1.private Form m_CurrentMdiChild;//宣告窗體
           
           
           
2.開啟你想要的視窗
private void ShowForm()
{
Form1 frm = new Form1();
frm.ShowMdiChild(frm);
}

3.private void ShowMdiChild(Form mdiForm)
{
    if (this.m_CurrentMdiChild != null)
    {
        this.m_CurrentMdiChild.Close(); //關閉當前窗體
    }
    this.m_CurrentMdiChild = mdiForm; //本窗體設定成為當前窗體
    mdiForm.MdiParent = this;
    mdiForm.WindowState = FormWindowState.Maximized;
    mdiForm.Show();