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LINQ 关键字

news 来源：原创 2024/5/2 6:58:56

排序

  IEnumerable<Student> studentQuery =
                        from student in students
                        where student.Scores[0] > 90
                        orderby student.Last ascending
                        select student;

ascending 升序排序，descending降序排序

分组

         var studentQuery3 =
                from student in students
                group student by student.Last[0];

  var studentQuery2 =
                from student in students
                group student by student.Last[0] into studentGroup
                orderby studentGroup.Key
                select studentGroup;

当运行此查询时，将爱那个会按照字母顺序排序。

使用let引入标识符

可以使用let关键字将任何表达式结果作为标识符引入到查询表达式。

在查询表达式中，存储子表达式的结果有时候很有用，这样可以在随后的子句中使用。可以使用let关键字完成这一工作。该关键字可以创建一个新的范围变量，并且用你提供的表达式的结果初始化该变量。一旦用值初始化了该范围变量，他就不能用于存储其他值。

可以通过存储表达式的结果提高性能。

           var studentQuery5 =
                from student in students
                let totalScore = student.Scores[0] + student.Scores[1] 
                            + student.Scores[2] + student.Scores[3]
                where totalScore / 4 < student.Scores[0]
                select student.Last + "  " + student.First;
            foreach (string s in studentQuery5)
            {
                Console.WriteLine(s);
            }

在查询表达式中使用方法语法

           var studentQuery6 =
                from student in students
                let totalScore = student.Scores[0] + student.Scores[1] +
                                student.Scores[2] + student.Scores[3]
                select totalScore;
            double averageScore = studentQuery6.Average();
            Console.WriteLine("classs average score = {0}", averageScore);

通过average()方法计算班级平均分。

在select 子句中转换或投影

            var studentQuery7 =
                from student in students
                let x = student.Scores[0] + student.Scores[1] +
                        student.Scores[2] + student.Scores[3]
                where x > averageScore
                select new { id = student.ID, score = x };

            foreach (var item in studentQuery7)
            {
                Console.WriteLine("Student ID :{0},Score:{1}",item.id,item.score);
            }