无法从非主线程隐藏WinForm窗体的技术问题求助

主线程包含FrmMain和FrmWait两个窗体，另有一个执行耗时任务的工作线程。FrmWait仅显示带“Downloading”动态GIF的PictureBox，希望在工作线程完成任务（或触发异常时）隐藏它，但尝试Forms/Controls的跨线程调用方式均无效。

现有代码 #

FrmMain 代码 #

Private Sub StartThread()
    Debug.Print("Main ThreadId is: " & System.Environment.CurrentManagedThreadId)

    FrmWait.Show()
    Dim trd As New Thread(Sub()
                              Flag.Agisci(Num, Value1, value2)
                          End Sub)
    trd.Start()
End Sub

Private Sub FrmMain_HandleCreated(sender As Object, e As EventArgs) Handles Me.HandleCreated
    Debug.Print("FrmMain Handle has been created: " & Me.Handle.ToString)
End Sub

FrmWait 代码 #

Private Delegate Sub CloseForm(hide As Boolean)

Public Sub CloseWait(hide As Boolean)
    Debug.Print("******************* Entring in CloseWait")
    Try
        Dim h As IntPtr = Me.Handle
        Debug.Print("................... InvokeRequired is: " & Me.InvokeRequired.ToString & " on Handle: " & h.ToString)
        If Me.InvokeRequired Then
            Debug.Print("----------- Invoke is Required !!")
            Me.Invoke(New CloseForm(AddressOf CloseWait), hide)
        Else
            Debug.Print("+++++++++++ Calling Me.Visible = " & hide.ToString)
            Me.Visible = hide
        End If
    Catch ex As Exception
        Debug.Print(ex.ToString)
    End Try
End Sub

Private Sub FrmWait_HandleCreated(sender As Object, e As EventArgs) Handles Me.HandleCreated
    Debug.Print("FrmWait Handle has been created: : " & Me.Handle.ToString)
End Sub

工作线程类（Flag）代码 #

Public Sub Agisci(Num As Integer, Value1 As String, Value2 As String)
    Try
        Debug.Print("Task ThreadId is: " & System.Environment.CurrentManagedThreadId)
        Sleep(2000)
        Throw New Exception("Foo")
  
        'some stuff bypassed by the exception

    Catch ex As Exception
        Debug.Print(ex.Message)

        FrmWait.CloseWait(True)

        'this is another Form that is correctly displayed
        FrmMessBox.SetText(ex.Message)
        FrmMessBox.ShowDialog()
    End Try
End Sub

输出日志 #

FrmMain Handle has been created: 4785876
Main ThreadId is: 1
FrmWait Handle has been created: 655546
Task ThreadId is: 5
发生异常: 'System.Exception' in MyApp.exe -Foo
******************* Entring in CloseWait
FrmWait Handle has been created: 917984
................... InvokeRequired is: False on Handle: 917984 '****** 为什么是FALSE?????
+++++++++++ Calling Me.Visible = False '************* 无效!

问题根源 #

日志里的关键线索是FrmWait出现了两个不同的句柄（655546和917984）：

• 主线程调用FrmWait.Show()时，创建的是第一个句柄（属于主线程的窗体实例）
• 工作线程调用FrmWait.CloseWait()时，因为VB的默认窗体实例化机制（直接用类名访问会自动创建新实例），这里实际操作的是全新的FrmWait实例，而非主线程显示的那个。

这个新实例的句柄是在工作线程中创建的，所以InvokeRequired返回False，但修改它的Visible属性对主线程的窗体完全没有影响。

解决方法 #

方法1：显式传递FrmWait实例到工作线程 #

这是最可靠的方式，确保操作的是同一个窗体：

1. 修改Flag类的Agisci方法，接收FrmWait实例作为参数：

Public Sub Agisci(Num As Integer, Value1 As String, Value2 As String, waitForm As FrmWait)
    Try
        Debug.Print("Task ThreadId is: " & System.Environment.CurrentManagedThreadId)
        Sleep(2000)
        Throw New Exception("Foo")

        'some stuff bypassed by the exception

    Catch ex As Exception
        Debug.Print(ex.Message)

        waitForm.CloseWait(False) '传False对应隐藏窗体，原代码传True是设置Visible=True，和需求相反
        FrmMessBox.SetText(ex.Message)
        FrmMessBox.ShowDialog()
    End Try
End Sub

2. 修改FrmMain的StartThread方法，传递FrmWait实例：

Private Sub StartThread()
    Debug.Print("Main ThreadId is: " & System.Environment.CurrentManagedThreadId)

    Dim waitForm As New FrmWait()
    waitForm.Show()
    Dim trd As New Thread(Sub()
                              Flag.Agisci(Num, Value1, value2, waitForm)
                          End Sub)
    trd.Start()
End Sub

方法2：使用主线程同步上下文 #

如果必须保留类名访问的方式，可以通过主线程同步上下文执行操作：

1. 在FrmMain中定义同步上下文变量：

Private mainSyncContext As SynchronizationContext

2. 在FrmMain的Load事件中初始化：

Private Sub FrmMain_Load(sender As Object, e As EventArgs) Handles MyBase.Load
    mainSyncContext = SynchronizationContext.Current
End Sub

3. 修改FrmWait的CloseWait方法：

Public Sub CloseWait(hide As Boolean)
    mainSyncContext.Post(Sub()
                             Me.Visible = Not hide '根据需求调整，隐藏则设为False
                         End Sub, Nothing)
End Sub

额外注意点 #

• 原代码中CloseWait(True)是设置Me.Visible = True，和“隐藏窗体”的需求相反，需要改为CloseWait(False)或者调整方法内的逻辑。
• 避免直接用窗体类名访问实例，显式创建并传递实例能彻底避免VB自动创建新实例的坑。

内容的提问来源于stack exchange，提问作者ezio