C# - Using a ListBox to populate other fields

Question

I'm trying to use the selected value of a ListBox control to populate a TextBox with its Text property, and a HiddenField with its value property. It sounds simple enough, and I went with this:

currentGroupTextBox.Text = currentSiteGroupList.SelectedItem.Text;
currentGroupHiddenField.Value = currentSiteGroupList.SelectedValue;

But upon execution, ASP.NET returns an error:

Object reference not set to an instance of an object.

And highlights the first line. currentGroupTextBox and currentGroupHiddenField are two controls which are enabled in the ASPX file so I'm not too sure why ASP.NET would complain about instancing them.

Answer 1

I'd be willing to bet that your first line is choking on referencing currentSiteGroupList.SelectedItem, as that seems the most likely candidate for being a null reference. Make sure your code is executing in the right place within the ASP.NET page lifecycle, such that the SelectedItem is set properly behind the scenes.

Answer 2

SelectedItem from your currentSiteGroupList.SelectedItem is likely to be null (what represents no selection). You need to test it before assigning it to currentGroupTextBox.Text

Answer 3

Is it currentGroupTextBox or currentGroupTextBox that is null? In debug if stop on that line..is it one or both that are not existing? A common issue I find is that controls are placed on an asp formview or similar and so the reference to that control is not actually its id/name, but more likely you need to do;

TextBox myTextBoxReference = (TextBox) formName.FindControl("currentGroupTextBox")
string theValue = myTextBoxReference.Text

Another often issue is the page life cycle. So if your object is not on a form but maybe you are referencing it before it exists in the postback. hth

Answer 4

I'm going to try and pull together answers to all of your questions, including those in the comments.

1. Even if SelectionMode="Single", the listbox starts out without anything being selected, unless you specify which item should be selected in your code.

2. To test if the SelectedItem is null, use the following code:

if (currentGroupSiteList.SelectedItem != null) {
    currentGroupTextBox.Text = currentSiteGroupList.SelectedItem.Text;
    currentGroupHiddenField.Value = currentSiteGroupList.SelectedValue;
}

1. What does your Page_Load code that loads the listbox look like? Is it wrapped with a if (!Page.IsPostBack) check? If not, pressing the button and initiating a postback will reload the listbox, thus losing the SelectedItem that the user selected.