Create button handler in F# for WinForms app

Question

I'm trying to make a very simple application in F#. I want just to make a tiny button with the click event, which will call the MessageBox.Show() method.

open System
open System.Windows.Forms

module FSI =
    [<System.Runtime.InteropServices.DllImport("user32.dll")>]
    extern bool ShowWindow(nativeint hWnd, int flags)
    let HideConsole() = 
        let proc = System.Diagnostics.Process.GetCurrentProcess()
        ShowWindow(proc.MainWindowHandle, 0)

FSI.HideConsole()

let form1 = new Form()
form1.Text <- "App"

let Test() = ( MessageBox.Show("Clicked.") )

let button1 = new Button( Text = "Click Me" )
button1.Click.AddHandler(fun _ _ -> Test())

form1.Controls.Add(button1)    

[<STAThread>]
do
   Application.Run(form1)

The errors occurred for the:

FSI.HideConsole() // warning
button1.Click.AddHandler(fun _ _ -> Test()) // compiler error

And they are similar.

For the 1-st:

Warning This expression should have type 'unit', but has type 'bool'. Use 'ignore' to discard the result of the expression, or 'let' to bind the result to a name.

For the 2-nd:

This expression was expected to have type unit but here has type DialogResult

Malvin Butterfinger Malvin Butterfinger · Accepted Answer · 2013-07-21T13:48:02

F# functions implicitly return the result of their last expression, so currently your function Test() returns the result of MessageBox.Show(string). To create a function that returns void/unit, you can use the ignore operator.

let Test() = ignore (MessageBox.Show("Clicked."))

It's a bit cumbersome to have to do the extra parenthesis wrapping, so typically one uses the pipeforward (|>) operator, which doesn't require that:-

let Test() = MessageBox.Show("Clicked.") |> ignore

This can also be applied to your call of HideConsole. The compiler uses the warning to inform you that the return value of HideConsole is not used.

Create button handler in F# for WinForms app

1 Answers