Linq 学习(3) 语法结构

Linq 语法结构

(来自Linq in action)

举个例子实现：
查询以a开头的字符串，按降序输出。
C#代码：

string[] list = { "adobe", "excel", "acer", "query", "async", "xianfen", "apple", "amazon", "player", "best", "wow" };

var result = from word in list
             where word.StartsWith("a")
             orderby word descending
             select word;

result.ToList().ForEach(str => Console.WriteLine(str));

// 输出：
// async
// apple
// amazon
// adobe
// acer

等价的面向对象语法为：

var result = list.Where(str => str.StartsWith("a"))
    .OrderByDescending(str => { return str; });

这里 Where 和 OrderByDescending都是IEnumerable<T>的扩展方法，因为数组类型实现了该接口，所以可以直接调用这些这些扩展方法。
这些扩展方法的实现是在Enumerable类型中实现的，因此，更直接的是调用Enumerable类型的静态方法方法：

var result =
    Enumerable.OrderByDescending(
    Enumerable.Where(
    list, str => str.StartsWith("a")), str => { return str; }
    );

从这些逆向演化可以看出，查询语法就是一种"语法糖"。前面提到过，编译后运行时版本还是2.0，从这可以看出一些端倪。如果想继续深入了解该实现可以用ILDASM查看IL源代码和反射类库源代码。
result类型为IEnumerable<TElement>或派生自IEnumerable<TElement>接口的类型。

与 Linq 相关的程序集

System.Linq 命名空间在System.Core.dll中，接口类型及扩展方法的定义在该命名空间中，如：
Enumerable
IGrouping<TKey, TElement>
ILookup<TKey, TElement>
IQueryable<T>
IOrderedQueryable<T>
Expressions
...

System.Data.DataSetExtensions.dll 包含 Linq to DataSet 的相关类型的定义。
System.Data.Linq.dll 包含 Linq to SQL 的相关类型的定义。
System.Xml.Linq.dll 包含 Linq to XML 的相关类型的定义。

全程数据

为了下面介绍方便，构造以下，
数据结构图如下：


数据定义如下：
public class Student
{
    public int StudentID { get; set; }
    public string Name { get; set; }

    override public string ToString()
    {
        return string.Format("Student ID:{0},Student Name:{1}", StudentID, Name);
    }
}

public class Course
{
    public int CourseID { get; set; }
    public string CourseName { get; set; }

    override public string ToString()
    {
        return string.Format("Course ID:{0},Course Name:{1}", CourseID, CourseName);
    }
}

public class Score
{
    public int StudentID { get; set; }
    public int CourseID { get; set; }
    public double Value { get; set; }

    override public string ToString()
    {
        return string.Format("Student ID:{0},Course ID:{1},Score:{2}", StudentID, CourseID, Value);
    }
}

public class DataSource
{
    public static List<Student> Students { get; private set; }
    public static List<Student> Students2 { get; private set; }
    public static List<Score> Scores { get; private set; }
    public static List<Course> Courses { get; private set; }

    static DataSource()
    {
        Students = new List<Student>
        {
            new Student{ StudentID=1, Name="Andy" },
            new Student{ StudentID=2, Name="Bill" },
            new Student{ StudentID=3, Name="Cindy" },
            new Student{ StudentID=4, Name="Dark" }
        };

        Students2 = new List<Student>
        {
            new Student{ StudentID=5, Name="Erik" },
            new Student{ StudentID=6, Name="Frank" }
        };

        Courses = new List<Course>
        {
            new Course { CourseID = 1, CourseName = "C Language" },
            new Course { CourseID = 2, CourseName = "Biophysics" },
            new Course { CourseID = 3, CourseName = "Fundamentals of Compiling" },
            new Course { CourseID = 4, CourseName = "Finance" },
            new Course { CourseID = 5, CourseName = "College English" }
        };

        Scores = new List<Score>
        {
            new Score { StudentID = 1, CourseID = 1, Value = 78 },
            new Score { StudentID = 1, CourseID = 2, Value = 60 },
            new Score { StudentID = 1, CourseID = 3, Value = 92 },
            new Score { StudentID = 1, CourseID = 4, Value = 85 },
            new Score { StudentID = 1, CourseID = 5, Value = 55.5 },
            new Score { StudentID = 2, CourseID = 1, Value = 59 },
            new Score { StudentID = 2, CourseID = 2, Value = 78 },
            new Score { StudentID = 2, CourseID = 3, Value = 86 },
            new Score { StudentID = 3, CourseID = 2, Value = 60 },
            new Score { StudentID = 4, CourseID = 4, Value = 96 },
            new Score { StudentID = 5, CourseID = 5, Value = 78 }
        };
    }
}

后续知识点将使用这里定义的数据进行解析。