Question

I have this code for adding selected items from one ListBox to another. How can I prevent the user from adding an item twice? I want the ListBox they are adding to lstBoxToUserProjects to only contain distinct items with no duplicate entries.

protected void btnAddSelectedItem_Click(object sender, EventArgs e)
{
    List<ListItem> itemsToAdd= new List<ListItem>();

    foreach (ListItem listItem in lstbxFromUserProjects.Items)
    {
        if (listItem.Selected)
            itemsToAdd.Add(listItem);
    }

    foreach (ListItem listItem in itemsToAdd)
    {
        lstBoxToUserProjects.Items.Add(listItem);
    }
}

EDIT:
Here’s what I ended up using

protected void btnAddSelectedItem_Click(object sender, EventArgs e)
{
    List<ListItem> itemsToAdd= new List<ListItem>();

    foreach (ListItem listItem in lstbxFromUserProjects.Items)
    {
        if (listItem.Selected)
            itemsToAdd.Add(listItem);
    }

    foreach (ListItem listItem in itemsToAdd)
    {

        if (!lstBoxToUserProjects.Items.Contains(listItem)) 
        {
            lstBoxToUserProjects.Items.Add(listItem);
        }
    }
}

Answer 1

If you bind the lstBoxToUserProjects list box to a datasource (HashSet) then you could do a simple check to see if the item proposed for selection was already in the destination:

foreach(ListItem itemToAdd in itemsToAdd)
{
    if (selectedItems.Contains(itemToAdd)) continue;
    lstBoxToUserProjects.Items.Add(itemToAdd);
}

Note I’m proposing a HashSet because then you can do a performant check on the set whereas a List would have to be enumerated to check for a match.