this.FormClosing += new System.Windows.Forms.FormClosingEventHandler(this.frmMain_FormClosing);

        private void frmMain_FormClosing(object sender, FormClosingEventArgs e)
        {
            if (MessageBox.Show("你确定要退出？数据可能会丢失！", "系统提示", MessageBoxButtons.YesNo, MessageBoxIcon.Information, MessageBoxDefaultButton.Button1) == DialogResult.Yes)
            {
                e.Cancel = false;     //关闭窗体
            } 
            else
                e.Cancel = true;     //返回窗体
        }