LINQ 学习路程 -- 查询操作 where

Posted 2020-09-06 蓝平凡

1.where

　　

Filtering Operators	Description
Where	Returns values from the collection based on a predicate function
OfType	Returns values from the collection based on a specified type. However, it will depend on their ability to cast to a specified type.

public static IEnumerable<TSource> Where<TSource>(this IEnumerable<TSource> source, 
                                                  Func<TSource, bool> predicate);

public static IEnumerable<TSource> Where<TSource>(this IEnumerable<TSource> source, 
                                                  Func<TSource, int, bool> predicate);

IList<Student> studentList = new List<Student>() { 
        new Student() { StudentID = 1, StudentName = "John", Age = 13} ,
        new Student() { StudentID = 2, StudentName = "Moin",  Age = 21 } ,
        new Student() { StudentID = 3, StudentName = "Bill",  Age = 18 } ,
        new Student() { StudentID = 4, StudentName = "Ram" , Age = 20} ,
        new Student() { StudentID = 5, StudentName = "Ron" , Age = 15 } 
    };


var filteredResult = from s in studentList
                    where s.Age > 12 && s.Age < 20
                    select s.StudentName;

 

方式2

Func<Student,bool> isTeenAger = delegate(Student s) { 
                                    return s.Age > 12 && s.Age < 20; 
                                };

var filteredResult = from s in studentList
                     where isTeenAger(s)
                     select s;

 

方式3

public static void Main()
{
    var filteredResult = from s in studentList
                         where isTeenAger(s)
                         select s;
}

public static bool IsTeenAger(Student stud)
{
    return stud.Age > 12 && stud.Age < 20;  
}

 

where的第二个扩展方法包含集合的index索引

IList<Student> studentList = new List<Student>() { 
        new Student() { StudentID = 1, StudentName = "John", Age = 18 } ,
        new Student() { StudentID = 2, StudentName = "Steve",  Age = 15 } ,
        new Student() { StudentID = 3, StudentName = "Bill",  Age = 25 } ,
        new Student() { StudentID = 4, StudentName = "Ram" , Age = 20 } ,
        new Student() { StudentID = 5, StudentName = "Ron" , Age = 19 } 
    };

var filteredResult = studentList.Where((s, i) => { 
            if(i % 2 ==  0) // if it is even element
                return true;
                
        return false;
    });

foreach (var std in filteredResult)
        Console.WriteLine(std.StudentName);

 

 

多个where从句

var filteredResult = from s in studentList
                    where s.Age > 12
                    where s.Age < 20
                    select s;

var filteredResult = studentList.Where(s => s.Age > 12).Where(s => s.Age < 20);

 

需要记住的几点：

1.Where根据特定条件来筛选集合元素

2.where扩展方法有2个重载，使用第二个重载方法可以知道当前元素在集合中的索引位置

3.方法语法需要整个lambda表达式，而查询语法仅仅需要表达式主体

4.在单一的LINQ查询中可以使用多个where从句