ListItem myListItem = new ListItem(Delete.Text.ToLower(),Delete.Text.ToLower());
// Check whether the 'ListItem' is present in the 'ListBox' or not.
if(ItemCollection.Contains(myListItem))
{
   String deleteString=Delete.Text;
   // Delete the listitem entered by the user in textfield.
   ItemCollection.Remove(deleteString.ToLower());
   Message.Text="<font color='green'><b>Deleted Successfully</b></font>";
}
else
{
  Message.Text="<font color='red'><b>No ListItem with the given name is present in the ListBox for deletion.</b></font>";
}

//Set the SelectedIndex to -1 so no items are selected.
// The new item will be set as the selected item when it is added.
DropDownList1.SelectedIndex = -1;
// Add the selected item to DropDownList1.
DropDownList1.Items.Add(ListBox1.SelectedItem);
// Delete the selected item from ListBox1.
ListBox1.Items.Remove(ListBox1.SelectedItem);