无法投射通用类 [英] Unable to cast Generic Class

问题描述

界面IFurnitureDao {...} 
 
 Class TableDao：IFurnitureDao {。 ..} 
 Class ChairDao：IFurnitureDao {...} 
 Class AlmirahDao：IFurnitureDao {...} 

我有类型IFurnitureDao的通用类...

类MyGenericClass< T> 其中 T：IFurnitureDao {...} 

MyGenericClass的对象存储在ArrayList中，如下所示：

 ArrayList myList =  new  ArrayList { new  MyGenericClass< TableDao> {..}， new  MyGenericClass< ChairDao> {..} ，MyGenericClass< AlmirahDao> {..}} 

在从ArrayList中重新获取数据时，我尝试将teh对象转换为IFurnitureDao。

列表< MyGeneRicClass< IFurnitureDao>> newList = myList.OfType< MyGenericClass< IFurnitureDao>>（）。toList（）; 

我收到一条错误，指出

无法从MyGenericClass< TableDao>转换对象到MyGenericClass< IFurnitureDao>。 

任何人都可以指导我。谢谢！



问候，

Divya

解决方案

你不能将具体对象转换为接口。您的具体对象实现接口，但也可以实现其他接口或包含其他属性/方法。因此你不能倒退。



另外，界面不是一个对象。它只是一个对象的描述（或蓝图），它将应用于实现该接口的对象。



如果要将特定对象强制转换为一个通用对象，您将需要创建一个通用类，除了您的接口之外什么都不实现，然后在迭代具体对象列表时分配每个属性。或者，从您的接口创建一个基类，然后从基类派生您的具体对象。像这样：

接口IFurnitureDao {...} 
 
 Class BaseDao：IFurnitureDao {... } 
 Class TableDao：BaseDao {...} 
 Class ChairDao：BaseDao {...} 
 Class AlmirahDao：BaseDao {...} 

如果这样做，那么只要没有真正的差异就可以向后投射：

类MyGenericClass< ; T> 其中 T：BaseDao {...} < /   t  >  

您尝试的类不是通用的。您已经将泛型实例化为完整的类。



说， MyGenericClass<> 是通用的， MyGenericClass< TableDao> （如果 TableDao 是一个完整的类，而不是通用参数）不是。



如果它们是泛型类，那么整个铸造的想法都没有任何意义：没有泛型类的实例。

你的类是非泛型的，但是你无法转换它们，因为它们不是赋值兼容的。目前尚不清楚你为什么要分配它们的实例。



对于某些背景，请看： http://msdn.microsoft.com/en-us/library/ee207183.aspx [ ^ ]。



但是，到了解它，你应该更好地理解更基本的编程主题：类和实例，泛型类，完整类型，遗传实例化和其他通用主题。



< DD> -SA

Hi,

Interface IFurnitureDao{...}

Class TableDao : IFurnitureDao {...}
Class ChairDao : IFurnitureDao {...}
Class AlmirahDao : IFurnitureDao {...}

I have a generic class of type IFurnitureDao...

Class MyGenericClass<T> where T:IFurnitureDao {...}

The objects of MyGenericClass is stored in an ArrayList as shown below :

ArrayList myList = new ArrayList {new MyGenericClass<TableDao>{..} , new MyGenericClass<ChairDao>{..}, MyGenericClass<AlmirahDao>{..} }

While retriving the data from ArrayList I am tryign to cast teh objects into IFurnitureDao.

List<MyGeneRicClass<IFurnitureDao>> newList = myList.OfType<MyGenericClass<IFurnitureDao>>().toList();

I am receiving an error stating

unable to cast objects from MyGenericClass<TableDao> to MyGenericClass<IFurnitureDao>. 

Can anyone guide me on this . Thanks !

Regards,
Divya

解决方案

You can't cast concrete objects to interfaces. Your concrete objects implement the interface but may also implement other interfaces or contain additional properties/methods. Therefore you can't go backwards.

Plus, an interface is NOT an object. It is only a description (or blueprint) for an object which will be applied to an object that implements that interface.

If you want to cast your specific objects back to a generic object, you will need to create a generic class which implements nothing but your interface then assign each property as you iterate the list of concrete objects. Or, create a base class from your interface then derive your concrete objects from the base class. Like this:

Interface IFurnitureDao{...}

Class BaseDao : IFurnitureDao {...}
Class TableDao : BaseDao {...}
Class ChairDao : BaseDao {...}
Class AlmirahDao : BaseDao {...}

If you do this, then you can cast backwards as long as there are no real differences:

Class MyGenericClass<t> where T:BaseDao {...}</t>

The classes you are trying to case are not generic. You already instantiated generics into a complete classes.

Say, MyGenericClass<> is generic, MyGenericClass<TableDao> (if TableDao is a complete class, not generic parameter) is not.

If they were generic classes, the whole idea of casting would not make any sense: there are not instances of generic classes.
Your classes are non-generic, but you cannot cast them because they are not assignment-compatible. It's not clear why are you trying to assign instances of them.

For some background, please see this: http://msdn.microsoft.com/en-us/library/ee207183.aspx[^].

But, to understand it, you should get better understanding of more basic programming topics: classes and instances, generic classes, complete types, instantiation of genetics, and other generic topics.

—SA

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