Public Class Form1
 
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As _
      System.EventArgs) Handles Button1.Click
        f1.Dock = DockStyle.Fill
        f1.TopLevel = False
        Panel1.Controls.Clear()
        Panel1.Controls.Add(f1)
        Panel1.Refresh()
        f1.Show()
 
    End Sub
 
    Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As _
      System.EventArgs) Handles Button2.Click
        f2.Dock = DockStyle.Fill
        f2.TopLevel = False
        Panel1.Controls.Clear()
        Panel1.Controls.Add(f2)
        Panel1.Refresh()
        f2.Show()
    End Sub
 
    Public Sub New()
 
        ' This call is required by the Windows Form Designer.
        InitializeComponent()
 
        ' Add any initialization after the InitializeComponent() call.
        f1 = New Form2
        f2 = New Form3
    End Sub
End Class
 
Module Module1
    Public f2 As Form3
    Public f1 As Form2
End Module