求助NullReferenceException

//string[] s = { "","","","","","",""};
            string[] s = null;
            lstSource.Items.CopyTo(s, 0);
            lstSource.Items.Clear();
            lstTarget.Items.AddRange(s);
我写了一段2个LISTBOX互选的小代码,这是里面全部移动的一段,string[] s = null;这句老是报NullReferenceException,于是我改成string[] s = { "","","","","","",""};但是如果以后比如要是加了几项又要改代码了,有什么好办法吗?