssocs(assocs) // ObjectQuery<Fruit> extension method.
               .ToList();
  }
  

  这是扩展方法:

  public static ObjectQuery<Fruit> IncludeAssocs(this ObjectQuery<Fruit> source, FruitAssociations[] assocs)
  {
     if (assocs.Contains(FruitAssociations.Color))
        query = query.Include("Color");
     // etc      
  }
  

  也工作正常.然而，我现在面临的问题是我对特定水果有关联.

  Also works fine. The problem i am now facing however is i have associations on particular Fruit.

  例如

  public enum OrangeAssociations
  {
     OrangeFamilies
  }
  

  而且我不确定如何拥有一个能够根据 T 的类型返回关联的 FindFruit 方法.

  And i'm not sure how to have a single FindFruit method that is capable of returning associations based on the type of T.

  这是我希望能够做到的最终结果:

  var oranges = _fruitServices.FindFruit<Orange>(new[] { OrangeAssociations.OrangeFamilies });
  var apples = _fruitServices.FindFruit<Apple>(new[] { AppleAssociations.AppleFamilies });
  

  但是如果没有为每种水果类型设置单独的 FindFruit 方法，我无法弄清楚如何做到这一点，因此无法解决通用 T 类型参数的问题.

  But i can't figure out how to do that without having seperate FindFruit methods for each fruit type, and hence defeating the point of the generic T type parameter.

  对于那些对我在这里做什么感到好奇的人 - 我正在使用 Entity Framework 4 的预先加载，因此使用 .Include 方法(它需要一个字符串来指定导航财产).因此，我的服务接受给定实体的枚举数组，然后我使用扩展方法将其转换为 .Include 语句中使用的字符串.

  For those who are curious as to what i'm doing here - i'm using eager loading with Entity Framework 4, and therefore using the .Include method (which takes a string designating the navigational property). So my service accepts an array of enumerations for a given entity, then i use an extension method to translate that to a string used in the .Include statement.

  我认为解决方案不是接受枚举数组，而是接受泛型类:

  I'm thinking the solution is to instead of accepting an array of enums, accept a generic class:

  public ICollection<T> FindFruit<T>(FruitAssociations<T> assocs) where T : Fruit
  

  像这样使用它:

  var associations = new FruitAssociations<Orange>(OrangeAssociations.OrangeFamilies);
  var apples = _fruitServices.FindFruit<Apple>(associations);
  

  但我不确定这将如何运作，或如何实施.

  But i'm not sure how that would work, or how to implement it.

  希望我的问题有意义并且不会太长/太复杂.

  Hopefully my question makes sense and is not too long/complicated.

  推荐答案

  Eric Lippert 会来我家用棍子打我，但它管用.

  Eric Lippert will come to my house and beat me with a stick for this, but it works.

  用法:

  // finds fruits of type Orange, includes Color and OrangeFamilies
  var result = FindFruit(OrangeAssociation.Color,
                         OrangeAssociation.OrangeFamilies);
  

  或

  // finds fruits of type Fruit, includes Manufacturers
  var result = FindFruit(FruitAssociation.Manufacturers);
  

  <小时>

  实施:

  static ICollection<TFruit> FindFruit<TAssoc, TFruit>(
      params FruitAssociation<TAssoc, TFruit>[] assocs)
      where TAssoc : FruitAssociation<TAssoc, TFruit>, new()
      where TFruit : Fruit
  {
      var query = _fruitRepository.Fruits.OfType<TFruit>();
  
      foreach (var assoc in assocs)
      {
          query = query.Include(assoc.Name);
      }
  
      return query.ToList();
  }
  

  与

  abstract class FruitAssociation<TAssoc, TFruit>
      where TAssoc : FruitAssociation<TAssoc, TFruit>, new()
      where TFruit : Fruit
  {
      public static readonly TAssoc Color = Define("Color");
  
      public static readonly TAssoc Manufacturers = Define("Manufacturers");
  
      protected static TAssoc Define(string name)
      {
          return new TAssoc { Name = name };
      }
  
      public string Name
      {
          get;
          private set;
      }
  }
  
  sealed class FruitAssociation : FruitAssociation<FruitAssociation, Fruit>
  {
  }
  
  sealed class OrangeAssociation : FruitAssociation<OrangeAssociation, Orange>
  {
      public static readonly OrangeAssociation OrangeFamilies =
          Define("OrangeFamilies");
  }
  

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