Public Class Form1
    '作者：章鱼哥，QQ：3107073263 群：309816713      
    '如有疑问或好的建议请联系我，大家一起进步 
    '声明SetParent函数，这是一个API函数
    Declare Function SetParent Lib "user32" Alias "SetParent" (ByVal hWndChild As IntPtr,ByVal hWndNewParent As IntPtr) As Long
    Private Sub Form1_Load(ByVal sender As System.Object,ByVal e As System.EventArgs) Handles MyBase.Load
        Me.IsMdiContainer = True '将Form1设置为父窗体容器
    End Sub
    Private Sub Button1_Click(ByVal sender As System.Object,ByVal e As System.EventArgs) Handles Button1.Click
        'Form1是父窗体，Form2是子窗体
        SetParent(Form2.Handle,Me.Handle)
        Form2.Show()
    End Sub
    Private Sub Button2_Click(ByVal sender As System.Object,ByVal e As System.EventArgs) Handles Button2.Click
        SetParent(Form3.Handle,Me.Handle)
        Form3.Show()
    End Sub
End Class