Interfaces specify sets of related members that other classes implement. Interface declarations resemble class declarations except that no members are implemented.
abstract.default keyword.The basic syntax to define an interface is as follows.
type type-name =
interface
inherits-decl
member-defns
end
The following example shows a very simple interface declaration.
type IDisplay =
abstract member Print : format:string -> unit
In the above code, an IDisplay interface contains the Print method and has a single parameter of the type string with the name format.
There are two ways to implement interfaces.
You can implement one or more interfaces in a class type using the interface keyword, the name of the interface, and the with keyword, followed by the interface member definitions, as shown in the following code.
type IDisplay =
abstract member Print : unit -> unit
type Point3D(x: float, y: float, z: float) =
interface IDisplay with
override this.Print() = printfn "X: %f, Y: %f, Z: %f" x y z
Interface methods can be called only through the interface, not through any object of the type that implements the interface.
You might have to upcast the interface type by using the :> operator or the upcast operator to call these methods.
let point = new Point3D(1.1, 2.2, 3.3)
(point :> IDisplay).Print()
Object expressions provide a short way to implement an interface. It is useful when you do not have to create a named type, and you want an object that supports the interface methods without any additional methods.
The following example shows the usage of an object expression.
type IDisplay =
abstract member Print : unit -> unit
let Point3D(x: float, y: float, z: float) =
{ new IDisplay with
member this.Print() = printfn "X: %f, Y: %f, Z: %f" x y z }
let point1 = Point3D(1.1, 2.2, 3.3)
point1.Print()
Interfaces can inherit from one or more base interfaces, as shown in the below example.
type Interface1 =
abstract member Method1 : int -> int
type Interface2 =
abstract member Method2 : int -> int
type Interface3 =
inherit Interface1
inherit Interface2
abstract member Method3 : int -> int
type MyClass() =
interface Interface3 with
member this.Method1(n) = n * 10
member this.Method2(n) = n + 100
member this.Method3(n) = n*n / 10