以下是代码片段：
private void subMenu1ToolStripMenuItem_Click(object sender, EventArgs e)
        {
            if (frm.sign == false)
            {
                frm f = new frm();
                f.MdiParent = this;
                frm.sign = true;
                f.Show();
               
            }
            else
            {
                MessageBox.Show("该窗体已经存在~");
                return;
            }

        }

以下是代码片段：
public static bool sign = false;

        public frm()
        {   
           InitializeComponent();
        }

        private void frm_FormClosing(object sender, FormClosingEventArgs e)
        {
            sign = false;
        }