在MDI窗口中, 生成子窗口时, 有些子窗口只想让它某一时刻只生成一个实例
改变类的结构来实现
public partial class winform1 : Form
{
        public static winform1 create;
        private winform1()
        {
            InitializeComponent();
        }
        public static winform1 Create
        {
            get
            {
                if (create == null)
                {
                    create = new winform1();
                }
                return create;
            }

        }
        
}