示例
var numbers = new[] {2,4,6,8,1,3,5,7};
var numbersOrDefault = numbers.DefaultIfEmpty();
Console.WriteLine(numbers.SequenceEqual(numbersOrDefault)); //True
var noNumbers = new int[0];
var noNumbersOrDefault = noNumbers.DefaultIfEmpty();
Console.WriteLine(noNumbersOrDefault.Count()); //1
Console.WriteLine(noNumbersOrDefault.Single()); //0
var noNumbersOrExplicitDefault = noNumbers.DefaultIfEmpty(34);
Console.WriteLine(noNumbersOrExplicitDefault.Count()); //1
Console.WriteLine(noNumbersOrExplicitDefault.Single()); //34
