cid = Int32.Parse(Request.Params["cid"].ToString());

cid 没有传参数时候 就错误.,传了cid参数 就正常了  为什么呢
我想不传CID参数和  传都可以这样的

public partial class standards_index : System.Web.UI.Page
{
    public int cid = 2;
    int teid;
    DataCommonProvider dp = DataCommonProvider.Instance();
    protected void Page_Load(object sender, EventArgs e)
    {
            cid = Int32.Parse(Request.Params["cid"].ToString());
        if (!IsPostBack)
        {
            DataBind();
            StandsDataBind();
        }
        if (cid ==2)
        {
            teid = 43;
        }
        else
        {
            teid = 44;
        }
    }

Params["cid"].ToString()
NULL强转string类型所以报错了

先判断一下取的值是否为空，然后在其他动作！
 public int cid ;
string shifouweikong；
shifouweikong = Request.QueryString["cid"];
if(shifouweikong != null)
{
cid = Int32.Parse(Request.Params["cid"].ToString());
        if (!IsPostBack)
        {
            DataBind();
            StandsDataBind();
        }
        if (cid ==2)
        {
            teid = 43;
        }
        else
        {
            teid = 44;
        }

}

protected int CID
{
    get
    {
        int temp = 0;
        if(!int.TryParse(Request.Params["cid"], out temp))
        {
            //做一些操作，提示输入的数据不正确
        }
        return temp;
    }
}

如果多次用到CID的话，建议这样：

private int _CID = 0;
protected int CID
{
    get
    {
        if(_CID == 0 && !int.TryParse(Request.Params["cid"], out _CID))
        {
            //做一些操作，提示输入的数据不正确
        }
        return _CID;
    }
}