from

var lowNums = from num in numbers
            where num < 5
            select num;

numbers 是数据源，而 num 是范围变量。             请注意，这两个变量都是强类型

List<Student> students = new List<Student>
        {
           new Student {LastName="Omelchenko", Scores= new List<int> {97, 72, 81, 60}},
           new Student {LastName="O‘Donnell", Scores= new List<int> {75, 84, 91, 39}},
           new Student {LastName="Mortensen", Scores= new List<int> {88, 94, 65, 85}},
           new Student {LastName="Garcia", Scores= new List<int> {97, 89, 85, 82}},
           new Student {LastName="Beebe", Scores= new List<int> {35, 72, 91, 70}}
        };       

        // Use a compound from to access the inner sequence within each element.
        // Note the similarity to a nested foreach statement.
        var scoreQuery = from student in students
                         from score in student.Scores
                            where score > 90
                            select new { Last = student.LastName, score };
where

条件

where    子句用在查询表达式中，用于指定将在查询表达式中返回数据源中的哪些元素。            

它将一个布尔条件（“谓词”）应用于每个源元素（由范围变量引用），并返回满足指定条件的元素。  一个查询表达式可以包含多个 where 子句，一个子句可以包含多个谓词子表达式。

Linq 关键字