如果孩子只是表格的副本，如何检测MDIChild？

Question

我有frm_Child和frm_Parent，我将isMDIContainer的{​​{1}}设置为frm_Parent

在frm_Parent中我有2个按钮

1. true - 创建另一个btn_Create
2. mdi child - 检测有效btn_Detect mdi Child

frm_Parent代码：

(When I say active it means that I set my focused on it)

然后我有了frm_Child，并且Public Class frm_Parent Public my_variable As String Private Sub btn_create_Click(sender As Object, e As EventArgs) Handles btn_create.Click my_variable += 1 Call New frm_Child() With {.MdiParent = Me}.Show() End Sub Private Sub btn_Detect_Click(sender As Object, e As EventArgs) Handles btn_Detect.Click 'Code to detect active mdi child End Sub End Class 内有label1.text label1.text的价值会自动获得form_load，这完全取决于my_variable对frm_Parent的价值{1}}

frm_Child代码：

Public Class frm_Child

   Private Sub frm_Child_Load(sender As Object, e As EventArgs) Handles MyBase.Load
      Label1.Text = frm_Parent.my_variable
   End Sub

End Class

如果我创建4个新的frm_Child

，则为屏幕截图



问题：如何以编程方式检测活动的mdi子节点并将label1.text显示给messagebox？ (note: active mean that I set my on it.)

我尝试过这样的事情，

Private Sub btn_Detect_Click(sender As Object, e As EventArgs) Handles btn_Detect.Click
    'Code to detect active mdi child
    Dim activeChild As Form = me.ActiveMdiChild
    MsgBox(activeChild.label1.text)
End Sub

此外，我还修改了the example from MSDN

Private Sub btn_Detect_Click(sender As Object, e As EventArgs) Handles btn_Detect.Click
    'Code to detect active mdi child
    Dim activeChild As Form = Me.ActiveMdiChild

    ' If there is an active child form, find the active control, which
    ' in this example should be a RichTextBox.
    If (Not activeChild Is Nothing) Then
        Try
            Dim theLabel As Label = CType(activeChild.ActiveControl, Label)
            MsgBox(theLabel.Text)
        Catch
            MessageBox.Show("You need to select an active form.")
        End Try
    End If
End Sub

Answer 1

因此，要总结答案中的注释，您需要通过父项的ActiveMdiChild属性获取子表单并将其转换为适当的类型，以便能够访问该类型的成员

Answer 2

就我个人而言，我认为你应该将“my_variable”值传递给你孩子表单的构造函数，但是按照jmcilhinney的建议 cast ，你应该像这（而不是使用默认实例）：

Private Sub frm_Child_Load(sender As Object, e As EventArgs) Handles MyBase.Load
    Dim parent As frm_Parent = DirectCast(Me.MdiParent, frm_Parent)
    Me.Label1.Text = parent.my_variable
End Sub

同样，这是你如何投射MdiChild并在“检测”按钮中抓取标签：

Private Sub btn_Detect_Click(sender As Object, e As EventArgs) Handles btn_Detect.Click
    If Not IsNothing(Me.ActiveMdiChild) Then
        Dim child As frm_Child = DirectCast(Me.ActiveMdiChild, frm_Child)
        MessageBox.Show(child.Label1.Text)
    End If
End Sub

当然，如果你要使用不同的类型的MdiChildren，那么你要么必须进行额外的检查并转换为正确的类型，要么使用接口，所有的孩子都会实现并转向。