算了 帮你找下了
private void Tool_Exit_Click(object sender, EventArgs e)
        {

            DialogResult rl = MessageBox.Show("确定退出？", "退出", MessageBoxButtons.YesNo,MessageBoxIcon.Question );
            if (rl == DialogResult.Yes)
            {
                Application.ExitThread ();//关闭项目
                //this.Close();
            }
            else
            {
                // return false;
            }
        }
 private void Main_FormClosing(object sender, FormClosingEventArgs e)
        {
            DialogResult rl = MessageBox.Show("确定退出？", "退出", MessageBoxButtons.YesNo,MessageBoxIcon.Question );
            if (rl == DialogResult.Yes)
            {
            Application.ExitThread ();
            }
            else
            {
                e.Cancel = true;
                // return false;
            }
        }
这样就可以了

如果你是在closed事件里面写会出现你说的问题，但你在closing事件里面写就不会出现了
前提是Application.ExitThread ();

但如果不是全部关闭
你可以在关闭按钮那个事件里面直接this.close()
然后再你的窗体closing事件里面写MessageBox.Show("确定退出？", "退出", MessageBoxButtons.YesNo,MessageBoxIcon.Question );

你这个在多窗体上有些不适合，因为主窗体不是form1！
应该是这样的：
 private void mainfrm_FormClosing(object sender, FormClosingEventArgs e)
        {
            if (run)
            {
                if (MessageBox.Show("  是否确定要退出本系统？  ", "图书馆管理系统----退出确认", MessageBoxButtons.YesNo, MessageBoxIcon.Question,MessageBoxDefaultButton.Button2) == DialogResult.No)
                {
                    e.Cancel = true;
                }
                else
                {
                    run = false;
                    Application.Exit();
                }
            }
            else
            {
                Application.Exit();
            }
        }

private void 退出系统ToolStripMenuItem_Click(object sender, EventArgs e)
        {
            if (MessageBox.Show("  是否确定要退出本系统？  ", "图书馆管理系统----退出确认", MessageBoxButtons.YesNo , MessageBoxIcon.Question,MessageBoxDefaultButton.Button2) == DialogResult.Yes )
            {
                run = false;
                Application.Exit();
            }
        }

结帖结帖

直接在关闭button的click方法中调用this.close();