如何从asp.net的列表框中获取所选项目？

Question

I have to 2 listboxes in my application and i cant get selected item from the first listbox. It says null refference error.

protected void Page_Load(object sender, EventArgs e)
{

    ListBox1.Items.Clear();
    List<Lekarna> lekarne = service.pridobiLekarne().ToList();

    foreach (Lekarna a in lekarne)
    {
        ListBox1.Items.Add(a.ID + " | " + a.imeLekarne + " | " + a.Kraj + " | " + a.Država + Environment.NewLine);
    }

}


protected void btnPoisci_Click(object sender, EventArgs e)
{

    string a = ListBox1.SelectedItem.ToString();
    int c1 = int.Parse(a[0].ToString());


    List<Zdravilo> zdravila = service.vrniVsaZdravilaGledeNaLekarno(c1).ToList();
    ListBox2.Items.Clear();
    foreach (Zdravilo b in zdravila)
    {

        ListBox2.Items.Add(b.ID + " | " + b.imeZdravila + " | " + b.letoIzdaje + " | " + b.proizvajalec + Environment.NewLine);
    }
}

Answer 1

Problem with your code is you are re-binding the ListBox on every postback. So when you click button btnPoisci a postback happends which rebinds the listbox and when btnPoisci_Click method is called there is no selected item thus the below line will throw NRE:-

 string a = ListBox1.SelectedItem.ToString();

Don't add items in ListBox after every postback. Check if it is initial get request on PageLoad using IsPostBack property and only add it once:-

protected void Page_Load(object sender, EventArgs e)
{
  if(!IsPostBack)
  {
    ListBox1.Items.Clear();
    List<Lekarna> lekarne = service.pridobiLekarne().ToList();
    foreach (Lekarna a in lekarne)
    {
        ListBox1.Items.Add(a.ID + " | " + a.imeLekarne + " | " + a.Kraj + " | " 
                                + a.Država + Environment.NewLine);
    }
  }
}

Answer 2

You are clearing the items in the list on each page load which means you are clearing out the list before your event fires. When you do that you clear the state of the list. Try loading Items on Page init or check to see if it IsPostBack.

Answer 3

You must only load the values in your ListBox1 if it is not a postback. Right now your first line of code in your page load event is clearing everything and refreshing. Do this:

if (!IsPostBack) {
    ListBox1.Items.Clear();
    List<Lekarna> lekarne = service.pridobiLekarne().ToList();
}

This will only load the ListBox1 on the first time to the page, keeping the value of any selected item on subsequent postbacks.