Part of the "Why use F#?" series (link)

Anything C# can do...

A whirlwind tour of object-oriented code in F#

As should be apparent, you should generally try to prefer functional-style code over object-oriented code in F#, but in some situations, you may need all the features of a fully fledged OO language – classes, inheritance, virtual methods, etc.

So just to conclude this section, here is a whirlwind tour of the F# versions of these features.

Some of these will be dealt with in much more depth in a later series on .NET integration. But I won’t cover some of the more obscure ones, as you can read about them in the MSDN documentation if you ever need them.

Classes and interfaces

First, here are some examples of an interface, an abstract class, and a concrete class that inherits from the abstract class.

// interface
type IEnumerator<'a> =
    abstract member Current : 'a
    abstract MoveNext : unit -> bool

// abstract base class with virtual methods
type Shape() =
    //readonly properties
    abstract member Width : int with get
    abstract member Height : int with get
    //non-virtual method
    member this.BoundingArea = this.Height * this.Width
    //virtual method with base implementation
    abstract member Print : unit -> unit
    default this.Print () = printfn "I'm a shape"

// concrete class that inherits from base class and overrides
type Rectangle(x:int, y:int) =
    inherit Shape()
    override this.Width = x
    override this.Height = y
    override this.Print ()  = printfn "I'm a Rectangle"

let r = Rectangle(2,3)
printfn "The width is %i" r.Width
printfn "The area is %i" r.BoundingArea

Classes can have multiple constructors, mutable properties, and so on.

type Circle(rad:int) =
    inherit Shape()

    //mutable field
    let mutable radius = rad

    //property overrides
    override this.Width = radius * 2
    override this.Height = radius * 2

    //alternate constructor with default radius
    new() = Circle(10)

    //property with get and set
    member this.Radius
         with get() = radius
         and set(value) = radius <- value

// test constructors
let c1 = Circle()   // parameterless ctor
printfn "The width is %i" c1.Width
let c2 = Circle(2)  // main ctor
printfn "The width is %i" c2.Width

// test mutable property
c2.Radius <- 3
printfn "The width is %i" c2.Width


F# supports generics and all the associated constraints.

// standard generics
type KeyValuePair<'a,'b>(key:'a, value: 'b) =
    member this.Key = key
    member this.Value = value

// generics with constraints
type Container<'a,'b
    when 'a : equality
    and 'b :> System.Collections.ICollection>
    (name:'a, values:'b) =
    member this.Name = name
    member this.Values = values


F# supports not just classes, but the .NET struct types as well, which can help to boost performance in certain cases.

type Point2D =
      val X: float
      val Y: float
      new(x: float, y: float) = { X = x; Y = y }

let p = Point2D()  // zero initialized
let p2 = Point2D(2.0,3.0)  // explicitly initialized


F# can create exception classes, raise them and catch them.

// create a new Exception class
exception MyError of string

    let e = MyError("Oops!")
    raise e
    | MyError msg ->
        printfn "The exception error was %s" msg
    | _ ->
        printfn "Some other exception"

Extension methods

Just as in C#, F# can extend existing classes with extension methods.

type System.String with
    member this.StartsWithA = this.StartsWith "A"

let s = "Alice"
printfn "'%s' starts with an 'A' = %A" s s.StartsWithA

type System.Int32 with
    member this.IsEven = this % 2 = 0

let i = 20
if i.IsEven then printfn "'%i' is even" i

Parameter arrays

Just like C#’s variable length “params” keyword, this allows a variable length list of arguments to be converted to a single array parameter.

open System
type MyConsole() =
    member this.WriteLine([<ParamArray>] args: Object[]) =
        for arg in args do
            printfn "%A" arg

let cons = new MyConsole()
cons.WriteLine("abc", 42, 3.14, true)


F# classes can have events, and the events can be triggered and responded to.

type MyButton() =
    let clickEvent = new Event<_>()

    member this.OnClick = clickEvent.Publish

    member this.TestEvent(arg) =
        clickEvent.Trigger(this, arg)

// test
let myButton = new MyButton()
myButton.OnClick.Add(fun (sender, arg) ->
        printfn "Click event with arg=%O" arg)

myButton.TestEvent("Hello World!")


F# can do delegates.

// delegates
type MyDelegate = delegate of int -> int
let f = MyDelegate (fun x -> x * x)
let result = f.Invoke(5)


F# supports CLI enums types, which look similar to the “union” types, but are actually different behind the scenes.

// enums
type Color = | Red=1 | Green=2 | Blue=3

let color1  = Color.Red    // simple assignment
let color2:Color = enum 2  // cast from int
// created from parsing a string
let color3 = System.Enum.Parse(typeof<Color>,"Green") :?> Color // :?> is a downcast

type FileAccess = | Read=1 | Write=2 | Execute=4
let fileaccess = FileAccess.Read ||| FileAccess.Write

Working with the standard user interface

Finally, F# can work with the WinForms and WPF user interface libraries, just like C#.

Here is a trivial example of opening a form and handling a click event.

open System.Windows.Forms

let form = new Form(Width= 400, Height = 300, Visible = true, Text = "Hello World")
form.TopMost <- true
form.Click.Add (fun args-> printfn "the form was clicked")


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