例子
var numbers = new[] {1,2,3,4,5};
var firstNumber = numbers.FirstOrDefault();
Console.WriteLine(firstNumber); //1
var firstEvenNumber = numbers.FirstOrDefault(n => (n & 1) == 0);
Console.WriteLine(firstEvenNumber); //2
var firstNegativeNumber = numbers.FirstOrDefault(n => n < 0);
Console.WriteLine(firstNegativeNumber); //0
var words = new[] { "one", "two", "three", "four", "five" };
var firstWord = words.FirstOrDefault();
Console.WriteLine(firstWord); //一
var firstLongWord = words.FirstOrDefault(w =>w.Length> 3);
Console.WriteLine(firstLongWord); //三
var firstMissingWord = words.FirstOrDefault(w =>w.Length> 5);
Console.WriteLine(firstMissingWord); //空值
