LINQ 学习路程 -- 查询操作 where

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 = , StudentName = "John", Age = } ,
        new Student() { StudentID = , StudentName = "Moin",  Age =  } ,
        new Student() { StudentID = , StudentName = "Bill",  Age =  } ,
        new Student() { StudentID = , StudentName = "Ram" , Age = } ,
        new Student() { StudentID = , StudentName = "Ron" , Age =  }
    };

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

方式2

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

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 >  && stud.Age < ;
}

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 = , StudentName = "John", Age =  } ,
        new Student() { StudentID = , StudentName = "Steve",  Age =  } ,
        new Student() { StudentID = , StudentName = "Bill",  Age =  } ,
        new Student() { StudentID = , StudentName = "Ram" , Age =  } ,
        new Student() { StudentID = , StudentName = "Ron" , Age =  }
    };

var filteredResult = studentList.Where((s, i) => {
            if(i %  ==  ) // 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 >
                    where s.Age <
                    select s;

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

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