Question

I have a question about closing and disposing a hidden child form.

Parent form with two buttons:

Public Class Form1
    Dim F2 As Form2

    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        F2 = New Form2
    End Sub

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        F2.Show()
    End Sub

    Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
        F2.Hide()
    End Sub
End Class

Child form:

Public Class Form2
    Private Sub Form2_FormClosing(ByVal sender As Object, ByVal e As System.Windows.Forms.FormClosingEventArgs) Handles Me.FormClosing
            e.Cancel = True
            Me.Hide()
    End Sub

    Private Sub Form2_VisibleChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.VisibleChanged
        MsgBox("Form2.Visible = " & Me.Visible.ToString)
    End Sub

    Private Sub Form2_Disposed(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Disposed
        MsgBox("Form2 has been disposed.")
    End Sub
End Class

As long as Form1 is open, I don’t want to close Form2. So that part works.
But I do want to close Form2 when Form1 is closed. Do I have to close it explicitly from Form1? And add more logic to Form2_FormClosing()?
As is, Form2_Disposed() never gets called as far as I can tell (I never get the message box). Is this correct?
When Form1 is disposed, variable F2 no longer exists. Will the Form2 be disposed by the garbage collector later on or not?

Answer 1

I will try to move the Form2_FormClosing event to the Form1 class.
In this way you will be able to control the closing of the Form2 instance from the Form1 instance.

' global instance flag
Dim globalCloseFlag As Boolean = False

...
Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load  
    F2 = New Form2  
    AddHandler F2.FormClosing, New FormClosingEventHandler(AddressOf Form2ClosingHandler)
End Sub  

' This will get the event inside the form1 instance, you can control the close of F2 from here
Private Sub Form2ClosingHandler(ByVal sender As Object, ByVal e As System.Windows.Forms.FormClosingEventArgs) 
    if globalCloseFlag = false then    
        e.Cancel = True    
        F2.Hide()
    end if

End Sub 

' Form1 closing, call the close of F2
Private Sub Form1_FormClosing(ByVal sender As Object, ByVal e As System.Windows.Forms.FormClosingEventArgs) Handles Me.FormClosing      
      globalCloseFlag = True
      F2.Close()
End Sub      

Please, pay attention that this is an example, you need to handle special cases like windows shutdown using the FormClosingEventArgs.CloseReason property