List<int> lst1 = new List<int>(){1,2,3,4,5}; List<int> lst2 = new List<int>(){6,7,8,9}; foreach(var i in lst2){ lst1.Add(i); } // lst1 : {1,2,3,4,5,6,7,8,9} // lst2 : {6,7,8,9}