/* -----------frmMian----------- */
private frmS fs = null;
private frmW fw = null;

private void button1_Click(object sender, EventArgs e)
{
   frmS f = new frmS();
   fs = f;
   f.Show();
   frmW ff = new frmW();
   fw = ff;
   ff.Show();
}

private void button3_Click(object sender, EventArgs e)
{
   if (fs != null)
   {
      fs.Close();
      fs = null;
   }
}

/* --------frmW------- */
private frmS fs = null;

private void button1_Click(object sender, EventArgs e)
{
  frmS f = new frmS();
  fs = f;
  f.Show();
  this.Close();
}

------解决方案--------------------
private void frmMain_Load(object sender, EventArgs e)
{
     frmS fs = new frmS();
     fs.MaximizeBox = true;
     fs.MdiParent = this;
     fs.Show();
}
  private void button1_Click(object sender, EventArgs e)
{
     this.Close();
     frmW fw = new frmW();
     fw.MdiParent = this.MdiParent ;
     fw.Show();
  }
private void button2_Click(object sender, EventArgs e)
{
     this.Close();
     frmS fs = new frmS();
     fs.MdiParent = this.MdiParent ;
     fs.Show();
  }

------解决方案--------------------

我的这个可以通过，而且可以防止打开多个窗体，如果已经打开，就激活他
C# code
public partial class frmMain : Form
    {
        public frmMain()
        {
            InitializeComponent();
        }

        private void 登录ToolStripMenuItem_Click(object sender, EventArgs e)
        {
            foreach (Form form in Application.OpenForms)
            {
                if (form.Name == "frmLogin")
                {
                    form.Activate();
                    form.WindowState = FormWindowState.Maximized;
                    return;
                }
            }
            
            frmLogin login = new frmLogin();

            login.MdiParent = this;
            login.WindowState = FormWindowState.Maximized;
            login.Show();
        }

        private void 日志ToolStripMenuItem_Click(object sender, EventArgs e)
        {
            foreach (Form form in Application.OpenForms)
            {
                if (form.Text == "frmLogInfo")
                {
                    form.Activate();
                    return;
                }
            }
            
            frmLogInfo log = new frmLogInfo();
            log.MdiParent = this;
            log.WindowState = FormWindowState.Maximized;
            log.Show();
        }

        private void frmMain_Load(object sender, EventArgs e)
        {
            frmLogin login = new frmLogin();
            login.WindowState = FormWindowState.Maximized;
            login.MdiParent = this;
            login.Show();
        }

    }