MDI容器
 具体步骤如下：  
private void 销售ToolStripMenuItem_Click(object sender, EventArgs e)
        {
            VisibledForm();

            Form2 f2 = new Form2();                                           打开新窗口
            f2.MdiParent = this;                                                   
            f2.Name = "f2";                                                          
            OpenForm(f2);                                                            
        }

        private void VisibledForm()                                  
        {
            foreach (Control c in panel1.Controls)
            {
                if (c is Form)
                {
                    c.Visible = false;
                }
            }
        }

        private void 仓库ToolStripMenuItem_Click(object sender, EventArgs e)
        {
            VisibledForm();                                         引用方法
            Form3 f3 = new Form3();
            f3.MdiParent = this;
            f3.Name = "f3";                                         
            OpenForm(f3);                                          
        }

        private void 财务ToolStripMenuItem_Click(object sender, EventArgs e)
        {
            VisibledForm();                                          引用方法
            Form4 f4 = new Form4();
            f4.MdiParent = this;
            f4.Name = "f4";                                         
            OpenForm(f4);                                          
        }

        //传一个窗体对象过来，看看有没有打开，如果开打了，则还原已打开的窗口
        //如果未打开，则打开此新窗口
        //参数：要验证的窗体
        //返回值：无

        public void OpenForm(Form fff)                                         Form的参数fff，创建方法OpenForm
        {
            bool has = false;
            foreach (Control c in panel1.Controls)                            
            {
                if (c is Form)                                                               如果这个控件是窗口
                {
                    if (c.Name == fff.Name)                                        
                    {
                        has = true;                                                           如果走到这说明有
                        fff = c as Form;                                                    
                    }
                }
            }

            VisibledForm();                                                               
            if (has)                                                                             然后进行判断
            {
                 //fff.WindowState = FormWindowState.Maximized;    按说应该进行还原窗口，但是，这里如果还原的话，这一步并不会走，所以不用这一步，而是直接Show出来
                fff.Show();
            }
            else                                                                                  如果这个窗口没有打开，然后Show出来，也需要调取隐藏的方法，所以把这个方法写到外边
            {
                fff.Parent = panel1;                                                      
                fff.Show();
            }

        }

      

        private void Form1_Load(object sender, EventArgs e)
        {
            Form2 f2 = new Form2();
            f2.MdiParent = this;
            f2.Name = "f2";
            OpenForm(f2);
        }