1: foreach (string s in Enumerable.Empty<string>())

 2: Console.Write(s); // <nothing>

 1: int[][] numbers =

 2: {

 3: new int[] { 1, 2, 3 },

 4: new int[] { 4, 5, 6 },

 5: null // null会导致查询失败.

 6: };

 7:

 8: IEnumerable<int> flat =

 9: numbers.SelectMany (innerArray =>innerArray);

 1: IEnumerable<int> flat = numbers

 2: .SelectMany (innerArray =>

 3: innerArray ?? Enumerable.Empty <int>() );

 4:

 5: foreach (int i in flat)

 6: Console.Write (i + " "); // 1 2 3 4 5 6

 1: foreach (int i in Enumerable.Range (5,5))

 2: Console.Write (i + " "); // 5 6 7 8 9

 1: foreach (int i in Enumerable.Repeat (5,3))

 2: Console.Write (i + " "); // 5 5 5