Grouping Operators	Description
GroupBy	GroupBy操作返回根據(jù)一些鍵值進(jìn)行分組，每組代表IGrouping<TKey,TElement>對(duì)象
ToLookup	ToLookup is the same as GroupBy; the only difference is the execution of GroupBy is deferred whereas ToLookup execution is immediate.

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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 studentListgroup 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 collectionConsole.WriteLine("Student Name: {0}", s.StudentName);
}

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?可以使用foreach遍歷group，每個(gè)Group包含一個(gè)key和內(nèi)部的集合

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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);
}

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ToLookup和GroupBy一樣，唯一不同的是GroupBy是延遲執(zhí)行，而ToLookup是立即執(zhí)行?

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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);
}

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注意:GroupBy和ToLookup返回一個(gè)集合（包含key，根據(jù)key分組的內(nèi)部集合）

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轉(zhuǎn)載于:https://www.cnblogs.com/lanpingwang/p/6602449.html

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