ListBox没有获取选定的项目

Question

I have a ListBox which I am adding ListItems to in a codebehind. The problem I'm having is the ListBox is not seeing the selected items. I have the ListBox being populated dynamically depending on what the user selects from a DropDownList, so the DropDownList has AutoPostBack set to true. I think this is somehow causing the problem.

My SelectedIndexChanged method, which is used whenever an item in the DropDownList is selected, calls a method called PopulateListBox . Here's what those methods looks like:

protected void SelectedIndexChanged(object sender, EventArgs e)
{
    string typeStr = type.SelectedItem.Text;
    MyType = Api.GetType(typeStr);
    PopulateListBox();
}

private void PopulateListBox()
{
    listbox.Items.Clear();
    foreach (PropertyInfo info in MyType.GetProperties())
        listbox.Items.Add(new ListItem(info.Name));
}

For what it's worth, here are the DropDownList and ListBox:

<asp:DropDownList runat="server" ID="type" width="281px" OnSelectedIndexChanged="SelectedIndexChanged" AutoPostBack="true" />

<asp:ListBox runat="server" ID="listbox" width="281px" height="200px" selectionmode="Multiple" />

What I am trying to do is add a List of strings (strings being the selected items) as a session variable upon clicking a submit button. The button redirects to a new page after the List has been added to the session. Going through in debugger, the List of strings is empty at the point where I add it to the session.

listbox.GetSelectedIndices() returns nothing.

Update

I can access the selected items if I do not make a change in the DropDownList. The ListBox is initially populated on page load, and if I make selections they are recognized. If I select something from the DropDownList and the ListBox is repopulated, the selections are not recognized.

My Page_Load method does only two things. It initializes my Api variable and calls PopulateDropDown , which looks like this:

private void PopulateDropDown()
{
    foreach (Type t in Api.GetAllTypes())
        type.Items.Add(new ListItem(t.Name));
    string typeStr = type.Items[0].Text;
    Type = Api.GetType(typeStr);
    PopulateListBox();
}

Answer 1

The problem is that you call PopulateDropDown() on every single Page_Load() , which calls PopulateListBox() , which clears the listbox and repopulates it. By clearing the listbox, you clear the selection.

You need to replace your call to PopulateDropDown() in the Page_Load() with the following code. The issue that I think you don't realize is that the page is loaded on every postback -- and that in the page life cycle, the page load occurs before the event. So by selecting a drop down item, you execute the Page_Load() event first (which indirectly executes the LoadListBox method, clearing the selection). The following code will populate the drop down list the first time the page loads only . Keep it the same wherever else you are using the load dropdown method:

protected void Page_Load(object sender, EventArgs e)
{
    // Do your API code here unless you want it to occur only the first
    // time the page loads, in which case put it in the IF statement below.
    if (!IsPostBack)
    {
        PopulateDropDown();
    }
}

The IsPostBack returns a boolean indicating whether the server side code is running because the page is loading for the first time ("false") or as a post back ("true").

As I said elsewhere, keep in mind that a listbox with potential for multiple selected values must be handled differently than one with potential for a single selection. Don't reference listbox.SelectedItem , but rather:

foreach (ListItem item in lbFullNames)
{
    if (item.Selected)
    {
        // TODO: Whatever you are doing with a selected item.
    }
}

Answer 2

I have also found that if you disable the ListBox server-side, then use client side code to enable the list box using code like the following, then you cannot get the selected items server side.

$('.css-class-assigned-to-listbox').attr('disabled', '');

The fix is simply to make sure it is enabled server-side (the default), then disable it (see blow) or enable it (see above) using client-side code.

$('.css-class-assigned-to-listbox').attr('disabled', 'disabled');