// 查询语法
            IEnumerable<int> filteringQuery =
                from num in numbers
                where num < 3 || num > 7
                select num;

  　　　　　　// 方法语法.
            IEnumerable<int> largeNumbersQuery = numbers2.Where(c => c > 15); 

　　　　　　　//混合语法
            int numCount1 =
                (from num in numbers1
                 where num < 3 || num > 7
                 select num).Count();

            // Better: Create a new variable to store
            // the method call result
            IEnumerable<int> numbersQuery =
                from num in numbers1
                where num < 3 || num > 7
                select num;

            int numCount2 = numbersQuery.Count();

  　　　　　　　　// QueryMethhod1 returns a query as its value.
                IEnumerable<string> QueryMethod1(ref int[] ints)
                {
                    var intsToStrings = from i in ints
                                        where i > 4
                                        select i.ToString();
                    return intsToStrings;
                }

                // QueryMethod2 returns a query as the value of parameter returnQ.
                void QueryMethod2(ref int[] ints, out IEnumerable<string> returnQ)
                {
                    var intsToStrings = from i in ints
                                        where i < 4
                                        select i.ToString();
                    returnQ = intsToStrings;
                }

  　　　　static List<int> numbers = new List<int>() { 1, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20 };
        static void Main()
        {
            IEnumerable<int> queryFactorsOfFour =
                from num in numbers
                where num % 4 == 0
                select num;

            // Store the results in a new variable
            // without executing a foreach loop.
            List<int> factorsofFourList = queryFactorsOfFour.ToList();
        }    

//单个属性作为组键对源元素进行分组
 var queryLastNames =
                from student in students
                group student by student.LastName into newGroup
                orderby newGroup.Key
                select newGroup;

通过使用除对象属性以外的某个项作为组键对源元素进行分组, 在此示例中，键是学生姓氏的第一个字母
  var queryFirstLetters =
                from student in students
                group student by student.LastName[0];

/*
通过使用某个数值范围作为组键对源元素进行分组。 然后，查询将结果投影到一个匿名类型中，该类型仅包含学生的名字和姓氏以及该学生所属的百分点范围。 使用匿名类型的原因是没有必要使用完整的 Student 对象来显示结果。*/
Helper method, used in GroupByRange.
        protected static int GetPercentile(Student s)
        {
            double avg = s.ExamScores.Average();
            return avg > 0 ? (int)avg / 10 : 0;
        }

  var queryNumericRange =
                from student in students
                let percentile = GetPercentile(student)
                group new { student.FirstName, student.LastName } by percentile into percentGroup
                orderby percentGroup.Key
                select percentGroup;

//通过使用布尔比较表达式对源元素进行分组
  var queryGroupByAverages = from student in students
                                       group new { student.FirstName, student.LastName }
                                            by student.ExamScores.Average() > 75 into studentGroup
                                       select studentGroup;

/*
下面的示例演示如何使用匿名类型来封装包含多个值的键。 在此示例中，第一个键值是学生姓氏的第一个字母。 第二个键值是一个布尔值，它指定该学生在第一次考试中的得分是否超过了 85。 可以按照该键中的任何属性对多组值进行排序。*/
 var queryHighScoreGroups =
                from student in students
                group student by new { FirstLetter = student.LastName[0], 
                    Score = student.ExamScores[0] > 85 } into studentGroup
                orderby studentGroup.Key.FirstLetter
                select studentGroup;