VB.NET中关闭Form1导致Form2的DialogResult变为Cancel的问题

我创建了两个Form窗体，分别通过Show和ShowDialog方法显示：Form1用作Logo展示，显示并停留3秒；Form2为登录窗口，以模态方式显示等待用户登录。但在关闭Form1后，发现Form2的DialogResult变为了Cancel，这是否属于BUG？

相关代码如下：

Private WithEvents f1 As Form
Private WithEvents f2 As Form
Private WithEvents bg As System.ComponentModel.BackgroundWorker

Public Sub Run()
    Try
        bg = New System.ComponentModel.BackgroundWorker
        f1 = New Form
        f1.Name = "Form1"
        f1.Show()
        Console.WriteLine("Show Form1 and stay 3 seconds")
        bg.RunWorkerAsync()

        f2 = New Form
        f2.Name = "Form2"
        Console.WriteLine("Show Form2 (ShowDialog)")
        Dim rlt = f2.ShowDialog()
        If rlt = DialogResult.Cancel Then
            Console.ReadKey()
        End If
    Catch ex As Exception

    End Try
End Sub

Private Sub bg_DoWork(ByVal sender As Object, ByVal e As System.ComponentModel.DoWorkEventArgs) Handles bg.DoWork
    Dim stw = Stopwatch.StartNew
    Do
        Application.DoEvents()
    Loop Until stw.ElapsedMilliseconds > 3000
End Sub

Private Sub bg_RunWorkerCompleted(ByVal sender As Object, ByVal e As System.ComponentModel.RunWorkerCompletedEventArgs) Handles bg.RunWorkerCompleted
    Console.WriteLine("Form2's DialogResult = {0}", f2.DialogResult)
    f1.Close()
    Console.WriteLine("After Form1 close, Form2's DialogResult changed to ({0}). WHY?", f2.DialogResult)
End Sub

解答：这不是BUG，是WinForms的默认行为 #

当WinForms应用中最后一个非模态窗体被关闭时，系统会自动触发Application.Exit()逻辑，这个操作会强制关闭所有正在显示的模态窗体，同时将它们的DialogResult设置为Cancel。

在你的代码里，Form1是唯一的非模态窗体，执行f1.Close()后，系统检测到没有剩余的非模态窗体，就触发了Application.Exit()，导致正在显示的模态窗体Form2被强制关闭，DialogResult被自动设为Cancel。

另外，你用BackgroundWorker配合Application.DoEvents()的实现方式存在风险：Application.DoEvents()会打乱正常的消息循环，容易引发线程安全问题和UI异常。更合理的实现方式是使用WinForms的Timer控件来控制Form1的关闭，避免消息循环混乱：

Private WithEvents f1 As Form
Private WithEvents f2 As Form
Private WithEvents closeTimer As System.Windows.Forms.Timer

Public Sub Run()
    Try
        f1 = New Form
        f1.Name = "Form1"
        f1.Show()
        Console.WriteLine("Show Form1 and stay 3 seconds")

        closeTimer = New System.Windows.Forms.Timer()
        closeTimer.Interval = 3000
        closeTimer.Start()

        f2 = New Form
        f2.Name = "Form2"
        Console.WriteLine("Show Form2 (ShowDialog)")
        Dim rlt = f2.ShowDialog()
        If rlt = DialogResult.Cancel Then
            Console.ReadKey()
        End If
    Catch ex As Exception
    End Try
End Sub

Private Sub closeTimer_Tick(sender As Object, e As EventArgs) Handles closeTimer.Tick
    closeTimer.Stop()
    Console.WriteLine("Form2's DialogResult = {0}", f2.DialogResult)
    f1.Close()
    Console.WriteLine("After Form1 close, Form2's DialogResult changed to ({0}).", f2.DialogResult)
End Sub

改用Timer后，因为逻辑在主线程执行，不会干扰消息循环；同时只要Form2这个模态窗体还在显示，系统不会触发Application.Exit()，Form1关闭后Form2的DialogResult也不会被强制修改。

内容的提问来源于stack exchange，提问作者ROY