F# - 介面

介面提供了一種抽象的方式來編寫類的實現細節。它是一個模板，宣告類必須實現並公開的方法。

語法

介面指定其他類實現的相關成員集。它具有以下語法：

// Interface declaration:
[ attributes ]
type interface-name =
   [ interface ]
      [ inherit base-interface-name ...]
      abstract member1 : [ argument-types1 -> ] return-type1
      abstract member2 : [ argument-types2 -> ] return-type2
      ...
   [ end ]
	
// Implementing, inside a class type definition:
interface interface-name with
   member self-identifier.member1 argument-list = method-body1
   member self-identifier.member2 argument-list = method-body2
// Implementing, by using an object expression:
[ attributes ]
let class-name (argument-list) =
   { new interface-name with
      member self-identifier.member1 argument-list = method-body1
      member self-identifier.member2 argument-list = method-body2
      [ base-interface-definitions ]
   }
member-list

請注意：

• 在介面宣告中，成員沒有實現。

• 成員是抽象的，由`abstract`關鍵字宣告。但是，您可以使用`default`關鍵字提供預設實現。

• 您可以使用物件表示式或類型別來實現介面。

• 在類或物件實現中，您需要為介面的抽象方法提供方法體。

• `interface`和`end`關鍵字（標記定義的開始和結束）是可選的。

例如：

type IPerson =
   abstract Name : string
   abstract Enter : unit -> unit
   abstract Leave : unit -> unit

呼叫介面方法

介面方法是透過介面呼叫的，而不是透過實現介面的類或型別的例項呼叫的。要呼叫介面方法，您可以使用`:>`運算子（向上轉型運算子）向上轉型為介面型別。

例如：

(s :> IPerson).Enter()
(s :> IPerson).Leave()

以下示例說明了這個概念：

示例

type IPerson =
   abstract Name : string
   abstract Enter : unit -> unit
   abstract Leave : unit -> unit

type Student(name : string, id : int) =
   member this.ID = id
   interface IPerson with
      member this.Name = name
      member this.Enter() = printfn "Student entering premises!"
      member this.Leave() = printfn "Student leaving premises!"

type StuffMember(name : string, id : int, salary : float) =
   let mutable _salary = salary

   member this.Salary
      with get() = _salary
      and set(value) = _salary <- value

   interface IPerson with
      member this.Name = name
      member this.Enter() = printfn "Stuff member entering premises!"
      member this.Leave() = printfn "Stuff member leaving premises!"

let s = new Student("Zara", 1234)
let st = new StuffMember("Rohit", 34, 50000.0)

(s :> IPerson).Enter()
(s :> IPerson).Leave()
(st :> IPerson).Enter()
(st :> IPerson).Leave()

編譯並執行程式後，將產生以下輸出：

Student entering premises!
Student leaving premises!
Stuff member entering premises!
Stuff member leaving premises!

介面繼承

介面可以繼承一個或多個基介面。

以下示例顯示了該概念：

type Interface1 =
   abstract member doubleIt: int -> int

type Interface2 =
   abstract member tripleIt: int -> int

type Interface3 =
   inherit Interface1
   inherit Interface2
   abstract member printIt: int -> string

type multiplierClass() =
   interface Interface3 with
      member this.doubleIt(a) = 2 * a
      member this.tripleIt(a) = 3 * a
      member this.printIt(a) = a.ToString()

let ml = multiplierClass()
printfn "%d" ((ml:>Interface3).doubleIt(5))
printfn "%d" ((ml:>Interface3).tripleIt(5))
printfn "%s" ((ml:>Interface3).printIt(5))

編譯並執行程式後，將產生以下輸出：

10
15
5