windows - F# 使用 WinForms 拖放 : DragDrop event of a control does not call the referenced member function

Question

您知道为什么 F# 中的 DragDrop 事件在我的示例中无法正常工作吗？所有其他事件，如 DragEnter、DragLeave、DragOver...都以相同的方式正常工作。

只需编译此代码并进行尝试，将文件拖到表单中，然后在启动可执行文件的位置查看在控制台/终端中触发的事件。

open System
open System.Drawing
open System.Windows.Forms

type MainForm( args: string list ) as this =
    // subclassing
    inherit Form()

    // controls -------------------
    let dragDropImage = new PictureBox()
    // ----------------------------

    // "constructor" (not a real constructor)
    do this.initComponents()
    // link events to specific member function
    do dragDropImage.DragEnter |> Event.add this.onDragEnter
    do dragDropImage.DragDrop |> Event.add this.onDragDrop
    // this syntax don't work either: do dragDropImage.DragDrop.Add(fun _ -> printfn "dragDrop")
    do dragDropImage.DragLeave |> Event.add this.onDragLeave
    do dragDropImage.DragOver |> Event.add this.onDragOver

    member this.initComponents() =
        // main form attributes
        this.Text <- "Averest-GUI"
        this.ClientSize <- new Size(350,230)
        this.StartPosition <- FormStartPosition.CenterScreen
        // drag'n'drop field
        dragDropImage.Size <- new Size(330,210)
        dragDropImage.Location <- new Point(7,7)
        dragDropImage.AllowDrop <- true // allow Drag'n'Drop functionality
        // insert controls into MainForm
        this.Controls.Add(dragDropImage)

    member this.onDragLeave( e: EventArgs ) =
        printfn "DragLeave" //e.Effect <- DragDropEffects.Copy

    member this.onDragOver( e: DragEventArgs ) =
        printfn "DragOver" //e.Effect <- DragDropEffects.Copy

    member this.onDragEnter( e: DragEventArgs ) =
        printfn "DragEnter" //e.Effect <- DragDropEffects.Copy

    member this.onDragDrop( e: DragEventArgs ) =
        printfn "DragDrop"


let testForm =
    let temp = new MainForm( ["Test"] )
    temp

// single thread apartment model (interacting with COM components)
[<STAThread>]
do Application.Run(testForm)

Answer 1

从 onDragEnter 中删除注释。除非您将 e.Effect 设置为 e.AllowedEffects 之一，否则不允许放置。这也会改变光标。