LINQ 学习路程 -- 查询操作 GroupBy ToLookUp

Grouping Operators	Description
GroupBy	GroupBy操作返回根据一些键值进行分组，每组代表IGrouping<TKey,TElement>对象
ToLookup	ToLookup is the same as GroupBy; the only difference is the execution of GroupBy is deferred whereas ToLookup execution is immediate.

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

var groupedResult = from s in studentList
                    group s by s.Age;

//iterate each group        
foreach (var ageGroup in groupedResult)
{
    Console.WriteLine("Age Group: {0}", ageGroup .Key); //Each group has a key 
             
    foreach(Student s in ageGroup) // Each group has inner collection
        Console.WriteLine("Student Name: {0}", s.StudentName);
}

 可以使用foreach遍历group，每个Group包含一个key和内部的集合

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

var groupedResult = studentList.GroupBy(s => s.Age);

foreach (var ageGroup in groupedResult)
{
    Console.WriteLine("Age Group: {0}", ageGroup.Key);  //Each group has a key 
             
    foreach(Student s in ageGroup)  //Each group has a inner collection  
        Console.WriteLine("Student Name: {0}", s.StudentName);
}

ToLookup和GroupBy一样，唯一不同的是GroupBy是延迟执行，而ToLookup是立即执行 

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

var lookupResult = studentList.ToLookup(s => s.age);

foreach (var group in lookupResult)
{
    Console.WriteLine("Age Group: {0}", group.Key);  //Each group has a key 
             
    foreach(Student s in group)  //Each group has a inner collection  
        Console.WriteLine("Student Name: {0}", s.StudentName);
}
       

注意:GroupBy和ToLookup返回一个集合（包含key，根据key分组的内部集合）