Iterate through all items:

  foreach(ListItem item in ListBox1.Items)
    {
       if (item.Selected == True)
       {
          //item.Value //no to be confused with text
          //item.Text //text displayed on UI
       }
    }

Find specific item text:

  private string GetSelectedItem(ListBox container)
        {
            //string selecteditem = "";
            for (int i = 0; i < container.Items.Count; i++)
            {
                if (container.Items[i].Selected == true)
                {
                    //use .value alternatively if you are trying to get value instead of text
                    return ListBox1.Items[i].Text; //breaks the loop or
                    //break; //if you decide to use without a function
                }
            }
            return ""; //error handled in calling func, remove return value if used without a function
            //return selecteditem ; //if you want to iterate all items anyway
        }

More generalized for re-usability to get text or value, though a little less readable and alot more code:

private static class GetSelectedItem {
private enum ListBox_ContentType {
Text,
Value
}

public string Text(ListBox container) { Get_ContentType(container,ListBox_ContentType.Text);}
public string Value(ListBox container) { Get_ContentType(container,ListBox_ContentType.Value);}

private string Get_ContentType(ListBox container, ListBox_ContentType ContentType) {
  for (int i = 0; i < container.Items.Count; i++)
            {
                if (container.Items[i].Selected == true)
                {
if (ContentType == ListBox_ContentType.Value) {
return ListBox1.Items[i].Text;
}
else //ContentType == ListBox_ContentType.Value 
{
                    return ListBox1.Items[i].Value;
}
                }
            }
            return "";
}
}