MDI子フォームを最背面に移動する

private void FormA_Closing(object sender, 
	System.ComponentModel.CancelEventArgs e)
{
	e.Cancel = true;
	this.SendToBack();
}

public class FormA
{
    enum GetWindow_Cmd {
        GW_HWNDFIRST = 0,
        GW_HWNDLAST = 1,
        GW_HWNDNEXT = 2,
        GW_HWNDPREV = 3,
        GW_OWNER = 4,
        GW_CHILD = 5,
        GW_ENABLEDPOPUP = 6
    }
    
    [DllImport("user32.dll")]
    static extern IntPtr GetWindow(IntPtr hWnd, uint uCmd);
    
    private void FormA_Closing(object sender, 
        System.ComponentModel.CancelEventArgs e)
    {
        // (省略)
        IntPtr hwnd =GetWindow(this.Handle, GetWindow_Cmd.GW_HWNDNEXT);
        // (省略)
    }
}

    Private WithEvents formA As formA
    Private WithEvents formB As formA
    Private WithEvents formC As formA

    Private _MdiChildren As New System.Collections.ArrayList
    Private _MdiZorder As New System.Collections.ArrayList

    Private Sub Form1_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles MyBase.Load
        formA = New formA
        formA.MdiParent = Me
        formA.Text = "A"
        formA.Show()
        _MdiChildren.Add(formA)

        formB = New formA
        formB.MdiParent = Me
        formB.Text = "B"
        formB.Show()
        _MdiChildren.Add(formB)

        formC = New formA
        formC.MdiParent = Me
        formC.Text = "C"
        formC.Show()
        _MdiChildren.Add(formC)

    End Sub


    Private Sub Form1_MdiChildActivate(ByVal sender As Object, ByVal e As System.EventArgs) Handles MyBase.MdiChildActivate
        '自前でZorderの変更
        Try
            _MdiZorder.Remove(sender)
        Catch ex As Exception
        End Try
        _MdiZorder.Insert(0, sender)
    End Sub

    Dim _forceClose As Boolean = False
    Private Sub MdiChild_Closing(ByVal sender As Object, ByVal e As System.ComponentModel.CancelEventArgs) Handles formA.Closing, formB.Closing, formC.Closing
        If _forceClose Then Exit Sub

        Dim form As Form = sender
        form.SendToBack()
        Try
            _MdiZorder.Remove(form)
        Catch ex As Exception
        End Try
        _MdiZorder.Add(form)
        _MdiZorder(0).focus()
        e.Cancel = True
    End Sub

    Private Sub Form1_Closing(ByVal sender As Object, ByVal e As System.ComponentModel.CancelEventArgs) Handles MyBase.Closing
        _forceClose = True
        For Each child As Form In _MdiChildren
            child.Close()
        Next
        e.Cancel = False
    End Sub