Question

我的数据库中有3个表"tbl_User", "tbl_City", "tbl_Town"。

在"tbl_City"，我有

 - cityno int [PK],
 - cityname nvarchar(50)

区域。

在"tbl_Town"，我有

 - townno int,
 - townname nvarchar(50),
 - cityno int

区域。

在"tbl_User"，我有

- userid int [PK],
 - email nvarchar(50),
 - password nvarchar(50),
 - city int,
 - town int

区域。

当用户在网站上注册时，他必须选择城市和城镇。所以我可以将城市和城镇保存为"tbl_User"中的号码。我想要做的是：当用户转到“profile.aspx”时，我希望有选择地在DropDownList中看到城市和城镇。我的“profile.aspx.cs”代码如下：

private void GetCityAndTownSelectively()
{
    if (Session["userid"] != null)
    {
        DataRow dr = function.GetDataRow("SELECT tbl_City.cityno, tbl_City.cityname, tbl_Town.townno, tbl_Town.townname FROM tbl_User LEFT JOIN tbl_City on tbl_City.cityno = tbl_User.city LEFT JOIN tbl_Town on tbl_Town.townno = tbl_User.town WHERE userid=" + Session["userid"].ToString());
        if (dr == null)
        {
            Response.Redirect("default.aspx");
        }
        else
        {
            DrpDwnLstCity.ClearSelection();
            DrpDwnLstCity.Items.FindByValue(dr["cityno"].ToString()).Selected = true;
            DrpDwnLstTown.ClearSelection();
            DrpDwnLstTown.Items.FindByValue(dr["townno"].ToString()).Selected = true;
        }
    }
    else
    {
        Response.Redirect("default.aspx");
    }
}

我更改了我的代码，但我仍然收到错误：aytasarim.dll中发生了'System.NullReferenceException'类型的异常，但未在用户代码中处理。

Answer 1

您的Sql查询错误

SELECT tbl_City.cityname, tbl_Town.townname FROM tbl_User 
LEFT JOIN tbl_City on tbl_City.cityno = tbl_User.city 
LEFT JOIN tbl_Town on tbl_Town.townno = tbl_User.town 
WHERE userid="";

由于cityno表中没有tbl_User，并且您在1个表中具有值int的列上创建join，而在其他表中具有Nvarchar

Answer 2

尝试以下

private void GetCityAndTownSelectively()
{
    if (Session["userid"] != null)
    {
        DataRow dr = function.GetDataRow("SELECT tbl_City.cityno, tbl_City.cityname, tbl_Town.townno, tbl_Town.townname FROM tbl_User LEFT JOIN tbl_City on tbl_City.cityno = tbl_User.city LEFT JOIN tbl_Town on tbl_Town.townno = tbl_User.town WHERE userid=" + Session["userid"].ToString());
        if (dr == null)
        {
            Response.Redirect("default.aspx");
        }
        else
        {
            DrpDwnLstCity.ClearSelection();
            DrpDwnLstCity.Items.FindByValue(dr["cityno"].ToString()).Selected = true;
            DrpDwnLstTown.ClearSelection();
            DrpDwnLstTown.Items.FindByValue(dr["townno"].ToString()).Selected = true;
        }
    }
    else
    {
        Response.Redirect("default.aspx");
    }
}

我无法选择性地将数据带到DropDownList（System.NullReferenceException错误）

2 个答案: