接口提供了一种抽象的方式来描述类的实现细节。 它是一个模板，声明类必须实现和公开公开的方法。

语法

接口指定其他类实现的相关成员集。 它有以下语法 

// 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

请注意

1、在接口声明中，成员未实现。

2、成员是抽象的，由default关键字声明。 但是，您可以使用default关键字提供默认实现。
3、您可以通过使用对象表达式或使用类类型实现接口。
4、在类或对象实现中，您需要为接口的抽象方法提供方法体。
5、关键字interface和end（标记定义的开始和结束）是可选的。
例如，

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

调用接口方法

接口方法通过接口调用，而不是通过实例的类或类型实现接口。 要调用接口方法，您可以使用：>运算符（upcast运算符）转换为接口类型。
例如，

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

下面的例子说明了这一概念 

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()

当你编译和执行程序，它产生以下输出 

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

接口继承

接口可以从一个或多个基本接口继承。

下面的例子显示的概念 

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))

当你编译和执行程序，它产生以下输出 

10
15
5