如何在 ASP.NET 中获取列表框的选定项？

Question

I use a listbox to view some record of my database as an string, but I have two problems:

1. the selection change does not invoke with double click or single click, it only invokes when you select an item and press return.
2. even when the selection changed invokes, the selectedIndex returns -1 and selected value is null.

ASP code:

<form runat="server">
    <div class="clearfix">
        <asp:ListBox runat="server" ID="g1SeasonsListView" CssClass="adminSeasonsListView"  OnSelectedIndexChanged="g1SeasonsListView_SelectedIndexChanged" />
        <asp:ListBox runat="server" ID="g2SeasonsListView" CssClass="adminSeasonsListView" />
        <asp:ListBox runat="server" ID="g3SeasonsListView" CssClass="adminSeasonsListView" />
    </div>
    <div class="clearfix">
        <asp:Button ID="g1DeleteBtn" Text="Delete" runat="server" OnClick="g1DeleteBtn_Click" CssClass="adminDeleteBtn" />
        <asp:Button ID="g2DeleteBtn" Text="Delete" runat="server" OnClick="g2DeleteBtn_Click" CssClass="adminDeleteBtn" />
        <asp:Button ID="g3DeleteBtn" Text="Delete" runat="server" OnClick="g3DeleteBtn_Click" CssClass="adminDeleteBtn" />
    </div>
</form>

The CS Code, which I checked via watch window in VS2013; there is nothing special in it:

protected void g1DeleteBtn_Click(object sender, EventArgs e)
{    
}
protected void g2DeleteBtn_Click(object sender, EventArgs e)
{
}
protected void g3DeleteBtn_Click(object sender, EventArgs e)
{    
}
protected void g1SeasonsListView_SelectedIndexChanged(object sender, EventArgs e)
{    
}

and when I want to update listboxes, I use this code:

private void UpdateListeView()
{
    List<string> seasonsListTemp = null;

    g1SeasonsListView.Items.Clear();
    seasonsListTemp = (from s in database.Gs where s.grade == 1 select s.season).ToList();
    for (int i = 0; i < seasonsListTemp.Count; i++)
        g1SeasonsListView.Items.Add(seasonsListTemp[i]);

    g2SeasonsListView.Items.Clear();
    seasonsListTemp = (from s in database.Gs where s.grade == 2 select s.season).ToList();
    for (int i = 0; i < seasonsListTemp.Count; i++)
        g2SeasonsListView.Items.Add(seasonsListTemp[i]);

    g3SeasonsListView.Items.Clear();
    seasonsListTemp = (from s in database.Gs where s.grade == 3 select s.season).ToList();
    for (int i = 0; i < seasonsListTemp.Count; i++)
        g3SeasonsListView.Items.Add(seasonsListTemp[i]);
}

I even tried to get the selected index when I press the delete button, but the result are the same.

Answer 1

Bind your code in a !Page.IsPostBack block. That will ensure that code does not executes on postback. If you bind the ListView on PostBack you will loose the state of the GridView that includes the event being triggered, SelectedIndex etc.

Page.IsPostBack Property

Gets a value that indicates whether the page is being rendered for the first time or is being loaded in response to a postback.

if(!Page.IsPostBack)
{
     UpdateListeView();
}

Usually the controls are bound in Page_Load method and you probably need to check Page.IsPostBack there.