如何判断一个窗体是否打开

        FormSendQueue formSendQueue = new FormSendQueue();
        private void button1_Click(object sender, EventArgs e)
        {
            formSendQueue.ShowDialog();
        }

把窗体创建语句放到外面

            if (!formSendQueue.IsDisposed)
            {
                MessageBox.Show("formsendqueue窗体打开");
            }
            else
            {
                MessageBox.Show("formsendqueue窗体已经关闭");   
            }