F＃对象列表

Question

对于我的一个项目，我需要一个可以携带多种类型的列表，并且我不想进行任何投射，因此我尝试了使用obj list s。下面是一些示例代码来展示一些东西，应该工作，但由于某种原因没有：

type Fruit() = 
    member this.Kind = "I'm a tasty fruit!" 

type Apple() = 
    inherit Fruit() 
    member this.Kind = "I'm a crispy apple!" 

type Cherry() = 
    inherit Fruit() 
    member this.Kind = "I'm a juicy cherry!" 


> (new Fruit()).Kind 
val it : string = "I'm a tasty fruit!" 

... // And so on; it works as expected for all three fruits 

> let aFruit = [new Fruit()] 
val aFruit : Fruit list = [FSI_0002+Fruit] 

> aFruit.Head.Kind       // Works just fine 
val it : string = "I'm a tasty fruit!" 

> let fruits : obj list = [new Fruit(); new Apple(); new Cherry] 
val fruits : obj list = [FSI_0002+Fruit; FSI_0002+Apple; FSI_0002+Cherry] 

> fruits.Head        // Okay, so we can extract items just fine. It also kept the property! 
val it : obj = FSI_0002+Fruit {Kind = "I'm a tasty fruit!";} 

> it.Kind         // This doesn't work. Why? What am I missing? 
error FS0039: The field, constructor or member 'Kind' is not defined 

Answer 1

的问题是it的类型为obj的列表是一个obj list和obj没有.Kind成员。您可以使用父类型 - 像

let fruits : Fruit list = [new Fruit(); new Apple(); new Cherry()];; 

val fruits : Fruit list = [FSI_0003+Fruit; FSI_0003+Apple; FSI_0003+Cherry] 

，然后用访问：

fruits.Head.Kind;; 
val it : string = "I'm a tasty fruit!"