Interfaces (F#)

Interfaces specify sets of related members that other classes implement.

// 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 ]


Interface declarations resemble class declarations except that no members are implemented. Instead, all the members are abstract, as indicated by the keyword abstract. You do not provide a method body for abstract methods. However, you can provide a default implementation by also including a separate definition of the member as a method together with the default keyword. Doing so is equivalent to creating a virtual method in a base class in other .NET languages. Such a virtual method can be overridden in classes that implement the interface.

There are two ways to implement interfaces: by using object expressions, and by using class types. In either case, the class type or object expression provides method bodies for abstract methods of the interface. Implementations are specific to each type that implements the interface. Therefore, interface methods on different types might be different from each other.

The keywords interface and end, which mark the start and end of the definition, are optional when you use lightweight syntax. If you do not use these keywords, the compiler attempts to infer whether the type is a class or an interface by analyzing the constructs that you use. If you define a member or use other class syntax, the type is interpreted as a class.

The .NET coding style is to begin all interfaces with a capital I.

Implementing Interfaces by Using Class Types

You can implement one or more interfaces in a class type by 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 IPrintable =
   abstract member Print : unit -> unit

type SomeClass1(x: int, y: float) =
   interface IPrintable with
      member this.Print() = printfn "%d %f" x y

Interface implementations are inherited, so any derived classes do not need to reimplement them.

Calling Interface Methods

Interface methods can be called only through the interface, not through any object of the type that implements the interface. Thus, you might have to upcast to the interface type by using the :> operator or the upcast operator in order to call these methods.

To call the interface method when you have an object of type SomeClass, you must upcast the object to the interface type, as shown in the following code.

let x1 = new SomeClass1(1, 2.0)
(x1 :> IPrintable).Print()

An alternative is to declare a method on the object that upcasts and calls the interface method, as in the following example.

type SomeClass2(x: int, y: float) =
   member this.Print() = (this :> IPrintable).Print()
   interface IPrintable with
      member this.Print() = printfn "%d %f" x y

let x2 = new SomeClass2(1, 2.0)

Implementing Interfaces by Using Object Expressions

Object expressions provide a short way to implement an interface. They are useful when you do not have to create a named type, and you just want an object that supports the interface methods, without any additional methods. An object expression is illustrated in the following code.

let makePrintable(x: int, y: float) =
    { new IPrintable with
              member this.Print() = printfn "%d %f" x y }
let x3 = makePrintable(1, 2.0) 

Interface Inheritance

Interfaces can inherit from one or more base interfaces.

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) = 2 * n
        member this.Method2(n) = n + 100
        member this.Method3(n) = n / 10

See Also


Object Expressions (F#)

Classes (F#)

Other Resources

F# Language Reference