学步园

        private Form form2;
        private void button1_Click(object sender, EventArgs e)
        {
            if (form2 == null || form2.IsDisposed)
            {
                form2 = new Form();
                form2.Show();
            }
            else
            {
                form2.WindowState = FormWindowState.Normal;
                form2.Activate();
                form2.Show();
            }
        }

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打开一个既存的窗体， 比如说是FORM3.

        private Form3 form3;

        private void button2_Click(object sender, EventArgs e)
        {
            form3 = new Form3();
            form3.Show();
        }