判断dockpanel 避免重复加载同一个子窗体

　　　　UsersForm uf = null;
            foreach (DockContent frm in this.dockPanel1.Contents)
            {
                if (frm is UsersForm)
                {
                    frm.Activate(); //激活子窗体
                    return;
                }
            }
            uf = new UsersForm();
            uf.Show(dockPanel1);