在vb6.0版的机房收费系统中说道过这个问题，回顾请点击：http://www.jb51.cc/article/p-qcociuxt-vn.html这次在vb.net版本中想再次利用原先的代码，可是其中控件和语法都发生了一些小的变化，费了些周折，终于将这个问题解决了。

首先设置底衬窗体属性 FormBorderStyle=none

然后在父窗体加载时，同时加载底衬，并设置好大小

   Private Sub frmMain_Load(sender As Object,e As EventArgs) Handles MyBase.Load
        Me.WindowState = FormWindowState.Maximized
        frmCheckCard.MdiParent = Me
        frmCheckCard.Show()
        frmCheckCard.Width = Me.ClientRectangle.Width - 5
        frmCheckCard.Height = Me.ClientRectangle.Height - 30
    End Sub

每个子窗体都这样设置

Public Class frmShowOnlineRecord
    Private Sub frmShowOnlineRecord_Activated(sender As Object,e As EventArgs) Handles Me.Activated
        Dim a As Form
        For Each a In Application.OpenForms '遍历所有已经打开的窗体
            If a.Name <> frmCheckCard.Name And a.Name <> frmMain.Name And a.Name <> Me.Name Then
                a.WindowState = FormWindowState.Minimized '将除父窗体、底衬、本身之外的窗体最小化
                frmCheckCard.SendToBack() '并且，将底衬永远放在父窗体的z顺序的最底层
            End If
        Next
    End Sub

    Private Sub frmShowOnlineRecord_Load(sender As Object,e As EventArgs) Handles Me.Load
        Dim a As Form
        For Each a In Application.OpenForms
            If a.Name <> frmCheckCard.Name And a.Name <> frmMain.Name And a.Name <> Me.Name Then
                a.WindowState = FormWindowState.Minimized
                frmCheckCard.SendToBack()
            End If
        Next
    End Sub
End Class

每个子窗体都这样设置这个功能的实现，从全局出发，将每个打开的窗体遍历，找到该显示窗体的独特条件“Name”和actived 事件触发，以此限制了其他窗体的显示状体。