1.一个应用程序只能被用户打开一次
 Process mobj_pro =Process.GetCurrentProcess();
            Process[] mobj_proList=Process.GetProcessesByName(mobj_pro.ProcessName);
            if(mobj_proList.Length>1)
            {
                MessageBox.Show("当前的应用程序已打开！", "系统提示", MessageBoxButtons.OK, MessageBoxIcon.Exclamation, MessageBoxDefaultButton.Button1);
                return;
            }
2.一个框架窗口下只打开一个子窗口
CustomerAdd pobj_CustomerAdd;  
            Form pobj_CustomerAdd_Return=CheckIsExit("CustomerAdd");
            if(pobj_CustomerAdd_Return==null)
            {
                pobj_CustomerAdd=new CustomerAdd();
                OpenSheet(pobj_CustomerAdd);
            }
            else
            {
                OpenSheet((CustomerAdd)pobj_CustomerAdd_Return);
            }  
void OpenSheet(Form pobj_form)
        { 
            pobj_form.MdiParent=this;
            pobj_form.Show(); 
        }

        /**//// <summary>
        /// 判断窗口是否存在
        /// </summary>
        /// <param name="ps_windowName">窗口的名称</param>
        /// <returns>存在返回此窗口的实例 不存在返回null</returns>
        Form CheckIsExit(string ps_windowName)
        {
            for(int i=0;i<this.MdiChildren.Length;i++)
            {
                if(this.MdiChildren[i].Name==ps_windowName)
                {
                    return this.MdiChildren[i];
                }
            }
            return null;
        }
3.弹出式窗口显示渐变效果
在页面上添加一个timer控件fadeTimer,interval