Unable to cast CheckBoxList.Items to a ListItem - Type or namespace could not be found

Question

I want to concatenate the selected value of a CheckedListBox to a single comma-separated string and store it to a database, but this code gives me an error:

var items = from member in checkedListBox1.Items.Cast<ListItem>()
            where member.Selected == true
            select member.Value;

string mylist = String.Join(",", items);

Error:

The type or namespace name 'ListItem' could not be found (are you missing a using directive or an assembly reference?)

Answer 1

You're mixing platforms. CheckedListBox is a WinForms control, but ListItem is a web control.

That's why it's displaying that error - you're not referencing System.Web and you don't have using System.Web.UI.WebControls; at the top of your file. And even if you had all that, it wouldn't be the right thing to do anyway.

---

The CheckedListBox control already keeps track of checked and selected items for you.

To retrieve the checked items, use CheckedItems and cast the collection back to the type of data you populated it with originally:

var checkedItems = checkedListBox1.CheckedItems.Cast<YourClass>();

If you actually wanted selected items (probably not), use SelectedItems instead:

var selectedItems = checkedListBox1.SelectedItems.Cast<YourClass>();

After casting the items correctly, the last part you have should work just fine (assuming it's a collection of strings or, if you're using your own class, that it overrides ToString()):

string myFlattenedList = String.Join(",", checkedItems);