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open System
open System.Windows.Forms
open System.Threading

type MainForm () as self =
    inherit Form()

    let lblHello = new Label(Text = "Hello", Height = 20, Dock = DockStyle.Top)
    let btnClickMe = new Button(Text = "Click Me", Height = 30, Width = 50, Dock = DockStyle.Bottom)

    let updateLabelText() =
        if lblHello.InvokeRequired then
            lblHello.Invoke(MethodInvoker(fun () -> lblHello.Text <- "World!")) |> ignore
        else
            lblHello.Text <- "World!"


    let clickHandler (e:EventArgs) =
        let thread = new Thread(updateLabelText)
        thread.IsBackground <- true
        thread.Start()

    do        
        btnClickMe.Click.Add(clickHandler)
        self.Controls.Add(lblHello)
        self.Controls.Add(btnClickMe)


[<STAThread>]
do
    let mf = new MainForm()
    Application.Run(mf)

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label.BeginInvoke(MethodInvoker( fun () -> label.Text <- "Hello" ))