列表框中选择asp.net问题 [英] list box asp.net selected problem

问题描述

当我从kategorije选择上午项目，新项目被装载到SUB_kategorije，但是当我选择SUB_kategorije项目，当我点击链接它表明我这个错误：

When I choose am item from "kategorije", new items are loaded into "SUB_kategorije", but when I choose the item from SUB_kategorije and when i click on button it shows me this error:

对象引用未设置到对象的实例。

Object reference not set to an instance of an object.

Line 101: kom.Parameters.Add("@podkategorija", SqlDbType.Text).Value =
          SUB_kategorije.SelectedItem.ToString();

这是我的源...

dod_pit.ascx

dod_pit.ascx.cs

推荐答案

问题是你的的Page_Load 。将其更改为：

The problem is your Page_Load. Change it to:

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        SqlDataSource df = new SqlDataSource();
        df.ConnectionString = conn;
        df.SelectCommand = "SELECT [ID], [Kategorije] FROM [kategorije] ";
        kategorije.DataSource = df;
        kategorije.DataTextField = "Kategorije";
        kategorije.DataValueField = "ID";
        kategorije.DataBind();
    }         
}

问题是你重新绑定 kategorije 每次你的code做了回发时间，所以在时间的button1_Click 运行时， kategorije 已重新绑定和重置，让的SelectedValue 为null。

The problem is your rebinding kategorije every time your code does a postback so by the time Button1_Click runs, kategorije has been rebinding and reset so the SelectedValue is null.

当回传发生在的Page_Load code第一次运行，那么的button1_Click 之后运行。移动该code到！的IsPostBack 检查将导致它只能运行在第一时间你的页面加载，而不是之后再次让的SelectedValue 将可用。

When a postback happens your Page_Load code runs first, then the Button1_Click runs right after. Moving that code into a !IsPostBack check will cause it to only run the first time your page loads and not again after so the SelectedValue will be available.

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