C#/ASP .NET - Gain Item value with AutoPostBack and OnIndexChange

Question

So I'm not really familiar with C# and I am trying to learn the basics, but it's still all vague to me. What I'm trying to do this: when I select one item in a Listbox it will get that selected items value and put it into a textbox (this is just for testing when i want to use the value for MSSSQL).

So here is my markup (.aspx code): <asp:ListBox ID="Listbox1" runat="server" AutoPostBack="True" onselectedindexchanged="lstArtiesten_SelectedIndexChanged"></asp:ListBox>

And here is my code-behind (.aspx.cs code):

protected void Listbox1_SelectedIndexChanged(object sender, EventArgs e)
{
    txtArtName.Text = Listbox1.SelectedItem.Text;
} 

As you can see AutoPostBack is on. I know whenever I click on one of the items it is redirected to the "protected void Page_Load(object sender, EventArgs e) {}", however when I try to do it without AutoPostBack, then it won't send my selected information. I'd like to use AutoPostBack, but I am not sure when or when not to use it).

My problem is when I select my item the page will be refreshed and my selected value will be lost, thus a null as result...

I've tryed working with the if(isPostBack){ }, but failed to understand the structure of C#.

I hope somebody could help me with this. Thanks in advance! Kind regards, Nkmol

Answer 1

Apologies if I have misunderstoood the question, but from the sounds of it, what's happening is that when you change the value in the Listbox, this causes a postback which in turn is running the code you use to populate the listbox initially. (I assume that you are doing this in code-behind since I don't see anything in your markup that is specifying the Items in the list)

So given this markup:

 <asp:ListBox ID="Listbox1" runat="server" AutoPostBack="True" 
     onselectedindexchanged="lstArtiesten_SelectedIndexChanged"></asp:ListBox>
     <asp:TextBox runat="server" id="txtArtName"></asp:TextBox>

your code-behind should look something like this:

protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            //only call initialise during initial page load.
            initialise();
        }
    }

    protected void initialise()
    {
        //add some items to the list
        Listbox1.Items.Add("Something1");
        Listbox1.Items.Add("Something2");
        Listbox1.Items.Add("Something3");
        Listbox1.Items.Add("Something4");
        Listbox1.SelectedIndex = 0; //select the first item in the list, or whatever
    }

    protected void lstArtiesten_SelectedIndexChanged(object sender, EventArgs e)
    {
        txtArtName.Text = Listbox1.SelectedItem.Text;
    }

Note how the initialise method is only called if we're not in Postback -i.e the initial page load. By checking for that state and not re-loading the Listbox, we allow ViewState to work it's [terrible, dark, evil] magic and restore the selection you made on the Listbox control.

Hope that helps.

Answer 2

Are you familiar with the concept of ViewState? This is what allows ASP.NET controls to "remember" their values. I would recommend that you visit this tutorial and then, if you are still having issues, then ask/re-ask your question.

ASP.NET ViewState Tutorial