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Single()严格返回一个元素项。如果返回的元素集合为空，或是返回的元素集合多于一个，Single()就会抛出异常。当程序需要严格要求返回一个元素时，不妨尝试用用Single()。

举例说明：

    public class Person
    {
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public Person() { }
        public Person(string f, string l) { FirstName = f; LastName = l; }
    }
    ...
    var somePeople = new List<Person>
    {
        new Person { FirstName = "Bill", LastName = "Gates"},
        new Person { FirstName = "Bill", LastName = "Wagner"},
        new Person { FirstName = "Bill", LastName = "Johnson"}
    };

    var answer = from p in somePeople
                 where p.FirstName == "John"
                 select p;
    // The following code throw an exception
    // var answer2 = (from p in somePeople
    //                where p.FirstName == "John"
    //                select p).Single();