C# winform判断窗体是否已打开

1    Form1 form;
2        /// <summary>
3        /// 开始检测
4        /// </summary>
5        /// <param name="sender"></param>
6        /// <param name="e"></param>
7        private void button1_Click(object sender, EventArgs e)
8        {
9            if (form==null) //如果子窗体为空则创造实例 并显示
10            {
11                form = new Form1();
12                form.StartPosition = FormStartPosition.CenterScreen;//子窗体居中显示
13                form.Show();
14            }
15            else
16            {
17                if (form.IsDisposed) //若子窗体关闭 则打开新子窗体 并显示
18                {
19                    form = new Form1();
20                    form.StartPosition = FormStartPosition.CenterScreen;//子窗体居中显示
21                    form.Show();
22                }
23                else
24                {
25                    form.Activate(); //使子窗体获得焦点
26                }
27            }
28        }